Voice of the Genome

Eukaryotic Cells

  • Contains membrane bound organelles and a nucleus
  • Nucleus - control cell activities, chromatids, DNA + proteins - transcription, nuclear envelope contaie pores to allow movement 
  • Nucleolus - dense body that makes ribosomes 
  • Lysosome - round organelle with no clear internal structure, contains digestive enzymes and bound by a single membrane layer, destruction of unwanted structures within the cell and cell death 
  • Golgi apparatus and vesicles - flattened sac of fluid-filled membrane, processes and packages lipids and proteins makes lysosomes, vesicles transports lipids and proteins 
  • Cytosol - cytoplasm and organelles 
  • Secretory vesicles - fusion with cell membrane to transport molecules 
  • Ribosomes - site of photosynthesis, small organelles that float freely of attached to rER 
  • Endoplasmic reticulum - system of membrane bound interconnected flattened sacs,
    • rER - ribosomes attached - proteins made an ribosome transported through ER 
    • sER - makes lipids and steroids (no ribosomes) 
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Eukaryotic Cells

  • Mitochondrion - site of krebs cycle, double membrane folded into cristae, matrix contains enzymes for photosynthesis 
  • Centrioles - animal cells contain a pair - hollow cylinders made of nine microtubles, involved in formation of spindles during cell division 
  • Cytoplasm - fluid that fills the cell, many different molecules 
  • Phospholipid bilayer 
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Production and transport of protein

  • DNA is transcribed to mRNA in the nucleus 
  • mRNA moves through the nuclear pores to the cytoplasm 
  • Ribosomes are used to translate mRNA to form primary structure of protein 
  • Proteins move into rER 
  • Protein moves though the ER assuming three-dimensional shape en route 
  • Vesicles pinched off the rough ER contain the protein 
  • Vesicles from rER fuse to form the flatterend sacs of the Golgi apparatus 
  • Proteins are modified within the Golgi apparatus 
  • Vesicles pinched off the Golgi apparatus contain the modified proteins 
  • Vesicle fuses with cell surface membrane releasing proteins such as extracellular proteins 
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Prokaryotic Cell

Bacteria and cynobacteria (photosynthetic bacteria)  - cells do not have a nucleus or other membrane-bound organelle 

  • Circular DNA 
  • Cell wall - of peptidoglycan 
  • Cytoplasm 
  • 70s ribosome 
  • Mesosome - infold of the cell surface membrane, site of respiration 
  • Food reserves 

Structures not always present 

  • Capsule - slimy layer on the surface for protection and to pevent dehydration 
  • Pili - thin protein tubes that allow bacteria to adhere to surfaces 
  • Flagellum - hollow cylindrical thread-like structure rotates to move cellls 
  • Plasmid - small circle of DNA 
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Mammalian gametes

Egg cell (Ovum) - incapable of independent movement - it is wafted along by ciliated cells and muscular contractions of the oviducts 

  • Follicle cells from ovary 
  • zona pellucida (jelly-like coating) 
  • Cell surface membrane 
  • cytoplasm 
  • Lysosomes
  • Food reserves - lipids and proteins 
  • Haploid nucleus 

Sperm - motile - powered by energy from the mitochondria and the long flagellum, muscular contractions of the uterus wall helps the sperms passage to oviduct and ovum releases chemicals which attract the ovum 

  • head (5um) - haploid nucleus and acrosome - specialised type of lysosome 
  • Middle - mitochondrion 
  • Flagellum 
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Fertilisation

  • Sperm reach ovum 

Acrosome reaction 

  • Chemicals are released from the cells surrounding the ovum, triggering the acrosome reaction 
  • The acrosome swells fusing with the spem cell surface membrane
  • Digestive enzymes in the acrosome are released 
  • The enzymes digest through the follicle cells and the zona pellucida surrounding the ovum 

Cortical reaction 

  • Sperm fuses with the ovum membrane 
  • The sperm nucleus enters the ovum 
  • Enzymes released from lysosomes in the ovum (cortical granules) - thicken the jelly-like layer preventing the entry of other sperm 

Nuclei of the sperm and ovum fuse 

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Linkage

Linkage of genes - any two genes with a locus on the same chromosome are linked together and will tend to be passed as a pair to the same gamete 

