OCR biology - cellular control
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| What is a gene vs a genome? | Gene is a sequence of bases that occupies a particular locus on a chromosome and CODES FOR A SPECIFIC POLYPEPTIDE eg membrane proteins/enzymes/electron carriers. Genome is entire DNA sequence of an organism. Humans: 3billion base pairs, 20,000 genes
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| What are the features of the genetic code? | triplet code (64 codons), degenerate code (all amino acids excluding methionine have more than 1 code), 1 start codon (AUG -> methionine), 3 stop codons, widespread but not universal (codons code for same amino acids in most organisms)
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| Describe transcription | DNA unwinds, separate. Activated RNA nucleotides bond to comp. exposed bases on template strand, H bonding. 2 phosphoryl groups hydrolysed. RNA polymerase forms phosphodiester bonds (condensation). RNA separates+leaves via nuclear pore
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| Describe the structure + function of a eukaryotic ribosome | Made in nucleolus, rRNA and protein, 80s (small subunit 30s, large subunit 50s), binding site is a ribosomal groove where mRNA is accepted, 2 codons bind at a time. Function: translation, decodes mRNA and holds tRNA in place for polypeptide formation
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| Describe the structure + function of mRNA | Structure: single strand, RNA nucleotides (AUCG), complementary to template strand, identical to coding strand. Function: carries genetic info from nucleus to ribosome so genes are transcribed into polypeptide
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| Desribe the structure + function of tRNA | Structure: at 1 end, 3 exposed bases attach to a specific amino acid, at other end 3 exposed bases form an anticodon which binds temporarily to complementary codon on mRNA strand. Function: carries specific amino acids to mRNA in specific order
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| Describe translation | 2 codons of mRNA enter ribosomal groove, attach to small subunit, exposed to large. AUG - tRNA with methionine has comp anticodon so binds to codon via H bonding, ATP. tRNA with specific amino acid and complementary anticodon binds to 2nd codon...
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| Continued | Enzyme forms peptide bonds (condensation) between amino acid. tRNA binds to 3rd codon, 1st tRNA is released. Translation ends when ribosome reaches stop codon (UAA, UAG, UGA) as no tRNA with comp anticodon. Folding, modification by Golgi
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| What is the role of cAMP during translation? | Activates proteins after translation to alter tertiary structure, improve their fit to complementary molecules like substrates and ligands.
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| What is an operon? | length of DNA in prokaryotes, contains structural genes-code for 1+ polypeptides/mRNA, control sites: operator region switches genes on/off, promoter region where RNA polymerase binds, regulator gene controls expression of structural g, repressor protein
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| What is the lac operon + structure? | E. coli-lac operon-acts as molecular switch, ensures enzymes for lactose breakdown only synthesised in presence of lactose (interaction), saves ATP/amino acids. Structural: Z=B-galactosidase, Y=lactose permease. Control sites. Regulator gene=repressor p
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| What occurs in the absence of lactose? | The regulator gene is expressed, repressor protein synthesised. Binds to operator region, covering part of promoter region. RNA polymerase can't bind to promoter region to transcribe structural genes into mRNA. Structural genes aren't expressed
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| What occurs in the presence of lactose? | Lactose (inducer) binds to 2nd binding site of repressor protein, which is bound to O region. Confirmational shape change, repressor protein dissociates from O. RNA polymerase binds to P region, transcribes structural genes into mRNA...
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| Cont | Synthesis of lac enzymes (inducible enzymes). Lactose permease takes up lactose into cell by facilitated diffusion. B-galactosidase hydrolyses lactose into glucose (for respiration) and galactose
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| What is morphogenesis? | The embryonic process that causes an organism to take its shape e.g. cell differentiation and development of organ systems
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| What are homeobox genes and how do they work? | Control morphogenesis + development of body plan e.g. position + polarity of an organism. selector genes so act as master switches in development - code for homeodomain proteins e.g. transcription factors. 180 bases, Hox clusters
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| What is a morphogen +eg? | SIGNALLING MOLECULE: released by 1 tissue, diffuses, forms concentration gradients that determine pattern of development, cells respond (e.g. differentiating, dividing) according to the concentration of morphogen they detect. Retinoic acid - retinol - cra
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| Why does thalidomide cause birth defects? | Prevents expression of homeobox genes involved in limb development - enters DNA and prevents angiogenesis in limb buds = birth defects
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| What organism is useful for studying genetic control and why? | Drosophila melanogaster (fruit fly) - high reproduction rate, small (easy to store), abundant, only 4 chromosomes, readily exhibits mutations e.g. X-rays, interbreeding
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| What are some of the homeobox genes in fruit flies? | Maternal effect genes control polarity. Segmentation genes control polarity of segments. Ultrabithorax - expressed in T3, supresses wing growth, halters grow instead. Antennapedia - expressed in thorax, promotes leg formation
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| What is meant by "homeobox genes are highly conserved"? | They have remained essentially unchanged through evolution, showing that they are very important and organisms with mutations in homoebox genes are unlikely to survive
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| What is evidence for the similarity of homeobox genes in animals, plants and fungi? | HoxB7 in mice and Antp in drosophila only differs by 2 amino acids. Insert HoxB7 into drosophila head = fruit fly legs grow from head.
