Which portion of a chromosome contains Okazaki fragments?
a. helicase
b. lagging strand
c. leading strand
d. primer
The correct answer and explanation is:
Correct Answer: b. lagging strand
During DNA replication, the portion of a chromosome that contains Okazaki fragments is the lagging strand. DNA replication is a highly coordinated and semi-conservative process where each original strand of the DNA double helix serves as a template for the formation of a new complementary strand. Because DNA polymerase can only add nucleotides in the 5’ to 3’ direction, the two strands are synthesized differently.
On the leading strand, replication proceeds continuously in the same direction as the replication fork movement because the orientation of the strand allows DNA polymerase to follow the helicase without interruption. This means the leading strand does not form Okazaki fragments. Instead, the DNA polymerase synthesizes a continuous new strand by adding nucleotides one by one.
In contrast, the lagging strand runs in the opposite direction, 5’ to 3’ away from the replication fork. This creates a problem because the DNA polymerase must still synthesize in the 5’ to 3’ direction but the fork is opening in the opposite direction. To solve this, the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments. These short segments are formed as the replication fork opens more of the template strand. Each fragment begins with an RNA primer laid down by primase. DNA polymerase then extends the primer to form an Okazaki fragment. Later, the RNA primers are removed and replaced with DNA nucleotides, and the gaps are sealed by DNA ligase, creating a continuous strand.
Okazaki fragments are an essential feature that demonstrates the clever adaptation of the cell to manage the directional limitation of DNA polymerase. Without these fragments, cells could not properly replicate the lagging strand, which would halt cell division and organismal growth.