Which of the following statements about DNA replication is true?

Introduction / Context:
This question tests your understanding of key enzymes and proteins involved in DNA replication. Accurate replication of DNA is vital for cell division and genetic stability. Recognizing which statements correctly describe replication components helps you build a detailed picture of this fundamental process.

Given Data / Assumptions:

• We are considering DNA replication in a typical cell.
• The options involve single strand binding proteins, Okazaki fragments, direction of DNA polymerase action, and DNA gyrase.
• Only one statement is fully correct.
• Standard textbook descriptions of replication machinery apply.

Concept / Approach:
During DNA replication, the double helix is unwound, and each strand serves as a template for new synthesis. Single strand binding proteins bind to the separated strands to prevent them from reannealing and to protect them. Okazaki fragments are short DNA segments synthesized on the lagging strand, not the leading strand. DNA polymerase extends new DNA only in the five prime to three prime direction. DNA gyrase relieves supercoiling ahead of the replication fork but does not directly unwind the helix in the way helicase does. By comparing these facts with the options, we can identify the true statement.

Step-by-Step Solution:
Step 1: Evaluate the first statement about single strand binding proteins. They bind to unwound DNA strands and stabilize the open conformation, preventing them from coming back together, which is accurate.Step 2: Evaluate the second statement, which claims that Okazaki fragments are synthesized on the leading strand. In reality, they are synthesized on the lagging strand because this strand must be made in short segments.Step 3: Evaluate the third statement about DNA polymerase direction. DNA polymerase synthesizes new DNA by adding nucleotides to the three prime end of the growing chain, which means synthesis proceeds in the five prime to three prime direction, not three prime to five prime.Step 4: Evaluate the fourth statement about DNA gyrase. This enzyme reduces torsional strain and supercoiling ahead of the replication fork; helicase is the main enzyme that unwinds the double helix at the fork.Step 5: Only the first statement correctly describes the role of single strand binding proteins in replication.

Verification / Alternative check:
A quick way to check is to remember the typical sequence of events at a replication fork. Helicase unwinds the DNA, single strand binding proteins coat the separated strands, primase lays down primers, and DNA polymerase extends new strands. If you recall that Okazaki fragments are associated with the lagging strand and that DNA polymerase works in a five prime to three prime direction, you can confidently reject the incorrect statements.

Why Other Options Are Wrong:
Okazaki fragments on the leading strand: The leading strand is synthesized continuously, so short fragments are not needed. Okazaki fragments are characteristic of the lagging strand.
Polymerase adding monomers in the three prime to five prime direction: DNA polymerases add nucleotides to the three prime end of the growing chain, resulting in overall synthesis in the five prime to three prime direction.
DNA gyrase directly unwinding the helix at the fork: Gyrase alleviates supercoiling ahead of the fork but helicase unwinds the helix at the fork itself.

Common Pitfalls:
Students often confuse the roles of different enzymes, thinking that any enzyme involved near the replication fork might be responsible for unwinding or stabilizing DNA. Another common mistake is mixing up the directionality of DNA polymerase. Remembering that polymerase can only extend in the five prime to three prime direction and that Okazaki fragments belong to the lagging strand helps avoid these errors.

Final Answer:
The true statement about DNA replication is Single strand binding proteins stabilize the open conformation of unwound DNA.