A-LEVEL NOTES
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A-LEVEL NOTES

A2-LEVEL AQA BIOLOGY NOTES 
TOPIC 1: alternation of the sequence of bases in dna can alter the structure of proteins

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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

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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

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Deletion = the absence of a nucleotide, causing ‘frameshift’ where all the subsequent bases are shifted back 1 place relative to the twin DNA strand

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The effects of these mutations can be..
  • 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
  • Harmful
    • 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

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
  • Missense mutation
    • The mutation changes the code for 1 amino acid
    • 1 amino acid in the sequence is changes
  • 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

Mutations occur randomly but their frequency/likelihood is increased by mutagenic agents
  • X-rays
  • Benzopyrene found in tobacco smoke
  • Viruses
  • Gamma rays

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