  • Genes will only be seperated and go into different gametes if crossing over happens between the pair of genes
  • If two genes are very close together on the same chromosome, crossing over is very unlikely to happen - genes are said to be strongly linked 
  • Genes on the same chromosomes make a linkage group - 23 linkage groups 

Sex Linkage - sex chromosomes XY (male) and ** (female) - all the genes on the sex chromosomes are passed on with those that determine sex 

  • Red-green colour blindness occurs in 8% of men and only 0.5% of women 
  • Haemophilia - most common in guys - only carried on the X chromosome, if he inherits the recessive gene from mother he will have the disease 
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Meiosis and Genetic Variation

Meiosis - creates gametes
Chromosomes replicate before division - after replication each chromosome is made up of two strands of genetic material - chromatids 
Meiosis 1 - homologous chromosomes pair up then seperate 
Meiosis 2 - second division 

Genetic variation 
1.Random fertilisation
Independant Assortment 

  • The arrangement of the chromosome pair at meiosis 1 is completely random 
  • The daughter cells in meiosis II contain different assortments of chromosomes depending on alignment 

Crossing over 

  • Homologous chromosomes pair and all four chromatids touch - at point of touch, chiasmata, the chromatids break and rejoin, chromatids exchange corresponding sections of the DNA - new alleles 
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Cell cycle

Cell cycle (I Passed My Anatomy Test Calmly) 

  • Interphase
  • Prophase 
  • Metaphase
  • Anaphase
  • Telophase 
  • Cytokinesis 

Interphase - cell synthesises new cell components such as organelles, membranes and new DNA, G1 phase (cell grows large, copies organelles, centrioles replicate), S phase (DNA synthesis and duplication of the centrosome), G2 (Growth + makes proteins and organelles + reorganises its content) 

  • Length of interphase depends on the role of the cell 
  • No interphase in the first few divisions of zygote 
  • Individual chromosomes are unravelled - allowing access to genetic material, enabling new proteins to be synthesised 
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Mitosis

Prophase

  • Chromasomes condense and become visable as two strands called chromatids - identical except for occasional genetic mutation 
  • Two chromosomes joined by the centromere 
  • Microtubules form three-dimensional structure called the spindle 
  • Centrioles move and position themselves as the two poles of the spindle - involved in the organisation of spindle fibres 
  • The widest part of the spindle called the equator 
  • Breakdown of the nuclear envelope signals the end of prophase 

Metaphase 

  • Chromosomes centromeres attach to the spindle fibres at the equator

Anaphase

  • Centromeres split, spindle fibres shorten, pulling the two halves of each centromere in opposite directions 
  • Anaphase ends when the speratied chromatids reach the poles and the spindle breaks down 
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Mitosis 1

Telophase 

  • Chromosomes unreaval and the nuclear envelope forms - two sets of genetic information becomes enclosed within seperate nuclei 

Cytokinesis - cytoplasmic division 

  • Animal cells - cell surface membrane contricts around the centre of the cell, a ring of protein filaments bound to the inside surface of the cell is divided into new cells (actin and myosin?) 
  • Plant cells - synthesise a new cell plate between the two new cells

Mitotic index - proportion of cells undergoing mitosis, how quickly the tissue is growing

Mitotic index = number of cells with visable chromosomes/ total number of cells observed

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Importance of mitosis

Growth and repair

  • Ensures that a multicellular organism has genetic consistency - all the cells in the body have the same genetic information 
  • Indentical daughter cells created 

Asexual reproduction

  • Many organisms grow copies of themselves by mitosis producing offspring that are genetically identical to each other and to their parents 
  • Fungus budding, cacti

Prokaryotes do not carry out mitosis or meiosis because they do not posses chromosomes - binary fission - one cell splits  into two identical cells 

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Stem Cells Definitions

  • Stem cell  - an undifferentiated cell of a multicellular organism with the capacity to dividea nd form identical stem cells or differentiate into somatic tissue 
  • Differentiationthe process by which cells, tissue, and organs acquire specialized features, especially during embryonic development - selected genes beome active in certain cells which become increasingly specialised 
  • Totipotent - capable of giving rise to any cell type or a complete embryo - capable of creating a blastocyst which develops into the placenta and embryo 
  • Pluripotent embryonic stem cells - capable of giving rise to most cell types - (50 cells of the inner blastocyst are pluripotent and go on to form the tissues of the developing embryo) 

