Cell Biology Keywords

  • Created by: kpaul1234
  • Created on: 15-05-22 10:39
During evolution, a eukaryotic organism, already equipped with adequate cytoskeleton and
internal membrane systems for this purpose, engulfed/phagocytosed a prokaryotic organism
and they started to live together for their mutual benefit.
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central dogma
In living systems, the flow of genetic information is unidirectional and follows the path: DNA
→ RNA → protein → property (phenotype).
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The process in the nucleus by which the genetic information contained in a template strand of
DNA is copied into a single-stranded RNA molecule of complementary base sequence
(primary/ nascent transcript or preRNA or hnRNA).
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Exons are sequences of genes that code part of or the entire gene product. In eukaryotes, the
exons are separated by noncoding sequences called introns. During transcription, the introns
are also transcribed as messenger RNAs, but are subsequently cut out
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A nucleotide sequence in a gene that does not participate in encoding the protein product.
Introns occurring in eukaryotes are transcribed in messenger RNA, but are subsequently
removed from the transcript before translation.
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RNA maturation
Post-transcriptional modifications in eukaryotic cells by which primary transcript RNA is
converted into mature RNA. The process includes three major steps: addition of a 5' end
capping, addition of a 3' poly-adenylation tail, and RNA splicing.
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RNA splicing
A post-transcriptional process by which introns are removed and exons are covalently joined to
one another as part of generating the mature messenger RNA. In the case of multi-exon genes
exons can be joined in different combinations.
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5’ end capping
A post-transcriptional process by which a specially altered nucleotide (7-methylguanosine trior polyphosphate) is added to the 5′ end of the primary RNA transcripts to protect the mRNA
from degradation by RNases. It is a part of RNA maturation.
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poly (A) tailing
A post-transcriptional process by which ~ 200 adenine bases are attached to the 3’end of the
synthesized primary RNA transcripts. It is a part of the RNA maturation process. The tail is important for the nuclear export, translation, and stability.
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translation, genetic code
The process in the cytoplasm by which a polypeptide chain is synthesized from an mRNA
molecule containing the genetic information. In the cytoplasm, the ribosome reads the sequence
of the mRNA bases as triplets.
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The coordinated processes of material uptake, conversion of materials and excretion of
substances taking place between the living system and its environment, which provides the
living being with adequate building materials and energy.
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A process that requires the presence of oxygen in the air or a living organism whose metabolism
requires oxygen.
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A process that does not require oxygen to happen or a living organism that does not require
oxygen for its metabolism.
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An organism that synthetizes ATP by degradation of organic material produced by other
organisms. ATP is used to produce the organism’s own organic compounds and to carry out
other cellular processes. E.g all animals, protozoans, fungi, and most bacteria.
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An organism which uses the sunlight as energy sources (photosynthetic organism) or produces
ATP molecules by simple oxidative processes (chemosynthetic organism). The produced ATP
molecules are used to create its own organic compounds by biosynthesis.
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The glycolysis is a series of enzyme-catalyzed catabolic reactions in the cytosol of the cells, in
which sugars, especially glucose, are partially degraded. The energy released from the process is stored in ATP molecules (and NADH).
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oxidative phosphorylation
During the so-called terminal oxidation catabolic products (mainly the products of glycolysis),
protons accumulate outside the inner membrane of mitochondrion and flow back to the matrix
of the mitochondrion across specific proton channels of ATP synthase
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The cell in which there is no real cell nucleus separated from the cytosol by a nuclear membrane. The genetic material of the cell is found in the cell plasma, there are no cell membrane bound
organelles or cytoskeleton. E.g Bacteria and cyanobacteria.
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Those living organisms that have in their cells a nucleus separated by a membrane from the cell
plasma, have a structured internal membrane system (nuclear membrane, endoplasmic
reticulum, mitochondria, etc.) and cytoskeleton.
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primary biogenic elements
Primary biogenic elements include carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and
phosphorus (P). They constitute 98% of the cells, which is explained by the fact that these
elements form the bulk of the organic matter of living organisms.
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secondary biogenic element
The proportion of secondary biogenic elements in cells is about. 2%. This group includes,
among others, sulfur (S), iron (Fe), chlorine (Cl), sodium (Na), potassium (K), and calcium
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organic substances
Compounds formed by joining several carbon atoms. Organic substances of high energy content
formed through metabolism in living organisms include carbohydrates, fats and proteins.
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amino acids
Molecules containing amino and carboxyl groups. In α-amino acids, the amino group is attached
to the α-carbon of the carboxylic acid chain. The proteins of most living cells are built from 20
different α-amino acids
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deoxyribonucleic acid (DNA)
The hereditary material of cells is constructed of two polynucleotide chains, which form a
double-helix structure. In the nucleotides constituting the DNA, the sugar is deoxyribose and
the nitrogenous bases are A,G,C,T.
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ribonucleic acid (RNA)
A polymer molecule composed of ribonucleotide units. In the nucleotides constituting RNA,
the sugar is ribose and the nitrogenous bases are adenine (A), cytosine (C), guanine (G) and
uracil (U).
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Organic compounds formed in living organisms with varying composition and structure that
have a common feature that they are well soluble in apolar solvents. Hydrolysable lipids include
neutral fats and phospholipids. Neutral fats are built from glycerol
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An intensively stained area (or areas) in the nucleus of eukaryotic cells during the interphase of the cell division cycle. Nucleolus organizing regions (NOR) are particular segments of certain
chromosomes that contain tandem repeats of genes.
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euchromatin, heterochromatin
Euchromatin is a decondensed, transcriptionally active region of chromosome, whereas
heterochromatin is a region of chromosome that remains condensed and transcriptionally
inactive during interphase.
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Simple water-soluble proteins that are rich in basic amino acids. Their positive charges allow
them to associate with the negatively charged DNA molecule, and package the DNA into
structural units of chromatin called nucleosomes.
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The repeating structural units of chromatin, each consisting of approximately 200 base pairs of
DNA wound around a protein core. This core, called histone octamer, is composed of the
histones H2A, H2B, H3, and H4.
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chromatin remodeling
An ATP-dependent modification of chromatin architecture during the process of turning genes
on and off, which regulates the sliding of nucleosomes along the DNA molecule or the
attachment of nucleosomes to DNA by remodeling enzymes.
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DNase sensitivity
The relatively higher sensitivity of transcriptionally active regions of chromatin against certain
nucleases (e.g., DNase I enzyme), which is a consequence of chromatin decondensation
(euchromatisation). This sensitivity extends to the gene.
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DNase I hypersensitivity, DNase I hypersensitive sites
The highly increased sensitivity of transcriptionally active regions, with a length of some
hundred base pairs, of chromatin against DNase I nucleases, because of the absence of
nucleosomes in that particular stretch of DNA.
