A2-LEVEL AQA BIOLOGY NOTES
TOPIC 1: alternation of the sequence of bases in dna can alter the structure of proteins
Types of Gene Mutations
Gene mutations are natural occurrences that come about during DNA replication. There are 3 categories of gene mutations:
Substitution = a nucleotide base is replaced with another
Gene mutations are natural occurrences that come about during DNA replication. There are 3 categories of gene mutations:
Substitution = a nucleotide base is replaced with another
Insertion = an extra nucleotide base is inserted into the sequence causing ‘frameshift’ where all the subsequent bases are shifted down 1 place relative to the twin DNA strand
Deletion = the absence of a nucleotide, causing ‘frameshift’ where all the subsequent bases are shifted back 1 place relative to the twin DNA strand
The effects of these mutations can be..
Point mutation
Mutations occur randomly but their frequency/likelihood is increased by mutagenic agents
- Neutral
- May occur in phenotypically insignificant strand of DNA
- May not result in change of polypeptide primary sequence (because DNA is degenerate)
- May result in a change of polypeptide primary sequence that does not affect secondary/tertiary/quaternary structure of protein therefore protein function unaffected
- May results in a change of polypeptide secondary/tertiary/quaternary structure but where active site of protein remains the same therefore function still unaffected
- May occur in phenotypically insignificant strand of DNA
- Harmful
- May result in change in final protein shape where protein and active site is deformed and therefore cannot fulfil function
- May result in change in final protein shape where protein and active site is deformed and therefore cannot fulfil function
- Beneficial
- May result in change in final protein shape where the protein performs its function better than it would have without the mutation
- This is the basis of natural selection and evolution
- The individual is better suited to survival and will pass on the mutation to its offspring
- E.g. eye colour
- Blue eyes was a mutation that occurred about 7000 years ago
- In sunny areas, this would be harmful as the retina is more exposed
- However in cloudy regions this was beneficial as it enabled people to see better
- So the mutation was carried down generations and became widespread
- Blue eyes was a mutation that occurred about 7000 years ago
- May result in change in final protein shape where the protein performs its function better than it would have without the mutation
Point mutation
- Mutations can affect 1 nucleotide base, or more than one adjacent bases
- A point mutation is where only one base is affected
- There are 3 types: silent, nonsense, missense
- Silent mutation
- No change in amino acid sequence of polypeptide
- No change in amino acid sequence of polypeptide
- Missense mutation
- The mutation changes the code for 1 amino acid
- 1 amino acid in the sequence is changes
- The mutation changes the code for 1 amino acid
- Nonsense mutation
- The mutation changes the code turning the triplet into a stop codon
- Instructs the end of polypeptide synthesis
- The polypeptide is shorter than it would normally be
- The mutation changes the code turning the triplet into a stop codon
Mutations occur randomly but their frequency/likelihood is increased by mutagenic agents
- X-rays
- Benzopyrene found in tobacco smoke
- Viruses
- Gamma rays