Chapter 24, Problem 82A

Interpretation Introduction

Interpretation: A base sequence for DNA that codes for the tripeptide Ala-Gly-Ser needs to be written. The possible variation in the answers needs to be explained.

Concept Introduction: A base sequence for DNA can be written by using three-letter codes for amino acids. Here, for most of the amino acids, there are single first and second-letter codes but there are 4 possible third-letter codes due to which different numbers of base sequences for DNA are possible.

Explanation of Solution

The given tripeptide is Ala-Gly-Ser. Here, the first amino acid is alanine, the second is glycine, and the third is serine.

For alanine, the first letter code word is C, the second letter code is G, and the possible third letter code words are A, G, T, and C. Thus, possible three-letter code for Ala is CGA, CGG, CGT, and CGC.

For glycine, the first letter code word is C, the second letter code is C, and the possible third letter code words are A, G, T, and C. Thus, possible three-letter codes for Gly are CCA, CCG, CCT, and CCC.

For serine, the possible first letter code words are A and T and for these second letter, code words are G and C respectively. Now, for the first letter code A, the possible third letter code words are A, G, T, and C. For the first letter code T, the possible third letter code words are A and G. Therefore, the possible three-letter code for Ser are AGA, AGT, AGT, AGC, TCA, and TCG.

Thus, a possible base sequence for the tripeptide Ala-Gly-Ser is CGACCAAGA.

Other possible base sequences can be possible for a different combination of three-letter codes for amino acids. This is the reason why different students can have different answers for the base sequence of DNA.