Q.87 Codon bias is correlated with the relative frequencies of which one of the following
types of RNA?
(A) mRNA
(B) rRNA
(C) siRNA
(D) tRNA

The correct answer is (D) tRNA.

Codon bias refers to the non-random preference for certain synonymous codons within a genome, which directly impacts translation efficiency during protein synthesis. This bias correlates strongly with tRNA relative frequencies because organisms evolve codon usage to match the abundance of available tRNAs, ensuring optimal matching between mRNA codons and tRNA anticodons.​

Option Analysis

mRNA (A): mRNA carries the genetic code using codons, but its sequence reflects codon bias rather than determining it. Codon preferences in mRNA arise from selection pressures tied to translation machinery, not mRNA abundance itself.​

rRNA (B): rRNA forms the core of ribosomes and facilitates translation but does not decode specific codons. Its abundance influences overall translation rate, yet no direct correlation exists with codon bias patterns.​

siRNA (C): siRNA mediates RNA interference for gene silencing and has no role in protein translation or codon recognition. It operates post-transcriptionally on mRNA degradation, unrelated to codon usage.​

tRNA (D): tRNA isoacceptors vary in abundance across codons for the same amino acid. Genomes favor codons recognized by the most plentiful tRNAs to minimize translation errors and speed elongation, creating the observed correlation.​

Codon bias correlated with tRNA frequencies represents a key concept in molecular biology for CSIR NET aspirants. This phenomenon explains how genomes optimize protein synthesis by aligning codon usage with available tRNAs.​

Codon Bias Mechanism

Organisms exhibit codon bias where synonymous codons (e.g., UUU vs UUC for phenylalanine) occur at unequal frequencies. Highly expressed genes preferentially use “optimal” codons matched to abundant tRNAs, enhancing translation speed and accuracy. Mutational bias and GC content contribute, but natural selection for tRNA compatibility drives the primary pattern.​

• tRNA adaptation index (tAI) quantifies this by weighting codon-anticodon pairing efficiency against tRNA copy numbers.​

• In bacteria like E. coli, arginine codons CGU/CGC dominate due to high tRNA^Arg levels, while human genes favor AGA/AGG.​

Why tRNA, Not Others?

Codon bias directly mirrors tRNA pool composition because translation stalls on rare codon-tRNA mismatches. Studies across prokaryotes and eukaryotes show strong positive correlations (e.g., Pearson r=0.88 in Neurospora) between codon frequency and tRNA gene copies.​

CSIR NET Relevance

This MCQ tests understanding of translation optimization, a recurring theme in genetics units. Rare codons slow elongation, increasing error risks, while optimal ones boost expression—crucial for biotechnology applications like recombinant protein production. Practice by analyzing codon tables against tRNA abundances for exam mastery.​