Question

Which of the following figures correctly shows the replication fork formed during DNA replication?

• A. 1
• B. 2
• C. 3
• D. 4

Hint:

Always look for the template polarity.

• {3' $\rightarrow$ 5' Template:} Continuous synthesis (Leading).
• {5' $\rightarrow$ 3' Template:} Discontinuous synthesis (Lagging).

Answer 1

The Correct Option is D

Step 1: Concept of DNA Replication Polarity:

• DNA Polymerase can only synthesize DNA in the 5' $\rightarrow$ 3' direction.
• Therefore, the template strand must be read in the 3' $\rightarrow$ 5' direction.

Step 2: Analyzing the Replication Fork:

• Leading Strand: Synthesized continuously. The template strand polarity is 3' $\rightarrow$ 5' towards the replication fork opening.
• Lagging Strand: Synthesized discontinuously (Okazaki fragments). The template strand polarity is 5' $\rightarrow$ 3' towards the replication fork opening.

Step 3: Evaluating the Diagram (Option 4):
In Option 4:

• Top template strand has 5' $\rightarrow$ 3' polarity towards the right (fork opening). The new strand is shown growing discontinuously (short fragments) away from the fork. This is correct for the lagging strand.
• Bottom template strand has 3' $\rightarrow$ 5' polarity towards the right (fork opening). The new strand is shown growing continuously into the fork. This is correct for the leading strand.

Step 4: Final Answer:
Option 4 correctly depicts the polarity and the continuous vs. discontinuous synthesis.