IB Biology Topic D1.1.7: Differences Between Replication on the Leading Strand and the Lagging Strand Notes

One Strand Copies Smoothly, the Other in Fragments

1. DNA polymerase can only add nucleotides in the 5′ to 3′ direction.
2. This creates two distinct processes: continuous replication on the leading strand and discontinuous replication on the lagging strand.

Continuous Replication on the Leading Strand

1. The leading strand is synthesized in the same direction as the replication fork moves.
2. Single primer initiation
   1. DNA primase adds one RNA primer at the start of the strand.
3. Continuous synthesis
   1. DNA polymerase III adds nucleotides without interruption, making one smooth, unbroken strand.

Discontinuous Replication on the Lagging Strand

1. The lagging strand is synthesized in the opposite direction to the fork's movement, so it needs a more complex process.
2. Multiple primers
   1. RNA primers are added at intervals along the strand.
3. Okazaki fragments
   1. DNA polymerase III makes short DNA segments, called Okazaki fragments, between the primers.
4. Fragment joining
   1. DNA polymerase I replaces the RNA primers with DNA, and DNA ligase connects the fragments into a continuous strand.

Key Differences at a Glance

Why the Two Strands Differ

1. The antiparallel structure of DNA is why one strand is copied continuously and the other in fragments.
2. Coordinating both strands lets the cell copy the whole molecule quickly and accurately.