The big idea: At each fork one new strand is built continuously (the leading strand) and the other discontinuously in short pieces (the lagging strand).
The points to remember
- Both new strands are built 5' to 3', but their templates run in opposite directions.
- Leading strand: built towards the fork, so it grows continuously as the fork opens.
- Lagging strand: built away from the fork, so it is made discontinuously.
- Its short pieces are called Okazaki fragments; they are joined up afterwards.
Remember it as: Leading follows the fork; lagging keeps starting again.
Real example: Okazaki fragments. Reiji and Tsuneko Okazaki discovered the short fragments in 1968 by labelling newly made DNA in E. coli. In bacteria each fragment is about 1,000 to 2,000 nucleotides long.
Not 3' to 5': The lagging strand is not built 3' to 5'. Every piece is built 5' to 3', just away from the fork.
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Primers: once or again and again: DNA polymerase cannot start a strand from nothing. It needs a short RNA primer to build from.
Leading strand
- Continuous
- Built towards the fork
- RNA primer laid down once
- One long new strand
Lagging strand
- Discontinuous
- Built away from the fork
- RNA primer laid down again and again
- Many Okazaki fragments, joined later
The primer rule
- Every new piece of DNA starts from an RNA primer.
- Leading strand: one primer. Lagging strand: a primer for every Okazaki fragment.
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How it is asked: A Distinguish or Explain on the two strands, part of a longer replication question, or a 1A item on a drawn fork.
IB-style questionDistinguish[3 marks]
Distinguish between replication on the leading strand and on the lagging strand.
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