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Basic units of the cell's chromatin. The structure and function of chromosomes vary during the
cell cycle. In G0/G1 phase, chromosomes are constituted of one chromatid each, their function
is to serve as template for transcription.
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interphase chromosome
Interphase chromosome: Each interphase chromosome consists of a linear DNA (giant)
molecule and its associated histone and non-histone proteins. The ratio of proteins and nucleic
acids is approximately 1:1. They are hierarchically packaged.
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metaphase chromosome
The metaphase chromosome is the most compact form of chromatin that contains two
identical DNA strands (that is two sister chromatids) coding the same information. This type of DNA material is inactive.
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It is made up of a DNA double helix and proteins are attached to it. Prior to cell division, the
gene pool of the cells is doubled and chromosomes, made of two chromatids are created.
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A sum of factors influencing gene activity that are over the information stored in sequences,
like as DNA methylation; posttranslational modifications of histones, e.g., methylation,
acetylation and phosphorylation (called histone code).
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histone code, with example
The pattern of posttranslational modifications of the histones in the nucleosomes along the
chromatin fiber. It is part of the epigenome, together with DNA methylation. There are various
histone modifications, for example methylation, acetylation.
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DNA region regulating transcription. It is a sequence located 5’ upstream from the associated
gene, at which RNA polymerase can bind and which designate the initiation site of transcription
of an associated gene.
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DNA region regulating transcription. It is a sequence, which enhances/amplifies the
transcription of a gene. It can be located long distance from the gene being regulated, but always at the same chromosome.
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nuclear envelope
The nuclear envelope is composed of two concentric lipid bilayers: the inner and outer nuclear membranes and in higher eukaryotes it is supported by the nuclear lamina composed of intermediate filaments. The nuclear envelope is punched by nuclear pores.
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nuclear membrane
It is a double membrane separating the nucleus from the cytoplasm. The two membranes have
different structure and function: The outer membrane is continuous with the endoplasmic
reticulum, and ribosomes can be found on its outer surface
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nuclear lamina
A protein meshwork composed of intermediate filaments beneath the inner nuclear membrane
that mechanically supports the nuclear membrane. It plays an important role in maintaining the structure and stability of nuclear pores.
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nuclear pores, NPC (nuclear pore complex)
A large molecular complex composed of nucleoporins (approximately 100 proteins in
mammals), that spans the double nuclear membrane. It ensures bidirectional, selective transport between the cytoplasm and the nucleus.
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Proteins forming the nuclear pore complex (approximately 50-100 proteins in mammals) are
called nucleoporins. Certain nucleoporins contain unstructured side-chains composed of repeats
of phenylalanine-glycine dipeptides.
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importin beta
A transport receptor protein which facilitates the import of nuclear proteins (cargo molecules)
into the nucleus through the nuclear pore complex by binding either to the cargo directly or to the cargo - importin α complex.
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importin alpha
An adapter molecule (one component of the importin α/β heterodimer) involved in the nuclear
import process. Importin alpha binds to those nuclear localization signals that cannot be
recognized by the importin beta.
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Ran-GTP and Ran-GDP gradient
Ran, a small (monomer) G protein, is involved in the synchronization of nuclear transport
process. Localization of Ran-GDP to the cytosol and Ran-GTP to the nucleus results from the
localization of two Ran regulatory proteins: Ran-GAP and Ran-GEF.
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NLS (nuclear localization signal)
The selectivity of the nuclear import process is based on the nuclear localization signals (NLSs),
which are present only in nuclear proteins. NLS is a (7-10 amino acid long) sorting signal
(motif) and it is recognized by importin molecules.
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NES (nuclear export signal)
Proteins exported from the nucleus bear a nuclear export signal (e.g., a leucine-rich sequence)
that is recognized by transport receptors called exportins.
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A GTPase activating protein localized in the cytoplasm (it is anchored to the cytoplasmic fibrils
of the pore complex). Ran-GAPs stimulate the GTPase activity of Ran, and GTP hydrolysis
occurs immediately when Ran arrives to the cytosol.
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Ran-GEF (RCC1)
Ran must release its bound GDP to acquire GTP. GDP dissociation from Ran is slow but is
stimulated by a nucleotide exchange factor (guanine exchange factor) named Ran-GEF, which
is exclusively localized in the nucleus.
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endoplasmic reticulum (= ER)
An intracellular membrane system characteristic of eukaryotic cells located in the
cytoplasm near the nucleus. The surface of the rough ER (RER) binds ribosomes, which
synthesize proteins that need to be in the membrane or in the lumen of the RER.
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A process which is typical for liver cells. Lipid-soluble /hydrophobic compounds, drugs and
dangerous substances arising in the body are transformed to water-soluble /hydrophilic
compounds by an enzyme system present in the SER.
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cytoplasm (cell plasma)
A gelatinous material that fills the cells where the cell’s metabolic processes take place.
Characteristic of all cells.
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A two-subunit cellular constituent built up from proteins and RNA (ribosomal RNA,
rRNA). It is found in both prokaryotic and eukaryotic cells. It catalyzes the synthesis of
proteins (polypeptides).
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Golgi complex
An organelle usually located near the nucleus, which is composed of flattened membrane
disks, the cisternae, stacked upon each other. It has a polarized organization with various
compartments characterized by different enzymes.
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A membrane-bound organelle with a variable appearance. It is where degradation of materials inside the cells takes place. The material to be degraded can be of intracellular and extracellular origin. Lysosomes contain acid hydrolases.
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A membrane-bound organelle, which may have an endosymbiotic origin and it is present in
large amount in liver and kidney cells. The enzymes of peroxisomes (e.g., peroxidases) are
used in oxidative decomposition processes (mainly fatty acids, alcohol.
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storage disease
Diseases characterized by lysosomal dysfunction arising as a result of an inherited mutation
of one of the lysosomal enzymes. The lack of enzyme activity results in accumulation of
undigested substrates in the lysosomes. Eg Tay – Sachs and Fabry disease.
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A process by which a cell decomposes its own components in its lysosomes. An organelle
or cellular constituent separated from the rest of the cell by a membrane is an
autophagosome which is converted to an autolysosome by fusion with a lysosome.
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A post-translational modification of proteins by which oligo/monosaccharide residues are
attached to certain amino acid residues of proteins in the rough endoplasmic reticulum and
the Golgi complex. The two most frequent types are N-&O-glycosylayion.
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ER signal sequence
A sequence composed of ~15-30, mainly hydrophobic, amino acids on the N-terminal of
certain polypeptides. The ER signal sequence binds to the SRP (signal recognition particle)
in the cytoplasm and directs the ribosome to the docking site.
