Suppose you could separate out the two strands of all replication bubbles in a cell into leading strands and lagging strands. Where would you expect to find the most primase enzymes?
The correct answer and explanation is:
Correct Answer: On the lagging strands
Explanation:
During DNA replication, the enzyme primase plays a crucial role by synthesizing short RNA primers. These primers provide the starting point for DNA polymerase to begin adding DNA nucleotides, since DNA polymerases can only add nucleotides to an existing strand and cannot start synthesis on their own.
DNA replication is semi-discontinuous. This means that one strand, called the leading strand, is synthesized continuously in the same direction as the replication fork is opening. The other strand, known as the lagging strand, is synthesized discontinuously in the opposite direction as the replication fork movement. The lagging strand is made in short segments called Okazaki fragments, each requiring a new RNA primer.
Because of this pattern, primase is much more active on the lagging strand. Every Okazaki fragment on the lagging strand needs its own primer, which primase must lay down before DNA polymerase can extend the strand. In contrast, the leading strand only needs one primer at the origin of replication because it is synthesized continuously.
If all replication bubbles in a cell were separated into their leading and lagging strands, you would expect to find the most primase enzymes associated with the lagging strands, where repeated primer synthesis is necessary. On the leading strand, the need for primase is minimal—only a single action at the start is typically sufficient. Therefore, the highest concentration and activity of primase would be found on the lagging strands of DNA during replication.