44. The consequences of a DNA base change in a mutation are maximum, if the base change is located
in the:
(a) First or second position of a codon ,
(b) Middle of an intron ,
(c) Repetitive DNA elements ,
(d) Third position of a codon

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How Codon Position Influences the Severity of DNA Mutations

Mutations in DNA can vary dramatically in their effects depending on where in the gene the base change occurs. Understanding codon position is crucial in predicting the functional consequences of such mutations, especially in protein-coding sequences.

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The Structure of a Codon

A codon is a triplet of nucleotides that encodes a specific amino acid in a protein. Each codon has three positions:

1. First position

2. Second position

3. Third position (also known as the wobble position)

Due to the redundancy in the genetic code (called codon degeneracy), the third position is often less critical — many amino acids are coded by multiple codons that differ only at this position.

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Evaluating the Options

Let’s break down each option to understand which leads to the maximum consequence:

(a) First or second position of a codon

Correct Answer — Mutations here often result in a missense or nonsense mutation, changing the amino acid or producing a stop codon, which can severely impact protein function.

(b) Middle of an intron

 Usually neutral — Introns are non-coding regions, and changes here often have no impact, unless they affect splice sites.

(c) Repetitive DNA elements

 Often non-functional — These are mostly non-coding and non-regulatory, so changes here generally don’t affect gene expression or protein structure.

(d) Third position of a codon

Least impactful — Due to the wobble effect, many changes at this position are synonymous (no change in the amino acid).

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Final Answer:

(a) First or second position of a codon

These positions are crucial for determining the amino acid, and mutations here are most likely to alter protein function significantly.