Below is a close-up of the portion of a DNA replication bubble.

Helicase is shown as a yellow triangle currently moving from left to right. Based on what you know about the creation of new DNA during replication, which is the lagging strand and why?

• A. A is the lagging strand, as DNA is always synthesized in the 5' to 3' manner.
• B. B is the lagging strand, as DNA is always synthesized in the 5' to 3' manner.
• C. A is the lagging strand, as DNA is always synthesized in the 3' to 5' manner.
• D. B is the lagging strand, as DNA is always synthesized in the 3' to 5' manner.
• E. It is impossible to tell, with the information provided.

Answer: (A)

Verified step by step guidance

1. Understand that DNA replication involves the synthesis of new DNA strands complementary to the original template strands.
2. Recognize that DNA polymerase can only add nucleotides to the 3' end of a growing DNA strand, meaning DNA synthesis occurs in the 5' to 3' direction.
3. Identify the direction of the helicase movement, which is from left to right in the image, unwinding the DNA double helix.
4. Determine the orientation of the template strands: Strand A is oriented 5' to 3' from left to right, and Strand B is oriented 3' to 5' from left to right.
5. Conclude that the lagging strand is synthesized discontinuously in short segments (Okazaki fragments) in the 5' to 3' direction, opposite to the direction of the replication fork movement. Therefore, Strand A is the lagging strand because it is synthesized in the 5' to 3' direction away from the replication fork.