Lagging DNA, more accurately referred to as the lagging strand, is one of the two strands of DNA that are synthesized during DNA replication. It's characterized by its discontinuous synthesis.
Understanding DNA Replication
Before understanding the lagging strand, it's essential to grasp the basics of DNA replication:
- DNA Double Helix: DNA exists as a double helix, with two strands running in opposite directions (antiparallel). One strand runs 5' to 3', and the other runs 3' to 5'.
- DNA Polymerase: DNA polymerase is the enzyme responsible for synthesizing new DNA strands. It can only add nucleotides to the 3' end of an existing strand. This directionality is crucial.
- Replication Fork: DNA replication begins at origins of replication, creating a replication fork – a Y-shaped structure where the DNA strands separate.
Leading vs. Lagging Strand
At the replication fork, the two DNA strands are replicated differently:
- Leading Strand: The leading strand is synthesized continuously in the 5' to 3' direction, following the replication fork as it opens. DNA polymerase can continuously add nucleotides to its 3' end.
- Lagging Strand: Because DNA polymerase can only add nucleotides to the 3' end, the lagging strand cannot be synthesized continuously in the direction of the replication fork.
Discontinuous Synthesis of the Lagging Strand
The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments. This is how it works:
- Priming: An enzyme called primase synthesizes a short RNA primer on the lagging strand. This primer provides a 3' end for DNA polymerase to attach to.
- Elongation: DNA polymerase adds nucleotides to the 3' end of the RNA primer, synthesizing an Okazaki fragment in the 5' to 3' direction, moving away from the replication fork.
- New Primer: As the replication fork opens further, another RNA primer is synthesized further down the lagging strand.
- Fragment Synthesis: DNA polymerase extends the second primer, creating another Okazaki fragment.
- Primer Removal: An enzyme removes the RNA primers.
- Gap Filling: Another DNA polymerase fills the gaps left by the removed RNA primers with DNA nucleotides.
- Ligation: DNA ligase joins the Okazaki fragments together, creating a continuous DNA strand.
Summary Table
| Feature | Leading Strand | Lagging Strand |
|---|---|---|
| Synthesis | Continuous | Discontinuous |
| Direction | Towards replication fork (5' to 3') | Away from replication fork (5' to 3') |
| Fragments | None | Okazaki fragments |
| Primers | One primer needed at the origin | Multiple primers needed for each Okazaki fragment |
In conclusion, the lagging strand is a necessary consequence of the antiparallel nature of DNA and the unidirectional activity of DNA polymerase. Its discontinuous synthesis ensures that both strands of DNA are replicated completely and accurately.
