Why leading and lagging strand

Revoke Cancel. Flag Inappropriate The Content is. Flag Content Cancel. Delete Content. Delete Cancel. The helicase unzips the double-stranded DNA for replication, making a forked structure. This enzyme can work only in the 5' to 3' direction, so it replicates the leading strand continuously. Lagging-strand replication is discontinuous, with short Okazaki fragments being formed and later linked together. This image is linked to the following Scitable pages:.

The decoding of information in a cell's DNA into proteins begins with a complex interaction of nucleic acids. Learn how this step inside the nucleus leads to protein synthesis in the cytoplasm. Comments Close. The Comment you have entered exceeds the maximum length. Submit Cancel. Comments Please Post Your Comment.

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DNA replication occurs at the replication fork, which forms when DNA is unwound by a helicase into strands that are copied by two polymerases into a leading strand and a lagging strand. The leading strand is synthesized in a continuous manner, whereas synthesis of the lagging strand requires a primase that makes RNA primers that are extended by the DNA polymerase to form Okazaki fragments — short DNA fragments that are processed to produce a continuous DNA strand.

Despite this complexity, the lagging strand is synthesized at the same rate as the leading strand, but how is this achieved? Pandey et al. The authors investigated the kinetics of RNA primer synthesis, DNA unwinding and DNA synthesis by the bacteriophage T7 replication complex the T7 replisome , which consists of the T7 gp4 a protein that contains a helicase and a primase domain and the T7 DNA polymerase, using synthetic replication fork substrates.

Single molecular fluorescence resonance energy transfer FRET — energy transfer between two fluorophores, which in this case are in close proximity before the DNA strands separate — shows a FRET decrease as the fluorophores move apart upon DNA unwinding and DNA synthesis, by the same rate in both the presence and absence of a priming sequence that allows primer synthesis.

This suggests that the T7 replisome does not pause during lagging strand primer synthesis. Pausing is not necessary because the nascent lagging strand template loops out between the helicase and primase domains of T7 gp4 so that it keeps the RNA primers near to the T7 replisome. As the leading strand T7 replisome does not pause during primer synthesis, this cannot explain why the leading and lagging strands are synthesized at the same rate.

This allows time for the lagging strand to attach to a new primer and initiate synthesis of another Okazaki fragment, and still keep up with leading strand DNA synthesis. This study suggests that primer synthesis does not pause DNA synthesis, and that primers are made concomitant with DNA synthesis, kept close to the replication complex by the lagging strand priming loop and extended by the lagging strand polymerase faster than the T7 replisome can copy the leading strand.

Pandey, M. Coordinating DNA replication by means of priming loop and differential synthesis rate. Nature 18 Nov doi Download references.

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