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quality control in the ER
The quality control system located in the ER lumen ensures that only correctly folded
proteins, whose appropriate 3D structure has formed spontaneously or with the help of
chaperon proteins (e.g., Bip), can leave the ER.
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protein sorting (in the trans Golgi)
Classification of proteins reaching the Golgi complex takes place in the trans-Golgi
network, whose sorting activity directs proteins into different pathways while trans-Golgi
resident proteins having a Golgi retention signal stay in the trans Golgi.
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SNARE proteins
A compartment-specific molecule pair whose members recognize each other during
vesicular transport. This recognition process ensures the specificity of docking and fusion
of vesicles.
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A process by which cells take up materials in a membrane-enclosed vesicles formed by the
invagination of the plasma membrane. According to the size and solubility of the
endocytosed material it can be classified to pinocytosis or phagocytosis.
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receptor-mediated endocytosis
Selective uptake of materials by cells. In its most important form, a clathrin coat is generated
at the cytoplasmic face of the plasma membrane. These clathrin-coated pits recruit the
transmembrane proteins and their cargo to be endocytosed.
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Transcellular vesicular transport in which materials taken up by endocytosis at the apical
membrane domain of cells are released without any modification by exocytosis at the
basolateral membrane domain. The process can be in the opposite direction.
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A vesicular transport process in which the membrane of a transport vesicle fuses with the
plasma membrane thereby releasing its content to the extracellular space.
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A special type of endocytosis in which larger particles are engulfed by single cell organisms
or cells of multicellular organisms (e.g.by macrophages and neutrophil granulocytes that
are subtypes of white blood cells).
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A coat-forming protein. A clathrin monomer composed of three heavy and three light chains
is arranged in special three-legged, so-called triskelion units, which can oligomerize to form
five-sided or six-sided rings (pentagons and hexagons).
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amphipathic molecule
Molecules (ions) containing apolar and polar (or ionic) moieties are called amphipathic
substances. Such molecules are e.g., the phospholipids.
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Molecules or parts of molecules with unequal charge distribution (dipoles). The polar
particles interact with water, i.e., they are "water-lovers", hydrophilic.
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A molecule or part of a molecule that does not undergo charge separation. Therefore, it does
not enter into significant interaction with water.
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lipid-water partitioning coefficient and its significance in transport
A quotient used for characterization of the hydrophobic character. It is the ratio of
equilibrium concentrations of a molecule measured in contiguous lipid- and water phases:
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passive transport
A material flow through biological membranes (cell membrane) that does not require
cellular energy because the concentration difference between the two sides of the membrane and the electric potential difference together provide the driving force.
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facilitated diffusion
Specific proteins help transfer certain substances (e.g., ions, glucose, some medicines)
through biological membranes (cell membrane, membrane of cell organelles). The process
does not require direct energy input from the cell.
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active transport
They transport ions against their electrochemical potential gradients or uncharged
molecules against their concentration gradients using the energy directly from ATP
hydrolysis (e.g., Na+/K+-pump, Ca2+
-pump, P-glycoprotein, V-types proton ATPase.
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secondary active transporters
In contrast with primary active transport, there is no direct coupling to ATP hydrolysis;
instead, the electrochemical potential difference of an ion is used. E.g., glucose-Na symport,
amino-acid-Na symport.
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Na/glucose coupled transport (secondary active) transport
Na/glucose symporter transports 2 Na+ and one glucose molecules into the cells
simultaneously and in the same direction, at the apical surface of epithelial cells of small
intestine (facing intestinal lumen) using the electrochemical potential of Na+.
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Na/Ca coupled transport (secondary active) transport
The Na+/Ca2+
-antiport transports 1 Ca2+ outward against its electrochemical potential
together with the inward transport of 3 Na+
simultaneously. The antiport uses the energy of
the electrochemical gradient of Na+
for the “uphill” transport of Ca2+.
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glucose uniport, with example
Glucose uniport is a type of facilitated transport, it does not require ATP hydrolysis, glucose
is transported in the direction of its concentration gradient. The transporter molecule
oscillates between two main conformations.
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V-type transporters
Vacuolar-type proton transporters in the membrane of membrane enclosed organelles which transport protons into these organelles. E.g., they are responsible for the low pH of
lysosomes. During ATP hydrolysis, the transporter does not become phosphorylated.
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P-type transporters (with 2 examples)
During their operation, they are phosphorylated transiently and the conformation change
elicited by this leads to the transport of the ion transported by the given transporter. The
Na+/K+-ATPase or plasma membrane Ca2+
-ATPase create ion gradients.
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adenylyl cyclase (adenylate cyclase)
A membrane-bound enzyme activated by the trimeric G-protein Gs which plays an
important role in signal transduction processes. It catalyzes the formation of the secondary
messenger cAMP from ATP.
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autocrine signaling
A type of signaling where the secreting/signaling cell and the target cell are the same, so
the same cell expresses the signal molecule (e.g., growth factor) and its receptor. It can be
important in coordinating groups of cells.
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It is produced inside the cell during signal transduction processes, e.g., upon the effect of
glucagon or epinephrine. It is produced by adenylyl cyclase (adenylate cyclase) from ATP.
It is not able the penetrate the cell membrane.
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cellular immunity
Cell mediated immune response. A subsystem of the adaptive immune system in which Tlymphocytes, activated in specific interactions, take part in the defense against antigens.
The T-cells mediating these effector functions belong to several groups.
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cytokine receptor superfamily
Dimer or trimer cell surface receptors of versatile structure, activated by cytokines. They
have no enzyme activity. Many of them act through the JAK/STAT pathway, but can also
activate other small, cytoplasmic tyrosine kinases of the src family.
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DAG (diacylglycerol)
A small signaling molecule (second messenger) in the membrane, produced by the cleavage
of membrane phosphatidylinositol bisphosphate (PIP2) by phospholipase C in response to
extracellular signals. It can activate protein kinase C.
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EGF receptor
It belongs to the group of cell surface receptors with intrinsic (own) enzymatic activity.
Binding of epidermal growth factor (EGF) leads to receptor activation only when the
receptor is in dimers or higher oligomers.
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endocrine signaling
The type of intercellular signaling where signaling cells in endocrine glands secrete the
messenger molecules (hormones) into the blood stream, which carries the message to every
part of the body. This is how the signal molecules reach remote target cells
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phospholipase C
An enzyme attached to the cytoplasma membrane. Some types are activated by receptor
tyrosine kinases via their SH2 domains. Others are activated by G proteins linked to 7-
transmembrane (serpentine) receptors (GPCR). They cleave PIP2 to produce DAG&IP3.
