Apentix/AP Biology/Unit 6: Gene Expression and Regulation/Question
AP Biologyhardmcq1 pt

Classical Sanger sequencing reactions include small amounts of dideoxynucleotides alongside normal deoxynucleotides. Why does incorporation of a dideoxynucleotide stop further extension of that strand?

A.Its sugar lacks the 3' hydroxyl group that must link to the next nucleotide
B.It carries three phosphate groups, jamming the polymerase active site
C.It binds every template base equally well, stalling the enzyme
D.It breaks the hydrogen bonds holding the template strand in place

Explanation

Core Concept

Chain elongation joins the incoming nucleotide to the free hydroxyl on the 3' carbon of the terminal sugar. Dideoxynucleotides carry hydrogen instead of hydroxyl at both the 2' and 3' positions. Once incorporated, such a nucleotide offers no reactive 3' hydroxyl, so no further phosphodiester bond can form. Fragments ending at each base position then reveal the template sequence.

Correct Answer

AIts sugar lacks the 3' hydroxyl group that must link to the next nucleotide

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