Consider a protein segment with the following amino acid seq

Consider a protein segment with the following amino acid sequence: Ala-Thr-Lys-Glu-Gly-Val-Ile

First, write out the nucleotide sequence of the mRNA segment that codes for this protein, using the Table of Codons.

Note that some nucleotide positions will be uncertain. In these positions use “X” to denote any base, “R” to denote a purine (A or G) and “Y” to denote a pyrimidine (U or C).

Now consider all possible single-base-change mutations in this region, and answer the following:

a) What fraction of all such mutations cause premature termination of translation?

b) What fraction cause a loss of a negative charge? (Note: This problem is more difficult than it seems. Be careful.)

c) In an experiment such as this, why would you see only two products with the polymer (GUA)n? What are they?

Solution

Consider a protein segment with the following amino acid sequence:

Ala-Thr-Lys-Glu-Gly-Val-Ile

Note that some nucleotide positions will be uncertain. In these positions use “X” to denote any base, “R” to denote a purine (A or G) and “Y” to denote a pyrimidine (U or C).

ACX- AAR-GGR-GGX-GUX-AUX

Question 1.

What fraction of all such mutations cause premature termination of translation?

Stop codon code for no amino acid and cause the termination of    translation.

Stop codons are UAA, UAG, UGA

Since the chain is in form of triplet so in order to have Premature termination of translation can happen only when there is mutation which convert the triplet into any of the stop codon

GCX-ACX-AAR-GGR-GGX-GUX-AUX

In case of mutation in GGX to UGX, when X is either A or G, there is possibility of premature translational termination

In case of mutation in GGR to UGR there is possibility of premature translational termination

In case of mutation in AGR to AGR there is possibility of premature translational termination

Question 2.

What fraction cause a loss of a negative charge? (Note: This problem is more difficult than it seems.

In the given sequence of protein Ala-Thr-Lys-Glu-Gly-Val-Ili

Glu code for amino acid glutamic acid is the only negatively charged amino acid the given peptide so if GGR is mutated into YGR or GYR then in that case peptide will lose its negative charge

Question 3.

In an experiment, such as this, why would you see only two products with the polymer (GUA)n? What are they?

GCX-ACX-AAR-GGR-GGX-GUX-AUX

GUA code for valine

UAG code for stop codon a

AGU code for serine.

When there is (GUA)_n only valine will be coded by during translation

Whereas in the case of (AGU)_n only Serine will be coded

When there is (UAG)_n there will be translational termination

So only two products will be formed when there is (GUA)_n polymer

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