Chapter 25, Problem 1P

Interpretation Introduction

Interpretation:

The nucleotide composition of coding triplets for the six amino acids should be explained based on the following experiment.

"A random polyribonucleotide sequence which is produced by polynucleotide phosphorylase with ADP and CDP in 1:5 molar ratio results in the incorporation of proline, histidine, threonine, glutamine, asparagine and lysine in proportion $100,23.4,20,3.3,3.3,1.0$ "

Concept Introduction:

The scientists Matthaei and Nirenberg used the copolymer mixed with ribonucleic acid (RNA) to decipher the code. The copolymer is synthesized by mixing two or more different types of ribonucleoside diphosphate in combination to form an artificial message.

In order to find out the frequency of any triplet code, the relative proportion of each type of ribonucleoside diphosphate must be known.

Explanation of Solution

When polyribonucleotide is formed by polynucleotide phosphorylase with ADP (adenine diphosphate) and CDP (cytidine diphosphate) in the molar ratio 1:5, it results in the incorporation of six amino acids, namely, proline, histidine, threonine, glutamine, asparagine and lysine. The proportion of six amino acids is $100,23.4,20,3.3,3.3,1.0$.

The proportion of proline is higher as it is coded by CCC codon, Thus, CCC forms the maximum percentage in the mixture.

The lysine is coded by AAA. The frequency of lysine is low as the mixture consists of cytosine diphosphate (CDP) and adenine diphosphate (ADP) in ration 5:1.

Frequency of AAA $={\left(\frac{1}{6}\right)}^3$

Frequency of CCC $={\left(\frac{5}{6}\right)}^3$

Thus, Frequency of CC: A = $=\left(\frac{1}{6}\right):{\left(\frac{5}{6}\right)}^2$

Also, histidine is coded by sequence CAC.

Thus, the above experiment tells us about the different codon recombination and the amino acid that codes them.