When helicase unwinds the parental DNA, there are two template strands; one that is 3' to 5' and the other from 5' to 3'. Now, DNA polymerase makes a complementary copy in the direction of unwinding or in other words, 5' to 3'. The top strand or the 3' to 5' template strand is where the DNA is synthesized continuously in the direction of unwinding. However, the other strand, the 5' to 3' template strand would mean the DNA polymerase has to add nucleotides to the opposite direction of unwinding. Instead of running in the opposite direction, it makes short lengths that are synthesized in the direction opposite of unwinding. Short lengths created by discontinuous replication is then joint together covalently into a single continuous polynucleotide chain. These short lengths are called OKAZAKI FRAGMENTS.
DNA SYNTHESIS: SUMMARY
DNA strand synthesized continuously in the direction of DNA unwinding is referred to as the leading strand. Template for that strand is the leading strand template which runs from 3' to 5'.
A strand synthesized discontinuously and in the opposite direction of DNA unwinding is called the lagging strand. Template strand for lagging strand is the lagging strand template which runs from 5' to 3'.
LAGGING STRAND: THERE'S MORE TO IT!
For the lagging strand, instead of having one RNA primer at the beginning of the strand to initiate DNA polymerase III, it consists of multiple RNA primers to allow for DNA polymerase III to add short lengths of DNA nucleotides called Okazaki fragments. For the lagging strand, DNA polymerase I removes the RNA primers at the 5' end of the previously newly synthesized Okazaki fragment, replacing the RNA nucleotides one by one with DNA nucleotides. RNA removal uses the 5' to 3'. DNA polymerase I stops replacing RNA and leaves the template when it encounters the first DNA nucleotide synthesized in the Okazaki fragments.
Another enzyme called DNA LIGASE joins the DNA nucleotides together.
SUMMARY:
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