121. Which one of the molecular processes utilize 2′, 3′ Dideoxy-ribonucleoside triphosphates?
A. DNA synthesis for primers.
B. Okazaki fragment formation during lagging strand synthesis.
C. DNA sequencing by synthesis.
D. Transcript chain elongation.

Detailed Explanation:

2′, 3′ Dideoxy-ribonucleoside triphosphates (ddNTPs) are synthetic nucleotide analogs that lack both the 2′-OH and 3′-OH groups on the sugar ring. Their absence of a 3’-hydroxyl group prevents the formation of a phosphodiester bond with the next nucleotide during DNA synthesis. This results in chain termination, making ddNTPs critical tools in molecular biology, particularly in DNA sequencing.

Where Are ddNTPs Used?

• DNA sequencing by synthesis, specifically in the Sanger method, is the primary use case.

  • In this method, a mixture of normal deoxynucleotides (dNTPs) and small amounts of ddNTPs are used.

  • When a ddNTP is incorporated into the growing DNA strand, synthesis stops at that point.

  • By using fluorescently or radioactively labeled ddNTPs, scientists can determine the DNA sequence by analyzing the lengths of terminated fragments.

Why Not in Other Processes?

• DNA synthesis for primers and Okazaki fragment formation require elongation and thus a 3’-OH group.

• Transcript chain elongation in RNA synthesis uses ribonucleotides (rNTPs), not dideoxynucleotides, and also needs a 3’-OH for polymerization.

Correct Answer:

C. DNA sequencing by synthesis.

This is the only listed process where 2′, 3′ dideoxy-ribonucleoside triphosphates are intentionally used to control and analyze DNA strand termination for sequencing purposes.