Mutations and variation

• Mutation= change in the sequence of bases in DNA.

  Protein synthesis can be disrupted if the mutation occurs within a gene.

  The change in sequence is caused by the substitution, deletion or insertion.
• The substitution of a single nucleotide changes the codon in which it occurs. If the new codon codes for a different amino acid this will lead to a change in the primary structure of the protein.

  The degenerate nature of of the genetic code may mean that the new codon still codes for the same amino acid leading to no change in the protein synthesized.
• Degenerate= more than one codon to code for a specific amino acid during protein synthesis.
• The position and involvement of the amino acid in R group interactions within the protein will determine the impact of the new amino acid on the function of the protein.
• The insertion or deletion of a nucleotide leads to a frame shift mutation. The triplets code means that sequences of bases are transcribed consecutively in non-overlapping groups of three. This is the reading frame of a sequence of bases.

  The addition or deletion of a nucleotide moves, or shifts, the reading frame of the sequence of bases.
• No effect mutations= there is no effect on the phenotype of an organism because normally functioning proteins are still synthesised.
• Damaging mutations= the phenotype of an organism is affected in a negative way because proteins are no longer synthesised or proteins synthesised are non-functional.
• Beneficial mutations= very rarely a protein is synthesised that results in a new and useful characteristic in the phenotype.
• Mutations can occur spontaneously, often during DNA replication but the rate of mutation is increased by mutagens.

  • Mutagen= is a chemical, physical or biological agent which causes mutations.
  • The loss of a purine base or a pyrimidine base often occurs spontaneously. The absence of a base can lead to insertion of an incorrect base through complementary base pairing during DNA replication.
  • Free radicals (oxidising agents) can affect the structures of nucleotides and also disrupt base pairing during DNA replication.
• Silent mutations= they no don't change any proteins or the activity of any proteins synthesised.

  • So they have no effect on the phenotype.
  • They may result in changes to the primary structure but do not change the overall structure or function of the proteins synthesised.
• Nonsense mutations= result in the codon becoming a stop codon instead of coding for an amino acid.

  • The result is a shortened protein being synthesised which is normally non-functional.
  • Normally have a negative effect on phenotypes.
• Missense mutations= result in the incorporation of an incorrect amino acid into the primary structure when the protein is synthesised.

  • Result depends on the role the amino acid plays in the structure and therefore function of the protein synthesised.
  • A conservative mutation occurs when the amino acid change leads to an amino acid being coded for which has similar properties to the original.
  • Non-conservative mutation is when the new amino acid coded for has different properties to the original, this is more likely to have an effect on protein structure.
• Chromosome mutations= affect the whole chromosome. They can also be caused by mutations and normally occur during meiosis.

  • Deletion= section of chromosome breaks off and is lost within the cell.
  • Duplication= section gets duplicated on a chromosome.
  • Translocation= section of one chromosome breaks off and joins another non-homologous chromosome
  • Inversion= section of chromosome breaks off, Is reversed, and then joins back onto the chromosome.