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G protein coupled receptors
Cell surface receptors without intrinsic (own) enzyme activity that bind ligands such as
glucagon and epinephrine. They have seven transmembrane alpha helices, 7-transmembrane receptors, or serpentine receptors.
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gap junction
Non-selective channels connecting neighboring cells. They are permeable to small
molecules like cAMP and ions like Na+ up to about 2000 Da size. Six connexin molecules
form a circular tunnel named connexon, & two connexons couple to form the gap junction.
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Extracellular signal molecules (primary messengers) secreted by endocrine glands that get into the blood stream and thereby reach remote target cells and alter their functions by
binding to their receptors.
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adaptive (acquired) immunity
Function implemented by lymphocytes (B and T cells), which provides specific protection
against foreign materials during the life of an individual, including the establishment of
immunological memory.
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natural (innate) immunity
A coherent defensive system of phagocytic cells and of proteins that act immediately upon
the appearance of any foreign (dissimilar to the own) substance inside the organism.
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memory cells
A specific immune response generates lymphocytes against a specific antigen. Of these, memory cells are long-lived, which allows a much faster detection of the antigen and activation of the immune system upon a repeated encounter with the antigen.
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humoral immunity
A subsystem of the adaptive immune system. Here, the major instruments of defense against
foreign materials (antigens) are the antibodies produced by plasma cells that have
differentiated from B-cells after those have been activated by the antigen.
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intracellular receptors / nuclear receptors / steroid receptor superfamily
Members of the steroid receptor superfamily are transcription factors located intracellularly.
Their ligands are small molecular weight, hydrophobic molecules (steroid hormones,
retinoids, thyroxine, vitamin D). They bind to specific DNA sequences.
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insulin receptor
A heterotetrameric cell surface receptor with intrinsic enzyme activity. It belongs to the
subgroup of receptor tyrosine kinases. It is activated by autophosphorylation upon binding
insulin. It also phosphorylates various intracellular proteins.
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Inositol trisphosphate. Second messenger produced by phospholipase C from the
membrane constituent phosphatidylinositol bisphosphate. IP3 diffuses in the cytosol
and releases free calcium from intracellular Ca2+ stores upon binding to the IP3 receptor.
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JAK (Janus kinase)
They belong to a family of intracellular, non-receptor tyrosine kinases that transduce
cytokine-mediated signals. They get activated on ligand bound cytokine receptors,
phosphorylate the receptor, each other, and downstream STAT.
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juxtacrine signaling
The subtype of paracrine signaling whereby the signal molecule is bound to the membrane
of the signaling cell and binds to its receptor which is in the membrane of the neighboring cell, generating a contact-dependent signal.
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calcium-calmodulin dependent kinase (CaM kinase)
Serine/threonine kinases activated by the Ca2+
- calmodulin complex. Phosphorylate serine
and threonine residues of proteins, such as the myosin light chain kinase (MLCK).
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Enzymes that link the high energy phosphate group of ATP to OH residues of target
molecules. The process is phosphorylation, it is reversible: the removal of phosphate by
phoshatases is dephosphorylation.
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MAPK pathway
The mitogen activated protein kinase pathway is one of the earliest recognized
signaling route regulating cell division (but also other metabolic processes). Its central
protein is the monomeric GTPase switch, Ras.
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second messenger
Molecules that relay the extracellular signal received by cell surface receptors to
intracellular target molecules. They are produced or released in specific reactions initiated by the ligand bound receptors, and are consequently degraded or taken up into
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Type I main histocompatibility complex proteins, which are expressed on all somatic cells
and play a role in antigen presentation. Endogenous proteins (the cells’ own, or viral) are
cleaved in proteasomes resulting in 8-15 amino acids long oligopeptides.
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Type II main histocompatibility complex proteins are expressed only on professional
antigen presenting cells, such as dendritic cells, macrophages and B lymphocytes, and play
a role in antigen presentation.
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the two major types of neurotransmitters (according to site of synthesis)
They are either clear synaptic vesicles that are synthesized in nerve terminals and taken up into the vesicles by proton antiporters (e.g epinephrine, dopamine), or neuropeptides (e.g oxytocin,vasopressin), which are synthesized in the RER in cell body.
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Types of effects, receptors and inactivation of neurotransmitters
Their effect can be activating (excitatory, e.g., ACH, glutamate) or inhibitory (e.g.,
GABA, glycine), receptors can be metabotropic (G-protein coupled) or ionotropic
(receptors with ion channel activity). Inactivation can be enzymatic degradation or reup
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Typically, a mutant or dysregulated form of a normal gene (proto-oncogene) involved in
the control of cell growth and division, such as EGFR, ras, src, abl, myc. Consequently,
oncogenes can cause dysregulated cell division and potentially, tumors.
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paracrine signaling
In this form of signaling, the secreted messenger molecule does not get into the blood
stream, rather, it reaches the target cells, maximally in a few mm distance, by diffusion
through the ic space. Its subtypes are: autocrine, juxtacrine, intracrine, syn
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protein kinase C
PKC enzymes are cytosolic serin threonine kinases, which are lipid dependent. They
translocate to the cell membrane and are activated by diacylglycerol formed there by
phospholipase C. Some subtypes also need high calcium concentration for translocation.
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Genes coding properly regulated proteins that take part in signaling feeding into cell
division and driving the cell cycle. For example, genes coding growth factors, their
receptors, switch and messenger proteins, cyclins and transcription factors.
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A monomeric G protein, which takes part e.g., in signaling by the EGF receptor as an
intracellular switch. It is active in GTP bound form,&it hydrolyzes GTP to GDP to
become inactive. GAPs (GTPase activating proteins) enhance its GTP hydrolytic activity.
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receptor tyrosine kinase
They are a subgroup of cell surface receptors with intrinsic (own) enzymatic activity. The
receptor is activated in dimers or higher oligomers. The dimerization partners crossphosphorylate each-other (auto-phosphorylation) on tyrosine residues.
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Signal transducers and activators of transcription: transcription factors active in the dimer
form, which is promoted by their phosphorylation on tyrosine by JAK kinases. For
dimerization, they bind each other’s phosphotyrosine with their SH2 domains.
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tumor suppressor
Negative regulators of growth signalling and the cell cycle, such as PTEN, p53, p21 and
retinoblastoma protein (Rb). Their inactivation, deletion, or loss of function mutations can
lead to development of tumors.
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ABC proteins
They consist of two ATP-binding sites (NBDs) responsible for binding and hydrolysis of
ATP molecules and two transmembrane domains (TMDs) that collectively form the
substrate binding sites.
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Cystic fibrosis transmembrane conductance regulator = CFTR=ABCC7
It is a channel type ABC protein. This is a Cl channel expressed in the apical
membrane of epithelial cells. Channel opening is induced by ATP binding of the nucleotide
binding domain (NBD) and the phosphorylation of the regulatory (R) domain by PKA.
