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