DNA is unique among all known molecules because it is the only one that is capable of duplicating itself. DNA replication is involved in the stages of meiosis and mitosis. Replicating DNA is crucial stage for the formation of identical daughter cells during the mitosis. This process of DNA duplication is called DNA replication.
DNA replication is somewhat complex, but what basically happens is that the two complementary strands which form the DNA molecule unzip and then are used as templates from which new strands are made as free nucleotides combine with their complementary bases.
Replication of DNA is semi-conservative. That is, one side of each new DNA strand is “old” and the other side is “new”.
The process involves large numbers of free nucleotides. These molecules will pair with the bases on the template strands during the replication process. The process of replication begins in the DNA molecules at thousands of sites called origins of replication. At these sites, which look like little bubbles, the hydrogen bonds between the bases are broken and the paired bases separate. The helix begins to pull apart or unwind. The unwinding of the helix is facilitated by an enzyme called helicase. As unwinding continues, they move in opposite directions creating two Y-shaped replication forks. Replication proceeds in both directions until the bubbles meet.
DNA Polyermerase III is the primary enzyme responsible for replication. Its main function is to add a new nucleotide to exposed nucleotides. A limitation of DNA polymerase III is that it can only add new bases to the 3’ end of a growing strand. New DNA strands always grow from the 5′ to the 3′ end (of the daughter strand). Since the original DNA strand unzips, replication can occur in a 5′ to 3′ direction on one side and in a 3′ to 5′ direction on the other side. But DNA polymerase works in a 3′ to 5′ direction.
As replication moves from an origin the addition and joining of bases on one strand can synthesized continually in a 5′ to 3′ direction. This strand is referred to as the leading strand.
The other parent strand is copied discontinuously, creating a lagging strand. The lagging strand is formed in fragments, between 1000 and 2000 nucleotides long – called Okazaki fragments. Synthesis of lagging strand does not really begin until an enzyme called RNA polymerase adds a primer of a few RNA nucleotides in a 5′ to 3′ direction. The primer is later removed and replaced by DNA. When DNA polymerase III meets the RNA primer from a previous segment, DNA polymerase I comes along and digests the primer, fills the gap with DNA, and leaves. The Okazaki fragments are joined together by ligase.
When each of the two new double-helix DNA molecules has one strand of the original DNA and one strand that is newly synthesized. The two DNA molecules rewind into their ‘cork-screw’ double helix shape again. Each double-helix is then coiled around histone proteins and further wrapped up to form separate chromatids, which are joined by centromere. This coiled DNA is seen under the microscope during mitosis and meiosis (prophase). The two chromatids will become separated in the cell division process (mitosis, meiosis) to form separate chromosomes.
| Terms: Semi-conservative, Anti-parallel, Helicase, Leading strand, DNA Polymerase III(Leading strand), Lagging strand, Okazaki Fragment, RNA Polymerase, RNA Primer, DNA Polymerase III(Lagging strand), DNA Polymerase I, DNA Ligase |
