1. DNA Polymerase's Directionality:
* DNA polymerase, the enzyme responsible for building new DNA strands, can only add nucleotides in the 5' to 3' direction. This means it can only attach a new nucleotide to the 3' hydroxyl group of the existing nucleotide.
2. Antiparallel Nature of DNA:
* The two strands of DNA run in opposite directions (antiparallel), one in the 5' to 3' direction and the other in the 3' to 5' direction.
3. The Replication Fork:
* DNA replication starts at a point called the origin of replication, and the two strands separate to create a replication fork.
Leading Strand:
* The leading strand is synthesized continuously because its 3' end faces the replication fork, allowing DNA polymerase to add nucleotides directly as the fork opens.
Lagging Strand:
* The lagging strand's 3' end faces away from the replication fork. This means DNA polymerase cannot synthesize it continuously. Instead, it's synthesized in short fragments called Okazaki fragments (around 100-200 nucleotides long).
The Process of Lagging Strand Synthesis:
1. RNA primer: An RNA primer is laid down by primase at the 5' end of the lagging strand.
2. DNA polymerase adds nucleotides: DNA polymerase extends the primer in the 5' to 3' direction.
3. Discontinous synthesis: The lagging strand grows in short fragments, each starting with a new primer.
4. Exonuclease removes primers: Once the fragment is complete, the RNA primer is removed by an exonuclease.
5. DNA ligase joins fragments: The gap between the fragments is filled by DNA polymerase, and the fragments are joined together by DNA ligase.
In Summary:
The leading strand can be synthesized continuously because its 3' end faces the replication fork, allowing for continuous nucleotide addition. The lagging strand, facing away from the fork, needs to be synthesized discontinuously in short fragments due to the 5' to 3' directionality of DNA polymerase.