Increased differentiation 

  • Multipotent stem cells -  can give rise to more than one type of cell  - adult stem cells and core blood stem cells 
  • Plant cells remain totipotent throughout the life of the plant 
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Uses of Stem Cells

Uses of human stem cells 

  • Regenerative medicine - concerned with replacing, engineering or regenerating human cells, tissues or organs to achieve normal function 
    • Embryonic stem cells - provision of new stem cells - greastest flexibility of development 
    • 'Spare embryos' from IVF - allowed to grow into blastocysts  - stem cells isolated from each embryo  - stem cells are then cultured and used for research 
    • Therapeutic cloning - nucleus of a diploid cell of a patient needing a transplant fused with an ovum (haploid nucleus removed - zygote like structure- stimulated to divide by mitosis - stem cells could be encouraged to develop into specific tissues - cell lines - organs?? 

Uses of adult stem cells 

  • Bone marrow transplants for the treatment of leukaemia and other related cancers
  • Cells for transplantation e.g. skin grafts 

Reprogramming somatic cells - reprogramming somatic (body) cells to make them pluripotent - induced pluripotent stem cells (iPSCs) resemble embryonic stem cells - would help overcome the problem of cell rejection and adress ethical concerns with the use of embryonic stem cells 

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Ethics of Stem Cells

Multipotent stem cells from adults - universally accepted - e.g. adult bone marrow - but these stem cells are less valuable for research and for the development of treatments than using pluripotent stem cells, which can only be derived from human embryos 

  • How similar are iPCSs to pluripotent stem cells 
  • Could iPCSs replace embryonic stem cells?

Moral principles - the use of embryonic stem cells is inethical and unacceptable as embryos should be accorded full human status from the moment of its creation 
Others argue embryos require and deserve no particular moral attention at all 
Special status of an embryo as a potential human being but that the respect due to the embryo increases as it develops 

UK Law - Use of embyros only when HFEA considers their use neccessary or desirable; to promote advances in the treatment of infertility, to increase knowledge about the cause of congential disease and miscarriage, to develop more effective methods of contraception, to develop methods for detecting gene or chromosonal abnormalities 

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Gene expression definitions

  • Gene expression - the process by which information from a gene is used in the synthesis of a functional gene product 
  • Stem cell - an undifferentiated cell from which certain other cells arise by differentiation 
  • Transcription - the process by which the information in a strand of DNA is copied into a new molecules of mRNA 
  • Translation - a step in protein synthesis - wherein the genetic code carried by the mRNA is decoded to produce a specific sequence of amino acids in a polypeptide 
  • Genetic code - the biochemical instructions that translate the genetic information present as a linear sequence of nucleotide triplets in mRNA into the correct linear sequence of amino acids for the synthesis of a particular polypeptide chain 
  • Codon - sequence of three nucleotides which together for a unit of genetic code in a DNA or RNA molecule 
  • Ribosomes - minute particle composed of protein and RNA  that serves as the site of polypeptide synthesis 
  • mRNA - moecules which transfer the information from DNA to the cell machinery responsible for protein synthesis 
  • tRNA - small RNA molecules that carry amino acids to the ribosome for polymerisation into a polypeptide 
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Transcriptional control and Epigenetics

  • For a gene to be expressed RNA polymerase must be able to bind to the promoter region 
  • Epigenetic markers control gene expression - methylation, deacetylation 
    • Transcription allowed if unmethylated cytosine and acetylated histone 
    • Transciption repressed if methylated cytosine and acetylated histone - wound tightly around the histone - genes inactive 
  • DNA is associated with a histone protein
    • Addition of a methyl group causes the DNA to associate more tightly with the histone protein
    • Addition of an acetyl group causes the DNA to associate less tightly 
      • Modification of histone protein using epigenetic markers controls gene expression 
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Lac Operon Model

