OCR A2 Biology F215 Cellular Control topic revision notes

Covers most of Cellular Control, apart from genetic crosses etc.

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  • Created on: 06-06-12 09:42
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CELLULAR CONTROL Kirsty Stephen
CELLULAR CONTROL REVISION NOTES
2.1.1 What is a gene?
A gene=length of DNA-so a sequence of nucleotide bases that codes for one or more polypeptides.
It is a unit of heredity. Around 25,000 genes in the human genome, few found in mitochondria,
most in linear chromosomes within nucleus. Each gene occupies specific places (locus) on the
chromosome. Each chromosome consists of one molecule of DNA and each gene is part of a DNA
molecule. DNA in chromosomes=associated with histone proteins.
Genes code for polypeptides eg:
Structural proteins eg. Collagen and keratin
Haemoglobin
Immunoglobulins (antibodies)
Cell surface receptors
Antigens
Enzymes
As genes code for proteins, they're involved in control of all metabolic pathways and therefore the
synthesis of all non-protein molecules found in cells.
DEFINITIONS
Polypeptide-polymer consisting of a chain of amino acid residues joined by peptide bonds
Genome-the genome of an organism is the entire DNA sequence of an organism.
Protein-large polypeptide-usually 100 or more amino acids. Some proteins consist of one
polypeptide chain and some consist of more than one polypeptide chain.
The genetic code
Sequence of nucleotide bases on a gene provides a code with instructions for building a polypeptide
or protein. Genetic code has number of characteristics:
Triplet code. Sequence of 3 nucleotide bases code for an amino acid, 4 different bases,
arranged in groups of 3, so the number of different sequences=64
Degenerate code. All amino acids except for methione have more than one code
Some codes don't correspond to an amino acid but indicate `stop'-end of the polypeptide
chain.
Widespread but not universal, eg. TCT codes for the amino acid serine in any organism-useful
for genetic engineering as can transfer gene from one organism to another and it will still
produce the same protein. However there are some variations eg. Stop codes in mammals
code for proteins in protoctistas.

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CELLULAR CONTROL Kirsty Stephen
Transcription-first stage of protein synthesis
Is the creating of a single-stranded mRNA copy of the DNA coding strand. First stage of protein
synthesis, an mRNA molecule is made; one strand of the length of DNA is used as a template. There
are free DNA nucleotides in the nucleoplasm and free RNA nucleotides in the nucleolus. The
nucleotides are activated-have 2 extra phosphoryl groups attached.…read more

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CELLULAR CONTROL Kirsty Stephen
The sequence of amino acids in a protein is essential because:
Forms the primary structure of a protein
The primary structure determines the tertiary structure-how the protein folds up into its 3D
shape and held into that 3D shape by hydrogen or ionic bonds and hydrophobic interactions
between the R groups of amino acids
The tertiary structure is what allows the protein to function-tertiary structure is dependent
upon the primary structure, and this primary sequence is determined by the genetic code…read more

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CELLULAR CONTROL Kirsty Stephen
In prokaryotes, DNA is not inside the nucleus, translation starts as soon as some mRNA has been
made.
Protein activation
Glycogen in muscle cells can be broken down by enzyme-glycogen phosphorylase and synthesised
by enzyme glycogen synthase. If both were happening at the same time, it would waste the cells
energy, so has to be a control mechanism to `make or break' glycogen according to the cells needs.…read more

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CELLULAR CONTROL Kirsty Stephen
This may change the final 3D shape of the protein so it doesn't work properly eg.…read more

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CELLULAR CONTROL Kirsty Stephen
methyl groups to DNA or through action of histone proteins Stem cells are undifferentiated and
become specialised as they undergo epigenetic programming in which certain genes are turned off
by methylation or association with histone proteins. It is also possible to turn genes back on using
epigenetic programming so if the epigenetic modifications that turned an embryonic stem cell into
a liver cell were removed-cell could be persuaded to develop into different types of cells.…read more

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CELLULAR CONTROL Kirsty Stephen
The regulatory gene is expressed and the prepressor protein is synthesised. Has 2 binding
sites-one that binds to lactose and one that binds to the operator region
The repressor protein binds to the operator region. In doing so it covers part of the
promoter region, where RNA polymerase normally attaches.
RNA polymerase cannot bind to the operator region so structural genes can't be transcribed
into mRNA.…read more

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CELLULAR CONTROL Kirsty Stephen
The proteins bind to DNA at the start of developmental genes, activating or repressing
transcription and so altering the production of proteins involved in the development of the
body plan
Retinoic acid & birth defects-retinoic acid = derivative of vitamin A. It activates homeobox genes in
vertebrates in the same order they are expressed in developing systems, eg. The axial skeleton and
the central nervous system-both of which run head to tail.…read more

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CELLULAR CONTROL Kirsty Stephen
Involved in development of body plans-mitosis and differentiation create most of body parts and
apoptosis refines the parts by removing unwanted structures eg.
When hands and feet first develop the fingers and toes are connected-only separated when
cells in connecting tissue are removed by apoptosis
As tadpoles develop into frogs, their tail cells are removed by apoptosis
An excess of nerve cells are produced during the development of the nervous system. Nerve
cells that aren't needed undergo apoptosis.…read more

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CELLULAR CONTROL Kirsty Stephen
Asexual and sexual reproduction- in asexual reproduction, all of the offspring are genetically identical
to the parent and each other-genetic variation is only introduced by random mutation. In sexual
reproduction-offspring are genetically different from each other and from the parents. Each parent
produces special reproductive cells, called gametes, these fuse to form a zygote.…read more

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