Replication

In replication, the DNA double helix is replicated into two copies. Replication is semiconservative and results in two double helixes consisting of a new stand and a parental strand. DNA replication begins with the replication fork. In the initial step of replication, DNA helicase, an enzyme, exposes a segment of the DNA strand by using ATP to unwind the DNA helix. This point is known as the origin of replication. To prevent reforming of the helix, the exposed strand is stabilized by single strand binding proteins. In addition, to prevent positive supercoils caused by unwinding DNA, DNA gyrase counteracts this effect by inserting negative supercoils. Of note, in bacteria, the origin of replication is known as the oriC and is a specific DNA sequence of about 250 bases recognized by DnaA. Two helicases are loaded in opposite directions and pairs of primase and DNA polymerase are loaded onto the DNA behind the helicases.

In DNA replication, replication always proceeds from the 5’ end to the 3’ end. This is due to the need of a hydroxyl group end only found on the 3’ end to continue building the nucleotide chain. Due to this fact, replication consists of a leading strand and a lagging strand. The leading strand continues as expected with no interruptions from the 5’ to 3’ direction. However, the lagging strand creates Ozaki fragments in a discontinuous process. These fragments are later joined together by DNA ligase by excising the RNA primer that was used as a starting guide for the Ozaki fragments.

In prokaryotes, chromosomes are found to be circular in nature and allows for bidirectional replication. This allows the organism to quickly replicate genetic material and overcome the potential limiting factor for growth.