ocr a level biology a module 6 - all specification notes
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Course
Modules 4,5,6
Institution
OCR
all specification specific notes for ocr a level biology module 6, these notes have allowed for me to get A* in all of my a level mock exams. These are recent notes as they have been created in 2023, I have used several textbooks and made these notes which specifically hit all specification marks, ...
(a) types of gene mutations and their possible effects on protein production and function
To include substitution, insertion or deletion of one or more nucleotides AND the possible
effects of these gene mutations (i.e. beneficial, neutral or harmful).
NOTES
MUTATIONS:
o substitution/point - one or more bases swapped for another,
o Three types of substitution mutations:
silent - change in base triplet does not affect amino acid produced,
missense - change in base triplet that changes amino acid sequence (can
severely effect protein produced),
nonsense - change in base triplet that becomes a termination triplet and
produces non-functioning, truncated protein.
o deletion - one or more bases removed (frameshift mutation),
o insertion - one or more bases are added (frameshift mutation - can change other
triplets).
DNA base order determines amino acid sequence in proteins. Mutations can alter primary
structure, affecting protein function (active site).
Some mutations can be neutral:
o genetic code is degenerate (other triplets can code for a specific amino acid),
o some amino acids are chemically similar so can perform the same function,
o mutated codon does not code for an amino acid involved with the protein’s function.
Some mutations can be beneficial:
o advantageous effect so a higher survival rate,
o bacteria can mutate to have enzymes which break down several antibiotics (antibiotic
resistance).
Some mutations can be harmful:
o disadvantageous effect on organism (decrease survival rate),
o example: deletion of bases in CTFR gene leads to cystic fibrosis.
(b) the regulatory mechanisms that control gene expression at the transcriptional level, post-
transcriptional level and post-translational level
To include control at the: • transcriptional level: lac operon, and transcription factors in
eukaryotes. • post-transcriptional level: the editing of primary mRNA and the removal of introns
to produce mature mRNA. • post-translational level: the activation of proteins by cyclic AMP.
NOTES
TRANSCRIPTIONAL LEVEL:
o Transcriptional factors control gene expression at transcriptional level,
o transcription factors are proteins (short non-coding RNA pieces) that bind to DNA and
switch genes on/off,
o factors that increase transcription rate = activators,
o factors that decrease transcription rate = repressors,
o In eukaryotes, TFs bind to specific sites near start of target genes,
o In prokaryotes, TFs bind to operons (group of structural genes controlled by a regulatory
mechanism),
o structural genes code for functional proteins,
, o operon contains both an operator (TF binds to) and a promoter (RNA polymerase binds
to),
o Regulatory gene codes for a repressor (TFs),
o Lac operon contains three structural genes (Lac Z, Lac Y and Lac A - produce B-
galactosidase and lactose permease),
o When no lactose present, regulatory gene (Lac I) produces repressor that binds to
operator and blocks RNA polymerase binding to promoter,
o When lactose present, it binds to repressor causing conformational change so RNA
polymerase can bind to promoter and begin transcription of structural genes.
POST-TRANSCRIPTIONAL LEVEL:
o genes in eukaryotic DNA contain sections that don't code for amino acids (introns),
o primary mRNA contains both introns and exons,
o mature mRNA formed through removal of introns by splicing (takes place in nucleus),
o mature mRNA then leaves for translation.
POST-TRANSLATIONAL LEVEL:
o some proteins not functional right after synthesis so have to be activated,
o protein activation caused by molecules that bind to cell membranes and trigger cAMP
inside cell,
o cAMP activates proteins by altering 3D structure which changes the active site (protein
kinase A).
1.signaling molecule binds to plasma membrane receptor,
2.activates transmembrane protein which activates G protein,
3.G protein activates adenyl cyclase enzyme,
4.adenyl cyclase catalyses ATP —> cAMP,
5.cAMP activates protein kinase A,
6.PKA catalyses phosphorylation of CREB protein which acts as a TF for
transcription.
(c) the genetic control of the development of body plans in different organisms
Homeobox gene sequences in plants, animals and fungi are similar and highly conserved AND
the role of Hox genes in controlling body plan development.
NOTES
o proteins that control ANIMAL body plan development coded for by Hox genes,
o homeobox gene sequences in plants, animals and fungi are similar and highly conserved,
o Hox genes have homeobox sequences that code for a part of a protein called the
homeodomain,
o homeodomain binds to specific DNA sites which enables protein to work as a TF,
o proteins then bind to DNA at the start of developmental genes, and activate/repress
transcription.
(d) the importance of mitosis and apoptosis as mechanisms controlling the development of body
form.
To include an appreciation that the genes which regulate the cell cycle and apoptosis are able to
respond to internal and external cell stimuli e.g. stress.
NOTES
o Apoptosis is programmed cell death.
, 7.first enzymes break down cell cytoskeleton,
8.cytoplasm becomes dense,
9.cell surface membrane changes and forms blebs (small protrusions),
10. chromatin condense, nuclear envelope breaks and DNA fragments,
11. cell breaks into vesicles which are digested by phagocytic cells.
2. Genes that regulate apoptosis and mitosis can respond to internal (DNA damage) and external
stimuli (stress).
(a) (i) the contribution of both environmental and genetic factors to phenotypic variation
To include examples of both genetic and environmental contributions – environmental
examples could include diet in animals and etiolation or chlorosis in plants.
NOTES
o phenotype - visible characteristics of an organism,
o physical and chemical agents (mutagens) can increase the rate of mutation,
o physical agent examples: x-rays, UV light,
o chemical agent examples: nitrous acid, mustard gas,
o chromosome mutations include: deletion, inversion (turns 180), translocation (breaks
and attaches elsewhere), duplication (overexpression of genes), non-disjunction (extra
chromosome),
o plants in dim light, or grown in magnesium-deficient soil can lack chlorophyll and have
yellow leaves (chlorosis) - the plant can’t photosynthesise,
o etiolation - condition where plants grow without light and elongate to reach light,
o diet in animals (CV) affected by genetics and environment, the mass is changing.
(ii) how sexual reproduction can lead to genetic variation within a species
Meiosis and the random fusion of gametes at fertilisation.
NOTES
o meiosis produces genetically different gametes,
o genetic variation in meiosis occurs because of: crossing over (P1), independent
assortment (M1&2) and random fertilisation,
o gametes are haploid - contain one of each pair of homologous chromosomes and one
allele for every gene,
o random fertilisation of gametes can lead to genetic diversity among resulting offspring.
(b) (i) genetic diagrams to show patterns of inheritance
To include monogenic inheritance, dihybrid inheritance, multiple alleles, sex linkage and
codominance.
NOTES
o allele - different version of the same gene,
o monogenic inheritance - determined by a single gene (hair colour) with two alleles
(blonde & black),
o phenotypic ratio of monohybrid heterozygous cross = 3:1,
o dihybrid inheritance - determined by two genes with four alleles,
o phenotypic ratio of dihybrid cross on different autosomes (unlinked) = 9:3:3:1,
o phenotypic ratio of dihybris cross on same autosome (linked) = 3:1,
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