Apentix/AP Biology/Unit 6: Gene Expression and Regulation/Question
AP Biologyeasymcq1 pt

In a PCR reaction, short single-stranded DNA primers anneal to the template strands during the cooling step. Why are these primers required?

A.They unwind the double helix so polymerase gains access to each strand
B.They provide paired 3' ends that Taq polymerase extends, setting the boundaries of the amplified segment
C.They donate the energy used to forge each new phosphodiester bond
D.They proofread newly built strands and cut out mismatched bases

Explanation

Core Concept

DNA polymerases cannot begin a brand-new chain; they only add nucleotides to an existing 3' hydroxyl. Primers bracket the chosen target and supply those starting points on each strand. Their sequence determines exactly which fragment accumulates exponentially across cycles.

Correct Answer

BThey provide paired 3' ends that Taq polymerase extends, setting the boundaries of the amplified segment

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