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MCAT Question A Day - 8/12/13 - Answer!

During replication, DNA ligase would be expected to be most active at

A.  the leading strand.
B.  the lagging strand.
C.  both strands.
D.  the site where the double helix uncoils.

The correct answer is (B). DNA ligase is needed to combine the okazaki fragments formed during replication of the lagging strand of DNA. Since the leading strand is synthesized continuously, the ligase enzyme is not needed. The “uncoiling” of DNA’s double-helix is accomplished by helicase enzymes.