D1.3 :

• Mutation :

  Change in the sequence of DNA.

  2 types of :

  • Synonymous.
  • Non-synonymous.
• Synonymous substitution :

  Neutral mutation.

  Changes in DNA doesn’t affect amino acid sequence.
• Non-synonymous substitution :

  Harmful/Beneficial mutation

  Changes in DNA affect amino acid sequence.
• 3 Types of mutations :

  • Substitution.
  • Insertion.
  • Deletion.
• Substitution mutation :

  A base is replaced with another base.

  E.g : A replaced with T.

  Changes code for 1 codon.
• Insertion mutation :

  1+ bases are inserted to the DNA sequence.

  E.g : T added between AG.

  Changes code for all codons after mutation.

  Causes frameshift mutation.
• Deletion mutation :

  1+ bases are deleted in the DNA sequences.

  E.g : AT removed from ATCT.

  Changes code for all codons after mutation.

  Causes frameshift mutation.
• Frameshift mutation :

  Occurs when insertion/deletion mutation occurs not in multiples of 3.

  Disrupts the sequence of DNA bases.

  Alters reading frame of codons.

  Can significantly impact protein structure & function.
• SNP :

  Replacement (substitution) of a single nucleotide with another.

  Found often in non-coding region of DNA.
• Degeneracy of DNA for mutations.

  Substitution mutation may not lead to a change in the protein structure due to many different codons coding for the same protein.
• Sickle cell disease :

  Substitution mutation of haemoglobin.

  Mutation alters shape of haemoglobin.

  Biconcave shape -> curved shape (sickle like).
• Causes of gene mutation :

  Mutations are caused by errors in DNA replication/repair or mutagens.
• Mutagens :

  Chemicals that can cause mutations.

  E.g : Nitrous oxide/Mustard gas/Benzene.

  Mutagenic radiation(ionising) are able to knock out base pairs (single/double strand break or chemical changes to bases).
• Randomness of mutations :

  Mutations can occur randomly anywhere in bases sequences of genome.

  Certain bases have a higher likelihood of mutating due to chemical properties -> cytosine undergo deamination (loses amine group to become uracil).

  Uncoiled DNA more likely to mutate.
• Mutations on purpose :

  Cells are unable to mutate on purpose to fix issues.