During DNA replication, why is an RNA primer required?

• A. Primers are required to prevent DNA ligase from sealing Okazaki fragments before replication is complete.
• B. Primers are required because helicase cannot unwind the DNA duplex unless an RNA segment is first synthesized.
• C. DNA polymerases cannot initiate DNA synthesis de novo and require a free $3\prime OH$ group to add nucleotides.
• D. DNA polymerases can only add nucleotides in the $3\prime \to 5\prime$ direction, so a primer reverses the direction of synthesis.

Answer: (C)

Verified step by step guidance

1. Understand that DNA polymerases are enzymes responsible for synthesizing new DNA strands during replication.
2. Recognize that DNA polymerases cannot start synthesizing a new DNA strand from scratch; they require a starting point with a free 3\' hydroxyl (OH) group to add nucleotides.
3. Know that an RNA primer is a short segment of RNA synthesized by primase that provides this free 3\' OH group.
4. Realize that once the RNA primer is in place, DNA polymerase can extend the DNA strand by adding nucleotides in the 5\' to 3\' direction.
5. Therefore, the RNA primer is essential because it allows DNA polymerase to initiate DNA synthesis, which it cannot do de novo.