AIIMS 2003 Botany DNA Replication MCQ Question

To address the question regarding the manner of replication of DNA, we need to understand the fundamental processes of DNA replication and the directionality of the DNA strands.

Correct Answer: D) 3' → 5'

Explanation of the Correct Answer:

DNA replication is a semi-conservative process in which each strand of the original DNA molecule serves as a template for the new strand. The key points are:

1. Directionality of DNA Strands: DNA strands have polarity, with a 5' end and a 3' end. The 5' end has a phosphate group, while the 3' end has a hydroxyl group.

2. Replication Mechanism: DNA polymerases, which are the enzymes responsible for synthesizing new DNA strands, can only add nucleotides to the 3' end of a growing DNA strand. This means that DNA synthesis occurs in the 5' to 3' direction. Therefore, while the new strand is synthesized in the 5' → 3' direction, the template strand is read in the 3' → 5' direction.

3. Conclusion: Hence, the correct representation of the manner of replication is that the template strand is read from 3' to 5', which corresponds to option D.

Clarification of Incorrect Options:

• Option A: 3' → 5': This option implies that the replication occurs in the direction of the 3' end to the 5' end, which is incorrect for the synthesis of a new strand. While the template strand is read from 3' to 5', the synthesis of the new strand occurs in the opposite direction (5' → 3').

• Option B: 5' → 3': This option suggests that replication occurs in the direction of the 5' end to the 3' end. While the newly synthesized strand grows in this direction, it does not accurately describe the reading of the template strand, which occurs from 3' to 5'.

• Option C: 5' → 3': Similar to option B, this option describes the direction of new strand synthesis but does not address the template strand's orientation during replication.

Summary:

In summary, the DNA replication process involves reading the template strand from 3' to 5' to synthesize a new strand in the 5' to 3' direction. This fundamental understanding of DNA directionality is crucial for comprehending various genetic processes. Therefore, the correct answer is indeed option D, which correctly represents the template strand's directionality during DNA replication.