This is a detailed revision note of 55 pages of Module 6 OCR Biology A level, including all the topics (Cellular control, Patterns of inheritance, Manipulating genomes, Cloning and biotechnology, Ecosystems, Populations and sustainability). It includes diagrams and mathematical equations used and n...
6.1.1 Cellular Control
Genes code for polypeptides, including enzymes;
Gene:
A length of DNA (sequence of nucleotide bases) that codes for one or more polypeptides
Each gene occupies a specific locus on the chromosome
o Each chromosome consists of one molecule of DNA
o Each gene is just one part of a DNA molecule
The genetic code;
TRIPLET CODE:
- Sequence of three nucleotides codes for an amino acid
DEGENERATE CODE:
- All amino acids (except methionine) have more than one code
Some codes don't correspond to an amino acid but indicate "STOP" - end of a polypeptide chain
Widespread but NOT universal
- Some base sequences code for the same amino acids in all organisms but it is not always the same
(they may code for different things/more than one thing)
Mutations cause changes to the sequence of nucleotides in DNA molecules (unpredictable)
Explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions;
Occur:
During DNA replication Can be SILENT MUTATION:
Meiosis Triplet still codes for same amino acid
No effect on polypeptide produced
Can be NEUTRAL MUTATION:
Substitution (point mutation): Resultant differentiation gives organism no
One base pair replaces another advantage/disadvantage
o Mutation causes change to structure of protein
o Another version of same gene - different ALLELE
o Different characteristic
OR mutation in non-coding region of DNA
OR silent mutation
Harmful = e.g pale skin in hot weather
Beneficial = e.g pale skin - absorb more vitamin D
Insertion: Causes a FRAME SHIFT:
Nucleotide base inserted into genetic code Inserted/deleted base pair changes entire coding of
DNA molecule
Deletion: Triplet codons change
One base removed from genetic code Alters all amino acids after mutation
State that cyclic AMP activates proteins by altering their three-dimensional structure;
Cyclic AMP is a nucleotide derivative
Activates proteins by changing their 3D shape
Shape is a better fit to their complementary shape
Explain genetic control of protein production in a prokaryote using the lac operon;
,Bacteria produce two enzymes in the presense of lactose – a potential respiratory substrate:
- B-galactosidase (break down lactose to glucose + galactose)
- Lactose permease (transport lactose into bacterial cell)
Bacteria need to produce this enzyme; only produced when lactose present. Lactose is the inducer for these
enzymes.
Lac operon
Operon:
- Length of DNA
- Made up of structural genes; code for proteins
o and control sites – operator/promotor region; regulate operon
Operator region –
Can switch structural genes on and off
Promotor region –
Enzyme RNA polymerase bind to here in order to being transcribing mRNA for synthesis of enzymes from Z/Y
Regulator gene (I) –
Not part of operon
It is expressed (transcribed/translated into protein)
Makes repressor protein/transcription factor
o Can bind to operator region
o Product switches structural genes on and off
Controls expression of structural gene
When lactose is absent from growth medium:
Regulator gene expressed and repressor protein produced
No lactose present to bind to repressor protein
o Repressor protein binds to operator region of
operon (which also binds to part of promotor
region)
RNA polymerase cannot bind to promotor region
Structural genes not transcribed into mRNA
Enzymes not synthesized
Why useful?
Useful resources/energy not wasted by making unnecessary enzymes
When lactose is present in growth medium:
Regulator gene expressed as normal, repressor protein still
produced
Lactose (inducer molecule) binds to repressor protein
Repressor protein changes shape
o Binding site no longer complimentary to operon
Repressor protein does not bind to operator region/breaks
away
RNA polymerase therefore binds to promotor region
Transcribes Z and Y into enzymes
Lactose can be brought into cell, broken down and respired
,e.g Suggest how the protein encoded by allele T regulates transcription
Ref promoter/operator/"on" switch
Allele T is regulator
Protein binds to DNA
Binds to repressor, preventing it binding to DNA
Allows RNA polymerase to bind
The genes that control development of body plans are similar in plants, animals and fungi, with reference to
homeobox sequences;
Homeobox genes:
Homeotic/regulatory gene
Contains 180bp/homeobox sequence
Codes for homeodomain (on protein)
Gene product binds to DNA
Intiates transcription/switch genes on/off
Control the development of the body plan of an organism including polarity (head/tail ends) and positioning
of the organs
Perform different functions: Mutation of these genes can change one body
Maternal genes; affect polarity (anterior/posterior) part to another
Segmentation genes; specify polarity of segments
Homeotic selector genes; specify the identity of each segment; direct development of individual body
segments
They regulate the expression of other genes
Switch genes on and off
Homeobox genes sequence for specific polypeptides (transcription factors/regulatory protein/repressor protein)
that in turn bind to genes and prevent the expression of them
Others act as TRANSCRIPTION FACTORS;
o Bind to gene
o Initiate transcription in protein synthesis, thus specialising the cell
Explain how the products are coded for by the genes interact to give different pigments
Gene products are enzymes
o Multi enzyme pathway
3 steps (specific to q)
Enzymes change tryptophan to red pigment
Product of one reaction is substrate of another
Dominant allele gives functional enzyme
Recessive gives non-functional/different enzme
(j) outline how apoptosis (programmed cell death) can act as a mechanism to
change body plans.
It is a speedy process
Important for cell development where mitotic divisions are regular
o Balance of too much apoptosis (cell loss/degeneration) and too little
(tumours)
Enzymes break down cell cytoskeleton
Cytoplasm becomes densely packed
o (Organelles bundle together)
Cell surface membrane alters in shape - blebs form
Chromatin in nucleus condenses
Nuclear envelope breaks down
DNA fragments
Cell packages its contents into vesicles
Vesicles taken up by phagocytosis
, Cancer:
If cells have a mutation to genes involved in regulating mitosis
Do not undergo apoptosis
Keep on dividing by mitosis
Tumour
Why?
Reuse cell components
No harmful enzymes released
Weeds out ineffective/harmful cells
Separation of limbs/digits during development
6.1.2 Patterns of Inheritance
Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the
associated behaviour of the nuclear envelope, cell membrane and centrioles. (Names of the main stages are
expected, but not the subdivisions of prophase);
Meiosis
Nuclear division where original number of chromosomes in the organism is halved
Cells produced are haploid
Means when two cells combined in sexual reproduction chromosome number is restored
Produces non-identical cells
Meiosis I
Prophase
Chromosomes supercoil and become visible
Chromosomes consist of two SISTER chromatids (genetically identical)
Chromosomes come together in their homologous pairs (each member has same
genes at the same loci) (one maternal, one paternal)
Form bivalent
Non-sister chromatids wrap around each other at points called chiasmata; "crossing
over"
o (they may break and swap sections of DNA - leading to alleles being switched
around)
Nucleolus disappears
Spindle forms
Nuclear envelope breaks down
Centrioles at opposite ends of cell
Metaphase
Bivalents line up at equator
Spindle fibres attached to bivalents at centromere
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