Q.38 In Sanger’s DNA sequencing, for chain termination of replication

1. 5′ – methylcytosine

2. 2′, 3′ -dideoxy analog

3. 2′, 3′ – deoxy analog

4. 2′, 3′ – cyclic cAMP

Sanger DNA Sequencing Chain Termination

Sanger sequencing uses chain termination via 2′,3′-dideoxy analogs for precise DNA base reading. The correct answer is option 2.

Sanger Sequencing Overview

Developed by Frederick Sanger in 1977, this first-generation method relies on DNA polymerase extending primers with normal dNTPs until a chain-terminating ddNTP incorporates randomly. Lacking a 3′-OH group, ddNTPs prevent phosphodiester bond formation, yielding fragments of varying lengths separated by electrophoresis. Fluorescent or radioactive labels on ddNTPs reveal the sequence via peak patterns.

Correct Answer: 2′,3′-Dideoxy Analog

Option 2: 2′,3′-dideoxy analog (ddNTPs)
These are dideoxynucleotide triphosphates (ddATP, ddCTP, ddGTP, ddTTP) missing hydroxyls at both 2′ and 3′ ribose positions. Incorporation halts extension, creating a nested set of fragments ending at each base type for ladder-based sequencing. Used in low ratios with dNTPs for controlled termination.

Why Other Options Fail

• Option 1: 5′-methylcytosine
  Modified base in epigenetics for DNA methylation studies; doesn’t terminate chains or block polymerase—used in bisulfite sequencing instead.

• Option 3: 2′,3′-deoxy analog
  Incorrect nomenclature; deoxy nucleotides (dNTPs) are standard building blocks with 2′-H but intact 3′-OH, enabling elongation, not termination.

• Option 4: 2′,3′-cyclic cAMP
  cyclic AMP (cAMP) is a signaling molecule regulating gene expression via second messengers; no role in DNA replication or sequencing.