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 from a template strand. This process occurs in the 5' to 3' direction, meaning new nucleotides are added to the 3' end of the growing strand.
2. Identify the direction of the helicase movement, which is from left to right in the image. Helicase unwinds the DNA double helix, creating a replication fork.
3. Examine the orientation of the strands labeled A and B. Strand A is oriented 5' to 3' from left to right, while strand B is oriented 3' to 5' from left to right.
4. Determine which strand is the lagging strand. The lagging strand is synthesized discontinuously in short segments called Okazaki fragments, because it is oriented in the 3' to 5' direction relative to the movement of the replication fork.
5. Conclude that strand B is the lagging strand, as it is synthesized in the 5' to 3' direction but opposite to the direction of the replication fork movement, requiring the synthesis of Okazaki fragments.