Practice DNA Replication

1.  a. What are the leading and lagging strands in DNA replication?

b. Describe the properties of DNA and DNA polymerase that make it necessary to have a leading and a lagging strand.

a. What are the leading and lagging strands in DNA replication?

The leading strand is synthesized continuously as the replication fork proceeds. The strand is labeled 3' to 5' so that the complementary bases that are added will be added in a 5' to 3' direction.

The lagging strand is synthesized discontinuously as the replication fork proceeds. The strand is labeled 5' to 3' so that the complementary bases that are added in the direction AWAY from the replication fork, but still in a 5' to 3' fashion.

See Figure 9.29 (page 230)

b. Describe the properties of DNA and DNA polymerase that make it necessary to have a leading and a lagging strand.

The property of DNA that causes the continuous/discontinuous synthesis is the antiparallel property of DNA. One strand has the directionality of 5' to 3' while the other strand has the directionality of 3' to 5'. See figure 9.27 (page 228) for a visual.

The property of DNA polymerase that causes the continuous/discontinuous synthesis is that the enzyme can only add nucleotides to a free 3' hydroxyl group. See figure 9.26 (page 228) for a visual. Because of this, synthesis MUST proceed from 5' to 3', but can never proceed 3' to 5' with this enzyme.