I created these notes throughout y12&13 and used them for my A level 2022 exams. I got a grade 9 in GCSE and am awaiting for my a level biology result. These notes were made using the CGP revision guide, AQA Biology textbook and class notes in order to cover everything in the course. I have also ad...
9.1 GENE MUTATION
02 November 2020
22:36
Mutation - Any change to quantity or sequence of bases in DNA.
Gene mutation - Any change to one or more nucleotide bases, or sequence of bases
in DNA.
Gene mutations can arise spontaneously during DNA replication and include base
deletion and substitution.
As genetic code is degenerate, not all base substitutions cause a change in the
sequence of encoded amino acids.
Mutagenic agents can increase the rate of gene mutation.
● High energy radiation.
● Tobacco tar.
If the base sequence changes, at least one codon will change.
● Could change amino acid sequence of polypeptide.
● Causing protein to change shape.
● Protein won't work properly.
A mutation changing a specific amino acid may not effect the enzyme because:
● Doesn't change shape of active site.
● Doesn’t change/slightly changes shape of tertiary structure.
However a different mutation changing a different amino acid can greatly effect the
enzyme:
● May have been involved in an important bond and so active site changes
shape.
● Amino acid may have been in the active site and so may have been required
for the binding of the substrate.
Substitution
A nucleotide in the DNA molecule is replaced by a different nucleotide with a
different base.
-Substitution will not cause a frame shift mutation.
● Polypeptide will differ by the single amino acid.
● The significance of this depends on the precise role of the original amino acid.
● If it was important in forming bonds to determine tertiary structure,
replacement may not form same bonds.
- Protein different shape, won't work properly.
, If the mutated triplet codes for the same amino acid, there's no change in the
polypeptide produced, so mutation has no effect.
● Because genetic code is degenerate.
Deletion
A nucleotide is lost from the normal DNA sequence.
The deletion of a base can cause a frame shift mutation, where all of the bases have
been shifted to the left by one.
● This changes many of the triplets, significantly changing the amino acid
sequence.
- Polypeptide won't function properly.
If there's a deletion mutation but no frame shift, 3 bases have been deleted.
● The rest of the amino acids will be the same.
Chromosomal mutations
Polyploidy - Changes in whole sets of chromosomes.
● Occurs when organism has 3 or more sets of chromosomes rather than 2.
Non-disjunction - Changes in the number of individual chromosomes.
● Homologous pairs of chromosomes fail to separate during meiosis.
● Gametes would therefore have on more, or less chromosomes.
Hybridisation - Combining sets of chromosomes from 2 different species.
Change / mutation in base / nucleotide sequence (of DNA / gene);
● Change in amino acid sequence / primary structure (of enzyme);
● Change in hydrogen / ionic / disulfide bonds;
● Change in the tertiary structure / shape;
● Change in active site;
● Substrate not complementary / cannot bind (to enzyme / active site) / no
enzyme-substrate complexes form.
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