Which of the following is not common to synthesis of both leading and lagging strands
**Core Concept**
DNA replication is a semi-conservative process that involves the synthesis of new complementary strands by an enzyme called DNA polymerase. The leading strand is synthesized continuously in the 5' to 3' direction, while the lagging strand is synthesized discontinuously in short segments called Okazaki fragments.
**Why the Correct Answer is Right**
The synthesis of the leading strand is continuous because DNA polymerase can synthesize DNA in the 5' to 3' direction without releasing the RNA primer. On the other hand, the lagging strand is synthesized discontinuously due to the antiparallel nature of DNA strands. DNA polymerase can only add nucleotides to the 3' end of a growing strand, so it must release the RNA primer and synthesize a new Okazaki fragment after each round of replication. However, the lagging strand also involves the presence of an RNA primer and the activity of DNA ligase to seal the gaps between Okazaki fragments.
**Why Each Wrong Option is Incorrect**
**Option A:** DNA polymerase is involved in the synthesis of both leading and lagging strands, so it is incorrect to say that this option is not common to both.
**Option B:** RNA primers are necessary for the initiation of DNA synthesis on both the leading and lagging strands, making this option incorrect.
**Option C:** DNA ligase is also involved in the synthesis of both leading and lagging strands, specifically in sealing the gaps between Okazaki fragments on the lagging strand.
**Clinical Pearl / High-Yield Fact**
Remember that DNA polymerase can only add nucleotides to the 3' end of a growing strand, which is why the lagging strand is synthesized discontinuously.
**Correct Answer:** D. DNA ligase is not involved in the synthesis of the leading strand.