MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
During the process of DNA replication, a new strand is synthesized continuously in one direction and discontinuously in the opposite direction. This phenomenon is explained by the fact that DNA polymerase can only synthesize DNA in which of the following directions?
- ARandomly
- B5' to 3'
- CBidirectionally
- D3' to 5'
Show answer & explanationAnswer & explanation
Correct answer: B. 5' to 3'
DNA polymerase can only add nucleotides to the 3' end of a growing DNA strand. This fundamental property dictates that synthesis proceeds in the 5' to 3' direction along the newly forming strand. This leads to the continuous synthesis on the leading strand and discontinuous synthesis (Okazaki fragments) on the lagging strand.
Why the other options are wrong
- A. DNA synthesis is a highly regulated and precise process, not random.
- C. DNA polymerase itself does not synthesize bidirectionally; the replication fork moves bidirectionally, but the enzyme's action is unidirectional.
- D. DNA polymerase cannot synthesize in the 3' to 5' direction; it can only add to an existing 3'-OH group.
DNA Polymerase Directionality
DNA polymerase enzymes can only add new nucleotides to the 3' hydroxyl group of a growing DNA strand, thus synthesizing new DNA in the 5' to 3' direction.
- Adds to 3' end.
- Synthesizes 5' to 3'.
- Crucial for leading/lagging strand synthesis.
- Requires a primer.
Memory trick: Polymerase: 5' to 3' is the only way to be!