  • E. Coli 
  • Only produce B-galactosidase to break down lactose when lactose is present in the surrounding medium - converts lactose into glucose and galactose
  • Lac Operon - involved in metabolism of lactose for respiration, synthesis of B-galactosidase 
  • The Operator Region - genes coding for related proteins are arranged into units known as operons
  • Promoter region - to which RNA polymerase binds before opening DNA 
  • Operator region - to which a repressor binds to, which would obstruct RNA polymerase from transcribing the genes 
  • Regulator region - codes for a repressor protein 
  • Structural genes - codes for a specific polypeptide  
  • When lactose is present - lactose binds to the repseros protein causing it to change shape and dissociate from the operator region - promoter region unblocked so RNA polymerase initiates transcription - E.coli can make make lactose permease and B-galactosidase to take up lactose and convert it into simple sugars
  • When lactose is absent - regulator gene expressed - lactose repressor protein is synthesised - repressor protein binds to Operator region - obstructs RNA polymerase 
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Environment and Gene Expression

Differences in phenotype caused by differences in genotype and differences in the environment in which the gene develops 

  • Discontinuous variation - controlled completely by the organisms genotype e.g. a person's blood group - phenotypes fall into discrete groups with no overlap 
  • Continuous variation - characteristics effected by both genotype and environment e.g. height
  • Polygenic inheritance - when a number of genes are involved in inheritance of a characteristic
  • Multifactorial - conditions where several genetic factors and one or more environmental factors and involved

Genetic and Environemental interactions 

  • Height e.g. genes for being tall but malnutrititon
  • Melanin - increased exposure to UV light - more MSH is produced - more MSH receptors form on the melanocyte, more melanin produced, more melanasomes collect around the nucleus of the skin cell and give more protection 
  • Seasonal colour change - artic foxes have white fur in winter and brown in summer - white winter coat grown in summer - foxes produce fewer MSH recpetors in the summer and so no melanin is made in the hair follicles 
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Environment influences the genome - Mice

Behaviour

  • Good mothers with offspring of bad mothers - high licking and grooming leads to - no methylation of the GR gene - GR gene expressed - offspring exposed to stress - high glucocorticoid levels - hormone binds to GR receptor - leads to low anxiety and high licking and grooming 
  • Bad mothers with offspring of good mothers - low licking and grooming leads to - methylation of the GR gene - GR gene not expressed- offspring exposed to stress - high glucocorticoid levels - hormone levels remain high as no GR receptor - leads to high anxiety and low licking andd grooming 

Agouti-Gene - Diet - overexpressed agouti gene - yellow obese mouse, normal - mottled skin, normal weight, inactive - dark brown, slim 
Mother with overactive agouti-gene 

  • Fo Mother fed folate - F1 shift towards psuedo-agouti - normal diet - F2 shift persists 
  • Fo Mother fed BPA - F1 shift distribution towards yellow - F2 distribution persostes 
  • Some epigenetic markers pass from mother to child despite creation of stem cells in embryo producion 
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Organisation

Cell - specialised for a particular function e.g. muscle cells (muscle fibres) 

Tissue - groups of specialised cells working together to carry out one function e.g. muscle tissue (sacromere) 

Organ - a group of tissues working together to carry out one function e.g. muscle 

Organ system - a group of organisms working together to carry out one function e.g. the circulatory system 

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When gene expression goes wrong

Growing bones in the wrong places - rare inherited disease FOP - bone cells normally produced in growing limbs and in other places where the skeleton develops, here genes are expresed that produce all the proteins needed to become a specialised bone cell - in people with FOP - one theses genes is not switched off in the WBC - effects all tissue that white blood cells come into contact with 

  • Cancer - rate of cell multiplication is greater than the rate of cell death - growth of a tumour 
  • Damage to DNA 
  • Oncogenes - code for the proteins that stimulate the transition from one stage in the cell cycle to the next, progression controlled - DNA mutations or epigenetic changes in these genes might cause the cell cycle to be continually active - excessive cell division 
  • Tumour suppressing genes - produce suppressor proteins that stop the cycle - lack of p53 in cancer cells means the cell cannot stop entry into the S phase - lost control of the cell cycle 
  • Inherited cancer - many gene defects have been indentified that make individuals predisposed to certain cancers e.g. BRCA1 gene mutations predispose a person to breast cancer 
  • Environment and cancer - chemical or physical damage - smoking, UV light, Dier, virus infection 
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