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Cystic fibrosis (CF)
It is a multiorgan disease caused by inactivating mutations of the CFTR Cl- -ion channel
expressed in epithelial cells. The high viscosity of secreted mucus causes symptoms
affecting the lungs, the gastro-intestinal system and the reproductive system.
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multidrug resistance
Resistance of cancer cells to numerous structurally or functionally unrelated anticancer
agents, which are extruded from the cells by ABC transporters and consequently cannot
reach lethal intracellular concentration.
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TAP1/TAP2 oligopeptide transporter
It is a heterodimeric transporter formed by half transporter molecules TAP1&TAP2. The
TAP1/TAP2 transporter is expressed in the ER membrane. It pumps oligopeptides formed upon the proteasomal degradation of cellular and certain and certain viral proteins
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ABCG2 (Breast Cancer Resistance Protein, BCRP)
It is an active transporter type ABC protein with a wide substrate spectrum partially
overlapping with the substrate spectrum of Pgp involving xenobiotics and numerous
structurally unrelated anticancer agents.
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sulfonylurea receptor 1 = SUR1
It is a channel regulator type ABC protein. Together with the pore forming Kir6.2 subunits
SUR1 molecules form an ATP-sensitive potassium channel localized in the plasma
membrane of pancreatic β cells. It is involved in the regulation of insulin secretion
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ion channel gating
Appropriate trigger causes a conformational change in the protein resulting in the transition
among different conducting and non-conducting states (closed, open, inactivated) of the channels.
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ion channel selectivity
The passage of one or some ion species is allowed through the pore of an open ion channel.
The ion conduction pore of a highly selective (e.g., K+, Na+, Ca2+, Cl-), mildly selective (e.g., acetylcholine receptor) and non-selective (e.g., gap junction.
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voltage sensor
A domain in voltage-gated ion channels composed of α helical segments and containing positively charged a.a side chains. Structural rearrangement of the voltage-sensors in response to changes in the membrane potential leads to conformational change in the
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The junction of a neuron with another cell. The stimulation can be spread from the presynaptic cell to the postsynaptic cell through neurotransmitters or directly through gap junctions of adjacent cell membranes.
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chemical synapse
Specialized junction between the nerve cell and another cell across which the nerve cell transmits its signal. The action potential in the
presynaptic cell causes the release of the neurotransmitters.
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ionotropic and metabotropic (neurotransmitter) receptors
receptors are ligand gated ion channels. The signal molecule binds to the ion channel itself, which then opens. It has fast action, Metabotropic receptors are not ion channels it regulates a channel through signaling pathway and it's fast actio
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ionotropic acetylcholine receptor (Nicotinic acetylcholine receptor)
A ligand-gated ion channel found at the neuromuscular junction in the plasma membrane
of skeletal muscle cells. The channel is made up of five subunits arranged around a central
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metabotropic acetylcholine receptor
It belongs to the family of G-protein-coupled receptors. The serpentine receptor activates a
G-protein upon Ach binding. The G-protein exchanges GDP to GTP and dissociates into α and βγ subunits which regulate effectors.
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electric synapse
A form of synapse between two cells (pre- and postsynaptic cells) in which electrical
impulse is transmitted rapidly due to the free passage of ions from one cell to another via
the gap junction.
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KATP channel
An ATP-sensitive potassium channel localized in the plasma membrane of pancreatic β
cells is composed of pore forming subunits (Kir6.2) and sulfonylurea receptor (SUR1, an
ABC protein) subunits.
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channelopathies (with two examples)
Diseases involving gain of function or loss of function of ion channels that are generally
caused by mutations in genes encoding ion channels or the proteins that regulate them,
however often resulted from autoimmune attack on ion channels.
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excitatory synapse (with 1 example)
A synapse, in which the activation of the neurotransmitter receptor on the postsynaptic cell
causes depolarization via the influx of cations (usually Na+, Ca2+). This shifts the membrane
potential closer to the action potential firing threshold.
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inhibitory synapse (with 1 example)
A synapse, in which the activation of the neurotransmitter receptor on the postsynaptic cell
causes hyperpolarization via the influx of anions (usually Cl-). This shifts the membrane
potential away from the action potential.
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plasma membrane Na+/Ca2+ antiport (NCX)
Electrogenic, secondary active transporter located in the cytoplasm membrane. 3 Na+
are transported into the cell according to their electrochemical gradient and 1 Ca2+ is transported out of the cytosol against its electrochemical gradient.
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plasma membrane Ca2+ ATP-ase (PMCA)
Electroneutral, primary active transporter, P-type ATP-ase, located in the cytoplasm
membrane. 1 Ca2+ -is transported from the cytosol to the extracellular space against its electrochemical gradient, accompanied by the inward transport of 2 H+.
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Sarco-endoplasmic reticulum Ca2+ ATP-ase, primary active, P-type ATP-ase transporter
located in the membrane of the SR&ER. The transporter transports 1 Ca2+ from the cytosol into the lumen of the ER/SR at the cost of the hydrolysis of 1 ATP.
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ryanodine receptor (RYR)
Intracellular ligand-gated Ca2+ channel located in the membrane of the SR&ER. The ligand activating the channel in skeletal muscle cells is a part
of the DHP receptor whereas in cardiac myocytes and neurons the ligand is Ca2+.
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IP3 receptor (IP3-R)
I.C ligand-gated Ca2+ channel located in the membrane of the ER. The ligand activating the channel is IP3 generated in the cell membrane upon receptor-ligand interaction. Upon opening of the channel Ca2+ is released from the ER store into the cytosol.
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Cytosolic Ca2+-binding protein with 4 Ca2+ binding pockets having EF-hand structure which
cooperatively bind Ca2+. Depending on the Ca2+-saturation of the binding sites the
conformation of calmodulin changes dramatically.
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pump-leak model of osmo- and volume regulation
This is a homeostatic regulation of the cell volume which operates in isotonic medium. The
tendency of inorganic ions to reach thermodynamic equilibrium results in a net influx of ions (Donnan effect) which is counterbalanced by the Na+/K+-pump.
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RVD (Regulatory volume decrease)
This cell volume regulatory mechanism is induced by cell swelling in hypotonic medium
and leads to the reduction of the cell volume and loss of water even if the hypotonic
condition is maintained.
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RVI (Regulatory volume increase)
This cell volume regulatory mechanism is induced by cell shrinkage in hypertonic medium
and leads to the increase in the cell volume by gaining water even if the hypertonic condition
is maintained.
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steady-state pH of the cytosol
At this pH the rate of base efflux (e.g., the Cl-
/HCO3-antiport) from the cells equals to the
rate of the acid efflux (e.g., by the Na+
/H+antiport), both classes of transporters work at the same speed and thus the cytosolic pH remains constant.
