Question

If the molecular weight of a polypeptide is \[15.3kDa\], what would be the minimum number of nucleotides in the mRNA that codes for this polypeptide? Assume that the molecular weight of each amino acid is \[90Da\].
A. \[510\]
B. \[663\]
C. \[123\]
D. \[170\]

Answer 1

Hint: Nucleic acids are a mixture of basic proteins and phosphorus-containing organic acid. There are mainly two types of nucleic acid, namely, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Nucleotides are the commonly known monomeric units of a nucleic acid. The nucleic acids are sometimes referred to as polynucleotides.

Complete answer:
Nucleotides are phosphate esters of nucleosides. These are generally made up of three components including a nitrogenous base, a \[5\]-carbon sugar, and a phosphate group. Polynucleotides are thus the polymers of nucleotides that are formed by the condensation of several nucleotides. The phosphodiester bonds form a linkage between the two nucleotides. Each nucleotide in the polynucleotide chain is therefore referred to as a nucleotide residue. The phosphate of polynucleotides has an acidic nature, and therefore, the polynucleotides are polyanions.
In this problem, the total molecular weight of a polynucleotide chain is given as \[15.3kDa\] (kiloDalton), and the molecular weight of each amino acid is given as \[90Da\]. During the process of formation of the polynucleotide chain from mRNA, \[3\]nucleotides sequence contributes to one amino acid. Therefore, the number of nucleotides in an mRNA that codes for this particular polynucleotide will be thrice the number of amino acids.
Molecular weight of \[1\] amino acid = \[90Da\]
Total weight of the polynucleotide chain = \[15.3kDa\]
 = \[15.3 \times 1000Da\]
= \[15300Da\]
Therefore,
Number of amino acids on the polypeptide chain = \[\frac{{15300}}{{90}}Da\]
= \[170Da\]
Since each amino acid is coded by \[3\] nucleotide, therefore
Number of nucleotides in the mRNA = \[3 \times 170Da\]
 = \[510Da\]

Hence, the correct answer is option (A).

Note: Each amino acid is coded by a set of three nucleotides known as the nucleotide codon. One amino acid can be encoded by one or more than one codon. Each codon is represented by three letters designating the nucleotides including purines or pyrimidines.