Chapter 13, Problem 1NST

In a mixed heteropolymer experiment using polynucleotide phosphorylase, 3/4G : 1/4C was used to form the synthetic message. The amino acid composition of the resulting protein was determined to be:

From this information,

1. (a) Indicate the percentage (or fraction) of the time each possible codon will occur in the message.
2. (b) Determine one consistent codon base composition assignment for the amino acids present.

Summary Introduction

To determine: The fraction of the time each possible codon will occur in the message formed in the given experiment.

Introduction: The heteropolymer is made up of two different monomers. The mixture of heteropolymers is used for the process of copolymerization. The amino acids are incorporated in a cell-free protein-synthesizing system in a test-tube. The process begins with a cell-lysate containing all the necessary factors for translation. This method was developed to follow the progress of translation for decoding the genetic code by radioactive labeling of one or more amino acids.

Explanation of Solution

In the given experiment, the amino acids are built from a polynucleotide sequence using only G’s and C’s. The proportion of G is ¾, and C is 1/4. The percentage that each codon will appear depends on its nucleotide proportion.

$\text{Chance of codon}=\left[\begin{array}{l}\left(\text{Proportion of first nucleotide}\right)\left(\text{Proportion of second nucleotide}\right)\\ \left(\text{Proportion of thirtd nucleotide}\right)\end{array}\right]$

The fraction for each codon that can occur in the synthetic message is represented as follows:

$\text{GGG}=3/4\times 3/4\times 3/4=27/64$

$\text{GGC}=3/4\times 3/4\times 1/4=9/64$

$\text{GCG}=3/4\times 1/4\times 3/4=9/64$

$\text{CGG}=1/4\times 3/4\times 3/4=9/64$

$\text{CCG}=1/4\times 1/4\times 3/4=3/64$

$\text{CGC}=1/4\times 3/4\times 1/4=3/64$

$\text{GCC}=3/4\times 1/4\times 1/4=3/64$

$\text{CCC}=1/4\times 1/4\times 1/4=1/64$

Thus, the fractions for the possible codons GGG, GGC, GCG, CGG, CCG, CGC, GCC, CCC are 27/64, 9/64, 9/64, 9/64, 3/64, 3/64, 3/64, and 1/64, respectively.

Summary Introduction

To determine: One consistent codon base composition assignment for the amino acids present.

Introduction: Codon is a sequence of three nucleotides that corresponds with a specific amino acid or termination signal during translation. DNA and RNA are written in a language of four nucleotides, while the protein language includes 20 amino acids.

Explanation of Solution

In this problem, we are assuming that the genetic code is not known to us. All the given amino acids except the proline are decoded by more than one codon.

Thus, the proline (CCG) is one consistent codon base composition assignment for the amino acids present.