Question

The sequence of nucleotides in an mRNA is 5’AUGACCCAUUGGUCUCGUUGGCUGAAGUCA 3’.
Hydroxylamine is a mutagen that results in the conversion of a GC base pair in the DNA to an AT pair. If hydroxylamine were applied to cells and caused a mutation (G to A) at position 20 of the DNA that codes for the mRNA, how many amino acids long would you expect the polypeptide to be in the mutant?

Answer 1

Answer:

Ths change represents a mutation of tryptophan (W) to a stop codon: W  to a stop codon  

Explanation:

Messenger RNAand protein sequence (below):

AUG ACC CAU UGG UCU CGU UGA CUG AAG UCA

  M     T      H      W       S      R     " W  "   L        K      S

Since the mutagen changes the sequence to produce a stop codon at position 20 of the DNA, it is expected the following polypeptide sequence:

AUG ACC CAU UGG UCU CGU UGA CUG AAG UCA

  M      T      H      W       S      R       -  

Stop codons such as UAG, UAA and UGA mark the end of the protein coding sequence

Answer 2

Answer:

The polypeptide chain will be six amino acids long after mutation has occurred on the mRNA strand.

Explanation:

The mRNA sequence to be transcribed into the polypeptide chain

5’AUGACCCAUUGGUCUCGUUGGCUGAAGUCA 3’

If hydroxylamine were applied to cells and caused mutation changing G to A at position 20. It will lead to a change in the amino acid to be produced and also a change in the expected number of amino acids.

Thus, if we have: 5’AUGACCCAUUGGUCUCGUUGGCUGAAGUCA 3’

A change from G to A will change the triplet codon to UAG which is one of the three stop codons signifying the end of translation, thus there will be six amino acids formed.