4.3 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 ar the causes for mutation?

  • Spontaneous mutations arise during DNA replication.
  • Increased rate caused by mutagenetic agents.
• What are mutagenetic 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 occurred 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 occurred 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 occurred 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 motion? [5]

  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.