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Understanding DNA Synthesis and Termination

Assertion (A): Termination of DNA synthesis occurs upon incorporation of a nucleotide lacking a hydroxyl group on the 3′ carbon of the pentose sugar into the growing DNA chain.

Reason (R): The hydroxyl group on the 3′ carbon of the pentose sugar is involved in phosphodiester bond formation.

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Explanation:

DNA synthesis involves the addition of nucleotides to the 3′ end of a growing DNA strand. This process relies on the nucleophilic attack of the 3′ hydroxyl (–OH) group on the incoming nucleotide’s 5′ phosphate group, forming a phosphodiester bond.

• If a nucleotide lacking the 3′ hydroxyl group, such as dideoxynucleotides (ddNTPs), is incorporated, no further extension can occur because there is no –OH group to form the next phosphodiester bond.

• This property is used in Sanger sequencing, where ddNTPs cause chain termination.

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Correct Answer:

1. Both A and R are true and R is the correct explanation of A

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Conclusion:

The termination of DNA synthesis upon incorporation of a nucleotide without a 3′ hydroxyl group is directly due to the essential role of that group in forming the phosphodiester bond. Both the assertion and reason are scientifically correct, and the reason directly explains the assertion.