Question

If Gobind Khorana added a synthetic mRNA with the sequence 5’-(CUAG)_n-3’ to the in vitro translation system, and then purified and analyzed long polypeptides (longer than 10 amino acids) that have been produced, how many different polypeptides (polypeptides with different amino acid sequences) would he detect?

Answer 1

The sequence require to code polypeptides that are more than 10 amino acids in length require 33 nucleotide sequences and hence the given 5'CUAG3' will be repeated for 9 times yielding a 36 nucleotide stretch of mRNA.

The RNA stretch that is repeated 9 times will yield the following sequence:

5' CUA GCU AGC UAG CUA GCU AGC UAG CUA GCU AGC UAG 3'

Three nucleotides code for one amino acid. These are called as codons. But among the above RNA stretch there is stop codon (UAG) at every 4th codon and hence the polypeptide that will be forming won't be greater than 3 amino acids in length. These 3 amino acids will be Leu(CUA)-Ala (GCU)-Ser (AGC)-stop (UAG). Hence the polypeptide of length 3 amino acids only will be formed and no polypeptide with length more than 10 polypeptides will be detected.

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