Biotechnology and Bioinformatics (1+1)

1.4.4. Leading strand and Lagging strand

The leading strand is the template strand of the DNA double helix so that the replication fork moves along it in the 3' to 5' direction. This allows the new strand

synthesized complementary to it to be synthesized 5' to 3' in the same direction as the movement of the replication fork. On the leading strand, a polymerase "reads" the DNA and adds nucleotides to it continuously. This polymerase is DNA polymerase III (DNA Pol III) in prokaryotes and presumably Pol ε in eukaryotes .

The lagging strand is the strand of the template DNA double helix that is oriented so that the replication fork moves along it in a 5' to 3' manner. Because of its orientation, opposite to the working orientation of DNA polymerase III, which moves on a template in a 3' to 5' manner, replication of the lagging strand is more complicated than that of the leading strand. On the lagging strand, primase "reads" the DNA and adds RNA to it in short, separated segments. In eukaryotes, primase is intrinsic to Pol α . DNA polymerase III or Pol δ lengthens the primed segments, forming Okazaki fragments .

Primer removal in eukaryotes is also performed by Pol δ. In prokaryotes, DNA polymerase I "reads" the fragments, removes the RNA using its flap endonuclease domain (RNA primer s are removed by 5'-3' exonuclease activity of polymerase I , and replaces the RNA nucleotide s with DNA nucleotides (this is necessary because RNA and DNA use slightly different kinds of nucleotides).

DNA ligase joins the fragments together.