Replication

Replication is a complex process in which the DNA of a chromosome is exactly copied prior to cell division. The ("parent" or template) strands of DNA in the double helix are separated (denatured) and each one is copied to produce a new ("daughter") strand. This process is said to be semi-conservative since one of each parent strand is conserved and remains intact after replication has taken place.

The origin of replication is the region of a DNA molecule at which replication is initiated.

• Helicase – enzyme that unwinds the DNA helix.
• DNA polymerase – enzyme that synthesizes DNA using one strand as a template to generate the complementary strand.
• DNA gyrase – enzyme that helps relieve the tension in DNA caused by the unwinding of the two strands of the DNA helix.

DNA can only be synthesized in the 5’ to 3’ direction, thus the DNA template can only be read in the 3’ to 5’ direction.

• The leading strand is synthesized continuously.
• The lagging strand is synthesized in discontinuous fragments.

Replication: the Lagging Strand

The lagging strand of DNA is synthesized discontinuously during replication because DNA synthesis can proceed only in the 5' to 3' direction.

• Primase – enzyme that synthesizes small fragments of RNA to serve as primers for DNA polymerase during DNA synthesis.
• DNA ligase – enzyme that joins the short fragments of DNA (Okazaki fragments) in the lagging strand to create a continuous complementary strand of the DNA template strand (oriented in the 5’ to 3’ direction from the origin of direction).

1. Primase initiates DNA polymerase. DNA polymerase synthesizes a complimentary strand from 5’ to 3’, until it meets the OH end of a synthesized segment of DNA.
2. Primase re-initiates DNA polymerase, which skips forward to the next segment of DNA (the enzyme reads the segment from 3’ to 5’ and synthesizes a complementary strand, adding nucleotides in the 5’ to 3’ direction). Again, the DNA polymerase will stop when it reaches the OH end of the previously synthesized fragment of DNA.
3. Primase will re-initiate DNA polymerase. And the process continues.