Before it was known how DNA was replicated there were two basic models for
DNA replication: conservative and semiconservative replications. In conservative
replication, the two strands of DNA do not unwind as a brand new copy is
produced and the old molecule remains intact. In semiconservative replication, however, the DNA strands unwind and each strand
serves as a template for the replication. So both of the new copies of DNA half
of the old DNA and half of newly synthesized DNA. It was later found in 1957 by
Meselson and Stahl that DNA followed the semiconservative model.
[Diagram illustraing the processes involved
in DNA replication.]
Many enzymes are necessary to successfully replicate DNA. First topoisomerase
is used to unwind the DNA strand. By cutting one strand the tension in the
supercoil is released. Next helicase is used to prevent the strand from
recoiling. DNA polymerase III then travels across the strand, from the five
carbon (5', or the fifth carbon on the phosphate group) to the third carbon (3',
or the third carbon on the phosphate group). DNA polymerase III places the
complementary nucleotide on the strand. So adenine would be put opposite of
thymine (and the reverse), and guanine would be opposite of cytosine (and the
reverse). DNA polymerase also needs a primer to begin attaching the nucleotides
on the strand. Primase acts as the primer for DNA polymerase. Primase attaches a
small RNA primer so that DNA polymerase can begin working. The RNA primer is
removed by RNase H and DNA polymerase I fills the spot. Ligase is then used to
reattach the strands with their new nucleotides.
Throughout the entire process single-strand binding proteins are used to keep
the DNA stable so that DNA polymerase can work.
It is important to realize that DNA polymerase can only work in the 5' to 3'
directions. The reason behind this is because 3' is more stable than 5' when
attaching a new nucleotides. If DNA polymerase ran in the other direction then
there is the risk that the phosphate group could break off.
So when DNA polymerase is working it only goes in the 5' to 3' direction.
However there are two sides, and one runs in the 5' to 3' direction, but the
other runs from 3' to 5'. So while DNA polymerase can work continuously in the
5' to 3' direction it has to work in spurts on the 3' to 5' direction, which are
called Okazaki fragments. The 5' to 3' is called the leading strand while the
other is called the lagging strand.