The amino acid changes noted in Parts A-C are the result of changes in DNA seque

Question

The amino acid changes noted in Parts A-C are the result of changes in DNA sequence.

For each of these amino acid changes, evaluate whether a single base change in the given codons could account for the change in the amino acid.

PART A - For which codon(s) could a single base change account for this amino acid change?
Lysine to Asparagine

Select all that apply.

A) AAA

B) AAG

C) UUU

D) NONE OF THESE

PART B -

For which codon(s) could a single base change account for this amino acid change?
Alanine to Phenylalanine

Select all that apply.

A) GCA

B) GCG

C) GCC

D) GCU

E) NONE OF THESE CODONS

PART C -

For which codon(s) could a single base change account for this amino acid change?
Leucine to Glutamine

Select all that apply.

A) UUA

B) CUC

C) CUG

D) UUG

E) CUU

F) CUA

G) NONE OF THESE CODONS

Explanation / Answer

From the Part A-C the single base that needs to be changed (if applicable) for the responsible codons are marked as BOLD in the correct option. Possible explanations follows each questions below.

Part A: Lysine to Asparagine

Answer: A) AAA

             B) AAG

Explanation: The genetic code for asparagine is AAU,and AAC. Hence, for the lysine codons change occurs in the third base (marked in bold) to give asparagine. Here, the third base purines adenine and guanine is changed to the pyrimidines uracil and cytosine. So, the correct options are (A) and (B)

Part B: Alanine to Phenylalanine

Answer: E) None of these codons

             Explanation: When we compare the genetic code of the two amino acids, the phenylalanine (UUU and UUC) can be obtained by changing two bases of the codons of alanine, viz., GCU and GAC. Here, the first two bases of both the codons i.e. GC and GA (marked in bold) must be changed to UU for converting alanine to phenylalanine. Hence, the question is asked for the single amino acid change, rather than two. The correct answer is justified as (E) None of these codons.

Part C: Leucine to Glutamine

Answer: C) CUG

             F) CUA

Explanation: The genetic codes for leucine are CUU, CUC, CUA and CUG. And the genetic codes for glutamine are CAA and CAG. Hence, for the changing leucine to glutamine, the second base, i.e. uracil of the two leucine codons namely CUA and CUG must be replaced by adenine. Hence the correct options are (C) and (F).

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