3.A.1 DNA and RNA Part II:Replication

DNA, and in some cases RNA, is the primary source of heritable information. DNA, and in

some cases RNA, is the primary source of heritable information.

DNA replication ensures continuity of hereditary information.

Replication occurs during S phase of the cell cycle.

Replication is a semiconservative process; that is,

one strand serves as the template

for a new, complementary

strand.

Replication requires DNA polymerase III plus many other essential cellular enzymes.

The copying of DNA is remarkable in its speed and accuracy.

In DNA replication, the parent molecule unwinds and two new daughter strands

are built following base-pairing rules.

A DNA strand is directional; it has a 3’ end and a 5’ end.

DNA polymerase III adds nucleotides to the 3 end of a growing strand. It can

only work in the 3’ 5’ direction.

DNA is antiparallel, so the two new strands have to be made differently.

Along the leading strand, DNA polymerase can synthesize a complementary strand

continuously, moving toward the replication fork in the 3’ 5’ direction.

To elongate the other new strand of DNA, the lagging strand, DNA polymerase must work in the direction away from the replication fork. The lagging strand is discontinuous, resulting in fragments: each with its own RNA primer.

The lagging strand is synthesized as a series of segments called Okazaki fragments, which

are then joined together by DNA ligase. The primers are then removed and replaced with

DNA.

DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides. In mismatch

repair of DNA, repair enzymes correct errors in base pairing.