Mutations

• What is a nondisjunction mutation?

  • If homologous chromosomes or sister chromatids do not separate during meiosis this can result in a chromosome mutation.
• What is a mutation?

  • A mutation is a change to the structure or quality of DNA, this includes nondisjunction mutations.
• What is a gene mutation?

  • A change to the sequence of bases in DNA.
• What does a gene mutation result in and what will happen if this mutation occurs during gamete formation?

  • This leads to the formation of a new allele.
  • If mutations occur during gamete formation the mutation can be inherited through sexual reproduction leading to a variation in a population.
• What are the causes for mutation?

  • Spontaneous mutations arise during DNA replication.
  • Increased rate caused by mutagenic agents.
• What are mutagenic agents?

  • Chemicals like benzene and tar in cigarettes and high energy radiation - ionising radiation like X-rays, gamma rays and UV light.
• What is substitution?

  • Substitution is where one nucleotide is replaced by another.
• What are the three possible consequences if substitution occurs?

  • Nonsense
  • Missense
  • Silent
• A substitution mutation occured which resulted in nonsense - what is nonsense?

  • If a base change results in formation of one of three stop codons then the production of polypeptides is stopped prematurely resulting in the protein not being able to perform normal functions.
• A substitution mutation occured which resulted in missense.

  What is missense?

  • This is when a base change results in a different amino acid being coded for, so the produced polypeptide differs by one amino acid.
  • Due to this the protein may be a different shape which is important if the amino acid makes up part of the active site, effecting substrate binding.
• A substitution mutation occured which resulted in silent.

  What is silent?

  • This occurs when the substituted base still codes for the same amino acid as before due to the degenerate nature of the genetic code.
  • The mutation has no effect.
• What is a deletion/addition mutation?

  • This occurs when a nucleotide is gained or lost from a DNA sequence.
  • Deletion changes the entire sequence of triplet codons from that point, this is known as a frameshift.
  • Ribosomes read an entirely different sequence effecting the whole protein.
  • Deletion/addition near the end of a sequence has less of a impact.
• What is a frameshift?


  • This is when one nucleotide is deleted but not been substituted, resulting in a shift of the sequence of triplets downstream of a mutation.
• What are the effects of a mutation?

  • Codons change due to a change in the DNA sequence resulting in a change of the amino acid sequence.
  • This changes the secondary structure as hydrogen bonds form in different places then the tertiary structure changes as the folding patterns changes due to ionic/disulphide bonds.
  • This may alter the function of the protein.