Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologymediummcq1 pt

Which statement about the directionality of nucleic acids is correct?

A.Nucleic-acid strands have 5′→3′ polarity, and polymerases generally extend a growing strand at its 3′ end
B.Both ends of a strand carry phosphates
C.DNA strands pair in parallel orientation
D.Directionality does not affect replication

Explanation

Core Concept

The 5′ and 3′ labels refer to carbon positions in the pentose sugar and define strand polarity. Standard DNA/RNA synthesis extends the growing strand by adding nucleotides to a 3′ hydroxyl. Saying that every nucleic-acid molecule must literally terminate in a free 5′ phosphate and a free 3′ hydroxyl is unnecessarily absolute because termini can have different chemical states or modifications.

Correct Answer

ANucleic-acid strands have 5′→3′ polarity, and polymerases generally extend a growing strand at its 3′ end

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