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Na+/H+ antiport
An electroneutral exchanger in the cytoplasm membrane that mediates the influx of Na+and
the efflux of H+ from the cytosol. The main function of the transporter is the regulation of
the cytosolic pH.
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An electroneutral exchanger in the cytoplasm membrane that mediates the influx of 1 Cl into, and the efflux of 1 HCO3-from the cytosol during one duty cycle. The main function of the transporter is the regulation of cytosolic pH.
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ATP (Adenosine triphosphate)
A nucleotide containing adenine, which is a vital
source of chemical energy in all living organisms. Adenine is bound to a D-ribose (making
an adenosine) and the D-ribose also carries three phosphate groups linked with linear
covalent bonds.
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ATP synthase
The ATP synthase is a molecule complex localized in the inner membrane of mitochondria
and catalyzes the synthesis of ATP from ADP and inorganic phosphate. It has two main
parts: the membrane spanning proton channel (F0) and the ATP synthetizing (F1) part
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Chemiosmotic theory
The energy released during the electron transport via the complexes of the respiratory chain
in the inner mitochondria membrane drives H+ actively to the intermembrane space (by
H+ pumps); therefore, the H+ concentration in the intermembrane is higher tha
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The mitochondrion is an organelle with variable shape, size and number in eukaryotic cells
and has endosymbiotic origin. The majority of energy production of cells happens in
mitochondria. They are bounded by two membranes having different compositions.
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respiratory chain
The respiratory chain is localized in the inner membrane of mitochondria and is composed
of proton pumping molecules which are easily oxidized and reduced and also hem
containing cytochrome proteins.
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Complex iron containing proteins, in which the iron is contained in the molecule called
hem, similarly to hemoglobin. The central iron ion of the hem can undergo redox reactions.
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The thermogenin is a protein in the inner membrane of mitochondrion which works as a
proton transporter in brown adipose tissues. Its main role is to produce heat by short cutting
the proton gradient and thus maintaining the constant core temperature.
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The porins are channel-forming transmembrane proteins localized in the outer membrane
of mitochondria (and also in the membrane of Gram-negative bacteria). The porins make
the outer membrane permeable to small molecules (≤5 kDa).
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The cardiolipin is a diphosphatidylglycerol, which has four fatty acid chains. This special
lipid component of the inner mitochondrial membrane reduces the proton-permeability of
the membrane and thus facilitates the production of proton gradients.
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mitochondrial DNA
Mitochondrial DNA is circular, double-stranded DNA and has many copies in each
mitochondrion. It is derived from the circular genomes of the bacteria that were engulfed
by the early ancestors of today's eukaryotic cells according to endosymbiotic theory.
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mitochondrial DNA coding
Some of the proteins of the mitochondrion are coded by its own genome, the mitochondrial
DNA. The greater number of proteins is coded by the genomic DNA&their synthesis is carried out on the cytoplasmic ribosomes&transported into the mitochondria.
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mitochondrial ribosome
The protein synthesis in the matrix of the mitochondria takes place on mitochondrial
ribosomes. 13 polypeptide chains are coded by mitochondrial DNA. Both the size (60S)&sensitivity to antibiotics of mitochondrial ribosomes are similar to that of bacteria
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protein import into mitochondria
Mitochondrial proteins synthetized in the cytoplasm have an N-terminal mitochondrial
targeting (signal) sequence which directs them into the matrix. They are imported in
unfolded form across protein channels spanning both mitochondrial membranes.
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proofs for bacterial origin of mitochondrion
1. Mitochondria have their own DNA
2. The size of mitochondria is similar to the size of the majority of bacteria (~ 1 µm).
3. Mitochondria are bounded by two membranes which are different from each other both
in their chemical compositions and structure.
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Fibronectins are multi-adhesive matrix proteins having heterodimer structure. Most of the
time they serve as typical extracellular matrix components of the connective tissue. Their
primary functions are to anchor the cells to the extracellular matrix.
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Integrins are cell surface receptors of multi-adhesive matrix proteins and have heterodimer
structure (α- and β-subunits). They are important in forming cell-cell or cell-matrix
interactions and in cell signalling.
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conformational change of integrin
In stimulated cells, some integrin molecules change their conformation after
phosphorylation of an intracellular region of the integrin molecules. Thereby the RGD
specific binding site is exposed on the surface of these integrins and can bind the ligand.
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Cadherins are members of a family of Ca2+
-dependent proteins that mediate the attachment
of one cell to another in animal tissues forming junctions between them. Cadherins form
homophilic interactions.
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RGD presentation
The RGD tripeptide motif is composed of arginine-glycine-aspartate amino acids and is
typical for ligands of integrins. The RGD is recognized by the binding site on the
extracellular region of integrins. The motif can be found in ECM macromolecules.
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Selectins have several transmembrane domains and bind the carbohydrate groups of
glycoproteins and glycolipids expressed on the surface of neighboring cells.
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The extravasation is the passage of white blood cells across intact capillary walls. Upon
injuries or infections, the expression of P-selectins increases on the surface of the
endothelial cells of capillaries.
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adhesion belt (zonula adherens)
The adhesion belt is the belt-like zone localized mainly around the epithelial cells, usually
under the tight junction, including the actin filaments attached to it from inside the cell. Ecadherin is the transmembrane protein.
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desmosomes (macula adherens)
Desmosomes are plaque-like junction structures between cells which link cells
mechanically to each other. The cell-cell connection is ensured by special cadherins
(desmoglein and desmocollin molecules).
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Hemidesmosomes attach the cell to the extracellular matrix. Hemidesmosomes are
asymmetrical and are found in epithelial cells connecting the basal surface of the cell to the
basal lamina. The transmembrane integrin molecules connect the proteins.
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focal adhesion
Focal adhesion plaques bind the microfilaments (actin fibers) of cells to extracellular matrix
molecules via integrin molecules. Vinculin-, talin- and α-actinin molecules are involved in
the attachment of the intracellular region of integrin molecules.
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The connexon is the structural unit of gap junctions. It is formed of six transmembrane
connexin proteins. The connexon units in the membrane of two neighboring cells associate
with each other and form a small diameter (1.5 nm), channel.
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tight junction (occluding zone)
Cell–cell junction composed of a double layer of claudin and occludin molecules that seals
adjacent epithelial cells tightly together, running around each cell body close to the apical
surface. It prevents the passage of most dissolved molecules.
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It is made up of filamentous proteins, provides mechanical stability for the cells and
restricts the position of intracellular organelles. It is involved in intracellular transport
processes, cell movement and cell division.
