Sanger dideoxy sequencing uses chain-terminating dideoxynucleotides (ddNTPs). Why do ddNTPs terminate DNA chain elongation?
- A They have a modified base that blocks DNA polymerase's active site
- B They carry fluorescent labels that sterically block polymerase progression
- C They lack the 3'-OH group required for nucleophilic attack on the next incoming dNTP's alpha-phosphate ✓
- D They form extra hydrogen bonds with the template, creating roadblocks
Correct answer: C. They lack the 3'-OH group required for nucleophilic attack on the next incoming dNTP's alpha-phosphate
Explanation
ddNTPs (2',3'-dideoxynucleotides) lack both the 2'-OH and the 3'-OH groups. After incorporation into the growing chain, the absence of the 3'-OH means there is no hydroxyl group to perform nucleophilic attack on the alpha-phosphate of the next incoming dNTP. The phosphodiester bond cannot form, and chain elongation terminates at that position. Each ddNTP type (ddATP, ddCTP, ddGTP, ddTTP) terminates the chain at the corresponding base position, generating a nested set of fragments whose sizes reveal the DNA sequence.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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