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| What is necrosis? | Untidy cell death induced by trauma that releases hydrolytic enyzmes
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| What is apoptosis? | Programmed cell death in multicellular organisms, initiated and executed by cell, quick, cellular debris doesn't damage other cells
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| Describe steps of apoptosis | enzymes break down cytoskeleton, cytoplasm becomes dense, blebs form on plasma membrane, chromatin condenses, nuclear envelop disintegrates, DNA fragments, cell splits into vesicles -> phagocyte
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| How is apoptosis regulated | Intracellular/ extracellular signals e.g. NO (makes inner mitochondrial membrane more permeable to H+), cytokines, growth factors, hormones
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| What is the result of mitochondrial membrane permeabilization? | The outer membrane becomes more permeable, releasing proteins into cytosol which bind to apoptosis inhibitor proteins to induce apoptosis
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| Give examples of when apoptosis occurs | All cell after 50 divisions. Separates digits in webbed hands/ feet of foetus. Open foetus eyelids. loss of tadpole tail. Destroy ineffective/ self-reactive T cells. Cytotoxic T cells-infected cells. Destroy cells that have replicated incorrectly
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| What can occur due to too much / too little apoptosis? | Too much - cell loss and degeneration e.g. Alzheimer's disease, Parkinson's disease. Too little -autoimmune diseases and cancers
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| Define mutation | Random change to the amount or arrangement of genetic material in a cell
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| What is the difference between a DNA and chromosome mutation? | DNA mutation - random change to the base sequence, creates new alleles. Chromosome mutation - random change to the structure of a chromosome e.g. inversion, deletion, translocation, non-disjunction e.g. Down's syndrome
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| Compare the effects of mutations during meiosis and mitosis | Meiosis - can be inherited, in gametes. Mitosis - limited to somatic cells, can't be inherited, can cause cancer/ aging
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| What is a point mutation/ substitution? | 1 base pair replaces another: silent (new codon codes for same amino acid as genetic code is degenerate), missense (different amino acid -> different protein), nonsense (stop codon halts transcription/ translation prematurely, protein is truncated)
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| What types of mutation cause a frameshift? | Inversion/ deletion mutations: 1 or more base pairs are added or removed
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| What is a frameshift? | After an inversion/ deletion mutation, base sequence from that point onwards is read incorrectly by a ribosome as it is read in triplets
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| How can mutations have beneficial effects? | Increase genetic variation, allowing directional natural selection and evolution e.g. antibiotic resistance in bacteria
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| How can mutations have neutral effects? | Mutation is in non-coding region of DNA, silent point mutation codes for same amino acid (genetic code is degenerate), mutation changes protein tertiary structure and phenotype but individual experiences no selective advantage/ disadvantage e.g. ear lobes
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| Describe mutations giving rise to: cystic fibrosis | Deletion of a base triplet from gene, so CFTR protein is missing 1 amino acid
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| Describe mutations giving rise to: sickle cell anaemia | Point mutation in gene coding for B-chains in haemoglobin replaces glutamate (hydrophilic R-group) with valine (hydrophobic R-group), valines bond to each other in adjacent haemoglobin molecules producing fibres, erythrocytes sickle
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| Describe mutations giving rise to: cancer | Point mutation in proto-oncogene -> oncogene - can't be switched off, promotes uncontrolled cell division
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| Describe mutations giving rise to: huntingdons | Huntingdon protein has over a threshold number of CAG repeats - stutter
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| Give an example of a mutation that could be harmful/ beneficial depending on environment + selection pressures | Pale skin in early humans - harmful in Africa (skin cancer, burning), beneficial as humans migrated north (less melanin in skin = easier to produce vitamin D, individuals with darker skin would have experienced vitamin D deficiency, rickets)
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