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Microtubules are approximately 25 nm in diameter. They are long cylindrical structures
built from 13 parallel protofilaments, each composed of heterodimers of alpha- and betatubulin molecules.
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Thin flexible fibers of approximately 7 nm in diameter, made from globular actin molecules.
The actin subunits assemble head-to-tail, using the energy of ATP, to generate filaments
with a distinct structural polarity.
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intermediate filament
Intermediate filaments have a diameter of about 10 nm and their major function is to provide
mechanical strength to cells and tissues. Cytoplasmic intermediate filaments are tissue
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An important feature of μ-tubules. The speed of polymerization of μ-tubules at their
positive end and the speed of depolymerization at their negative end is equal, thus the length
of the microtubules doesn't change, but the position of the tubulin dimers
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Tubulins are the protein subunits of microtubules. They have globular structure. Alpha and
beta tubulins can form a heterodimer with non-covalent binding, and these heterodimer
molecules form the microtubules after polymerization.
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GTP cap
A μ-tubule has GTP-containing tubulin subunits at its growing end, forming a GTP cap.
During polymerization the incorporation of GTP-bound tubulin dimers is faster than the
hydrolysis of GTP, which leads to the accumulation of GTP-bound tubulin dimers.
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It is plant alkaloid from autumn crocus (Colchicum autumnale). Via binding to the free
tubulin subunits, it can block the polymerization of microtubules and inhibit their dynamic
instability. It can cause fast dissociation of the microtubules.
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Flattened, sheet-like protrusions supported by a dynamically changing meshwork of actin
filaments, which are extended at leading edge of a crawling animal cell.
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Intermediate filament-associated protein, which can create cross-bond between the
intermediate filaments and microtubules or between the intermediate filaments and actin
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motor proteins
Motor proteins convert chemical energy (usually of ATP) into kinetic energy, resulting in
their movement along filaments. E.g: (1) Kinesin moves towards the (+) of a μ-tubule, (2) Dynein moves toward the (-).
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dynamic instability
μ-tubules and μ-filaments are dynamic structures that grow by adding tubulin dimers
(using GTP energy) or actin monomers (using ATP energy) and shrinking by
depolymerization. This is the sudden conversion from growth to shrinkage, or vice versa.
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cell cortex
It is a 3D network of actin filaments on the inner face of the plasma membrane. It is attached
to the cell membrane via membrane-anchoring proteins.
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Actin sequestering proteins
These proteins bind to actin monomers and/or filaments and modify the dynamic
assembly/disassembly of the microfilaments. Thymosin is an actin monomer sequestering
protein, which can block incorporation of monomers on both ends of the actin filaments.
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Actin binding toxins
Toxins that bind actin and can block dynamic actin assembly/disassembly. E.g: (1) Cytochalasin: binds to the plus end of the actin filament
and blocks polymerization. (2) Phalloidin is a toxin that binds to and stabilizes filamentous actin.
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cell cycle
The life of eukaryotic cells, which lasts from the formation of the cell to the next cell
division. It can be divided into two phases, the resting period and the subsequent cell
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Cell division which reduces the number of chromosomes. The number of chromosomes of
the 4 progeny cells is half of the number in the parent cell, and their genetic constitution is
a mixture of the parents.
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interphase – and its subphases
A period NOT containing the M-phase of the cell cycle of proliferative cells, which can be
divided into further parts: G1 phase is the growing stage of the cell; S phase is the part of
DNA and histone synthesis, and G2 phase.
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mitosis, karyiokinesis – and its subphases
Nuclear division, within the M-phase of cell cycle in proliferating cells, which produces
two nuclei with type and number of chromosomes identical to the parent cell.
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start (= commitment = restriction) point of the cell cycle
A checkpoint in the G1 phase of the mitotic cell cycle where the cell is determined to begin
DNA synthesis. Beyond this point (time point), the cell goes through to S-phase even under
unfavorable conditions (e.g., inhibition of protein synthesis).
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MTOC (centrosome)
It is composed of 2 centrioles and the pericentriolar material which surrounds them. The
gamma-tubulin rings embedded into the pericentriolar material are the nucleation sites of
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centriole cycle
Centrioles of the centrosome are duplicated in a semiconservative manner. In the S-phase
of the cell cycle the two centrioles localized in the centrosome separate and move away
from each other.
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It is the protein structure assembled on the centromere of chromatids where the kinetochore
microtubules of the spindle attach during mitotic and meiotic cell division, to pull the
chromatids apart.
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mitotic spindle
It consists of polar (interpolar), kinetochore and astral (unattached) microtubules. These
connect the poles to each other, to the kinetochore of chromosomes, and to submembrane cell cortex, respectively. Their role is to segregate the chromosomes.
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The M-phase of the cell cycle can be divided into two overlapping parts, mitosis (more
accurately called karyokinesis) and the division of the cytoplasm (cytokinesis). Cytokinesis
begins in the anaphase B and ends in the telophase of M-phase.
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cleavage furrow
In cytokinesis a groove, called the cleavage furrow, is formed due to the activity (1) of a
contractile actin-myosin ring developed under the cell membrane in the equatorial plane of
the cell, and (2) new membrane synthesis.
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ubiquitination and its role
Ubiquitin is a small protein. Protein complexes with ubiquitin ligase activity called
ubiquitinating complexes (e.g., APC, SCF) attach chains of ubiquitin covalently to certain
target proteins, labelling them to be degraded in the proteasome.
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APC (ubiquitinating complex)
Anaphase promoting complex. A regulatory molecular complex which is responsible for
the onset of the anaphase of mitosis. It becomes active after its phosphorylation by MPF
and when the complex binds an additional protein called cdc2. It targets securin.
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SCF (ubiquitinating complex)
An ubiquitinating complex responsible for the proteasomal degradation of certain CDK
inhibitor proteins (CDKIs), of cdc6 and of certain cyclins during the G1-S phases.
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dynamic instability, its role in mitosis
The dynamic instability is regulated in mitosis so that in the anaphase, depolymerization occurs at the kinetochores which, cooperating with (-) end motor proteins carrying chromosomes towards the poles.
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treadmilling, its role in the cell cycle
A phenomenon characteristic of μ-tubules and also of actin filaments the length of cytoskeletal elements does not change since the same number of units is added at the + end that dissociates at the - end, the position of each molecule is changing in time.
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cyclin dependent kinases (CDK-s)
Cyclin activated kinases which regulate the progression of the cell cycle. Their quantity
does not, but their activity oscillates simultaneously with the concentration of cyclins.
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cyclins (with an example)
Cyclins are involved in the activation of cyclin dependent kinases (CDK) and thereby help
to control the progression of cell cycle from one stage to the next. Cyclins also determine
the substrate spectrum of the CDK molecules.
