Okazaki fragments and the lagging strand
Okazaki fragments are the short pieces of new DNA that make up the lagging strand, built away from the replication fork (where the double helix opens).
Remember
DNA polymerase (the enzyme that builds new DNA) can only build in the 5' to 3' direction (the numbers name the two ends of a DNA strand). The two strands of the double helix run in opposite directions, so the lagging strand is made in short pieces, which DNA ligase (the enzyme that joins the fragments into one chain) later joins.
Memory aid
DNA polymerase is like a train that can only lay track forwards.
One strand can follow the opening fork, whereas the other must keep going back to start new pieces.
DNA polymerase can only add nucleotides (the building blocks of DNA) to the 3' end of a growing strand, so it builds new DNA in the 5' to 3' direction.
The two strands of the double helix run in opposite directions. Helicase (the enzyme that unzips the helix) opens the replication fork, and one template strand (the strand being copied) lets the new strand be built continuously towards the fork. This is the leading strand, made in one continuous piece.
The other template runs the opposite way, so its new strand has to be built away from the fork, in short stretches. Each stretch starts from its own RNA primer (a short RNA starter laid down by primase), because DNA polymerase cannot begin a strand from nothing. These stretches are Okazaki fragments, and the strand made this way is the lagging strand.
The lagging strand lags because the fork has to open further before each new fragment can start. DNA ligase later joins the fragments into one chain.
Reading and building are a common mix-up. DNA polymerase reads the template in the 3' to 5' direction, but it builds the new strand 5' to 3'.
Key facts
- Polymerase direction
- DNA polymerase can add nucleotides only in the 5' to 3' direction, on both strands.
- Leading strand
- it is made continuously towards the replication fork from a single primer.
- Lagging strand
- it is built as Okazaki fragments away from the fork.
- Priming
- the lagging strand needs a new primer for every fragment.
- Joining and unwinding
- the fragments are sealed by DNA ligase, while helicase unwinds the double helix at the fork.
- Reading and building
- DNA polymerase reads the template in the 3' to 5' direction and builds the new strand 5' to 3'.
Exam traps
- DNA synthesis cannot begin without a primer, because DNA polymerase cannot begin a strand from nothing and needs a free 3' end to extend, so the lagging strand needs a new primer for every fragment.
- Helicase does not join DNA fragments, because it unwinds the double helix at the fork; the fragments are sealed by DNA ligase.
- DNA polymerase reads a template in the 3' to 5' direction but builds the new strand 5' to 3', and it is the building direction that forces fragments on one strand.
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