Leading Strand & Lagging Strand
- Double-stranded DNA consists of two antiparallel strands (oriented in opposite directions)
- During DNA replication, the two strands are ‘unzipped’ and DNA polymerase moves along each template strand linking nucleotides together to form a new strand
- Crucially, DNA polymerase can only add new nucleotides in a 5’ to 3’ direction
- As the template strands are antiparallel, replication needs to proceed in opposite directions
- As the replication fork opens up in one direction only, each new strand is synthesised differently
- The leading strand is made continuously, following the fork as it opens
- The lagging strand is made discontinuously, in short fragments, away from the fork
- As more template strand is exposed, new fragments (called Okazaki fragments) are created
- Okazaki fragments are later joined together by DNA ligase to form a continuous complementary DNA strand
During DNA replication, synthesis of the leading strand is continuous but synthesis of the the lagging strand is discontinuous in small fragments (not all the enzymes involved are shown)