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cohesin, securin, separase
Proteins which play roles in the separation of sister chromatids in the anaphase of mitosis.
The APC is activated directly before the onset of anaphase&ubiquitinates securin, which
keeps the separase in inactive form, so it is degraded in the proteasome.
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positive feedback in cell cycle regulation
A direct substrate of MPF involved in the regulation of the cell cycle is a phosphatase which
removes the inhibiting phosphate group from the inactive MPF (containing both an
activating and an inactivating phosphate group), thereby activating the complex.
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negative feedback in cell cycle regulation
The active MPF, which is a cyclin-CDK complex, activates among other substrates the APC, which ubiquitinates and thereby degrades the cyclin component of MPF, therefore the cyclin concentration falls down, causing the MPF to become inactivated.
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checkpoint control mechanisms with 2 examples
The checkpoint mechanisms involved in cell cycle regulation halt the cell cycle in case of
any abnormality and give a chance to repair it. E.g., a DNA damage checkpoint operating
in G1 and the kinetochore checkpoint.
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kinetochore checkpoint
A checkpoint in the metaphase of cell division, which controls the correct attachment of the
kinetochore microtubules to the chromatids and the balance of forces pulling at the
kinetochores of the two chromatids.
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DNA re-replication control
A molecule (cdc6) necessary for the start of DNA replication from the replication origins,
is degraded after it has performed its function at the origins,&a new round of replication
cannot start until the synthesis of new cdc6 molecules in the next G1.
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A tumor suppressor gene (antioncogene) product, acting mainly at the G1/S transition. It is
a transcription regulator (transcription factor) of the p21 gene, which is a CDK inhibitor
(CDKI). It is involved in the cell’s response to DNA damage.
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p21 protein
CDK inhibitor protein, it binds to and inhibits the activity of G1-S cyclin-CDK complexes.
The expression of this gene is tightly controlled by the tumor suppressor protein p53. The
p21 protein mediates the p53-dependent cell cycle arrest.
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myc gene, Myc protein
The protooncogene myc codes a transcription factor (the Myc protein), which is necessary
for the expression of proteins that allow the cell to progress through G1 to S phase.
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origin recognition complex (ORC)
A protein complex binding to the replication origins, which stay bound to the origins during
the whole cell cycle, together with cdc6 and Mcm helicase makes up the prereplication complex from which DNA replication starts after phosphorylation of ORC.
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retinoblastoma (Rb) protein
The product of the retinoblastoma tumor suppressor gene. It is a nuclear protein that
normally acts as an inhibitor of the progression of cell cycle in G1 phase, by binding to and
keeping the E2F transcription factor inactive.
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It is the process by which a less specialized cell becomes a more specialized cell type. The
gene expression patterns change in a coordinated manner during these processes, while the
nucleotide sequence of the genome is unaltered.
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myoD (myogenic determination gene)
A transcription factor, that controls myogenesis i.e., muscle cell differentiation, acting on
myoblasts. It induces transcription of muscle specific genes. MyoD preferentially binds to
DNA in a heterodimer form.
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morphogen (with 2 examples)
A morphogen is a diffusible substance carrying positional information within a developing
organism, needed for the appropriate distribution of tissues in the body and specialized cell
types within the tissue. E.g.sonic Hedgehog, bone morphogenic
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sonic hedgehog (SHH)
A morphogen which is important for the formation of various body parts, particularly the
limbs and the nervous system during embryonic development. Secreted SHH binds to its
cell membrane receptor patched (Ptc) which ceases to inhibit the Smo serpentine.
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totipotent stem cell
Single cells that have the potential to form an entire organism. They have the capacity to
differentiate into extraembryonic membranes and tissues (including the trophoblast and the
placenta), the embryo, and all postembryonic tissues and organs.
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It is a hormone growth factor produced in the juxtaglomerular cells of kidney. It is released
in response to the decreased levels of O2 in the kidney&stimulates production of RBCs in the bone marrow by inducing proliferation of erythroid progenitor cells.
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stem cell niche
It refers to a microenvironment where stem cells can reside for an indefinite period of time
and produce progeny cells while self-renewing.
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replicative cell senescence
An aging-related process/condition of cells affecting their lifespan whereby, after a certain
number of divisions, the Hayflick limit, cells in the body or in cell culture cease to divide any further, due to telomere shortening in cell divisions.
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Cells become released from the control mechanisms enforcing the Hayflick limit/replicative cell senescence,&can divide continuously. It can be induced by the activity of the telomerase enzyme or the injury of the checkpoint control.
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Shelterin proteins
DNA-binding proteins on the telomere region, which both help to protect the chromosomal
ends from being recognized as free DNA ends (DNA breaks) by the DNA repair system of
the cell and play regulative roles in the maintenance of telomere length.
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telomerase enzyme
In each DNA replication cycle the telomerase enzyme refills the truncated telomeric regions of each chromosome by its reverse transcriptase activity and thereby prevents the cells from the replicative cell senescence.
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breakage-fusion-bridge cycle (BFB)
In the combined absence of telomerase activity and checkpoint mechanisms, the
chromosomal ends become “sticky” as a consequence of shortening in every cell division
and are often joined together via DNA repair processes even non-homologues
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beta catenin
A dual function protein. In cell-cell junctions, it binds the cadherins of adherent junctions
to the actin cytoskeleton but it also takes part in the regulation of cell proliferation. Its
degradation is regulated by the APC (adenomatous polyposis coli).
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A form of cell death in which cells take an active part. It also called programmed cell death
and has several different forms. The characteristic events of apoptosis lead to morphological changes.
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cytochrome c
A molecule released from mitochondria in apoptotic cells. It is a member of the electron
transport chain located in the inner membrane of the mitochondrion. When released from
the mitochondrion into the cytosol, it initiates a caspase cascade.
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Bcl-2 proteins
A protein family whose members take part both in the formation and regulation of the
apoptosis induced pores (channels) of the mitochondrial membrane. The pro-apoptotic
members of this protein family stimulate channel/pore opening.
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caspase cascade
An enzyme cascade composed of special proteases activated in apoptotic cells the
consecutive members of the cascade are activated by proteolytic cleavage carried out by the
upstream caspase which leads to the degradation of many cellular substrates.
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Hayflick limit
A normal cell, which does not express the telomerase enzyme, can divide about 50 times.
because the telomere regions of the chromosomes are shortened at every
division in the absence of this enzyme.
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The dominance of glycolysis over oxidative phosphorylation in spite of aerobic conditions.
It is typical of intensively growing normal, or of tumor cells. Less ATP is generated than in
oxidative phosphorylation.
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central dogma


In living systems, the flow of genetic information is unidirectional and follows the path: DNA
→ RNA → protein → property (phenotype).

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