3.8.1 ALTERATION TO BASE SEQUENCE

• Mutations = change to the base sequence of DNA.

  Can be caused by errors during DNA replication. 
• Types of mutations:

  • Substitution — one or more bases are swapped for another.
  • Deletion — one or more bases are removed. (FS)
  • Addition — one or more bases are added. (FS)
  • Duplication — one or more bases are repeated. (FS)
  • Inversion — a sequence of bases is reversed.
  • Translocation — a sequence of bases is moved from one location in the genome to another. Could be movement within same chromosome or movement to different chromosome.
• Mutations can result in a different amino acid sequence in the encoded polypeptide:

  Order of DNA bases in a gene determines amino acid sequence in a polypeptide. Mutation occurs in a gene - sequence of amino acids in polypeptide it codes for could be changed.
• • Degenerate nature of the genetic code means some amino acids are coded for by more than one DNA triplet. Means mutation doesn’t always result in changed amino acid sequence of polypeptide.
• Polypeptides make up proteins. A change in the amino acid sequence of a polypeptide may change the tertiary structure (final 3D shape) of the protein, which could mean it doesn’t work properly.
• Some mutations can increase the likelihood of developing certain cancers, some can cause genetic disorders (inherited disorders caused by abnormal genes or chromosomes).

  If a gamete containing a mutation for a type of cancer or a genetic disorder is fertilised, the mutation will be present in the new foetus formed — hereditary mutations.
• Frameshift mutations -

  Additions, deletions and duplications within a gene will almost always change the amino acid sequence of a polypeptide. 

  All change the number of bases in the DNA code. 

  Cause a shift (frameshift), changing all following base triplet codes downstream of mutation, so triplet code reads differently.
• Mutagenic Agents -

  Mutations occur spontaneously, e.g. when DNA is misread during replication.

  Mutagenic agents are things that can increase the rate of mutations. Eg. Ultraviolet radiation, ionising radiation, some chemicals and some viruses. 
• Mutagenic agents increase the rate of mutations in different ways:

  • By acting as a base - base analogs can substitute for a base during DNA replication, changing the base sequence in the new DNA.
  • By altering bases - some chemicals can delete or alter bases.
  • By changing the structure of DNA - Some types of radiation can do this, which causes problems during DNA replication.