In: Biology
In DNA replication, what is the difference between replicating the leading versus the lagging DNA strands?
DNA replication is the process by which a double-stranded DNA molecule is copied to produce two identical DNA molecules. Replication is an essential process because, whenever a cell divides, the two new daughter cells must contain the same genetic information, or DNA, as the parent cell.
Leading and lagging strand :-
The DNA polymerase that handles most of the synthesis is DNA
polymerase III. There are two molecules of DNA polymerase III at a
replication fork, each of them hard at work on one of the two new
DNA strands.
DNA polymerases can only make DNA in the 5' to 3' direction, and
this poses a problem during replication. A DNA double helix is
always anti-parallel; in other words, one strand runs in the 5' to
3' direction, while the other runs in the 3' to 5' direction. This
makes it necessary for the two new strands, which are also
antiparallel to their templates, to be made in slightly different
ways.
One new strand, which runs 5' to 3' towards the replication fork,
is the easy one. This strand is made continuously, because the DNA
polymerase is moving in the same direction as the replication fork.
This continuously synthesized strand is called the leading
strand.
The other new strand, which runs 5' to 3' away from the fork, is
trickier. This strand is made in fragments because, as the fork
moves forward, the DNA polymerase (which is moving away from the
fork) must come off and reattach on the newly exposed DNA. This
tricky strand, which is made in fragments, is called the lagging
strand.
The small fragments are called Okazaki fragments, named for the
Japanese scientist who discovered them. The leading strand can be
extended from one primer alone, whereas the lagging strand needs a
new primer for each of the short Okazaki fragments.