" The Synthesis or Formation of DNA from the template DNA is called Replication.
Timing Of Replication:
The replication of DNA takes place during the resting phase of the Cell particularly S-Phase of resting period.
(NOTE) What is a Replica??
The sign or mark of a template is called replica.
Dr. Griffith's Work:
DNA can transform the other DNA molecule, was proved by the crucial work of Dr. Griffith.
Amount Of DNA:
The amount of DNA in the life of cell varies from time to time. But it does not mean that the change in DNA is permanent. Because just the moment mitosis or dividing phase starts, the amount of DNA is equalized in the Daughter Cell.
DNA duplicates In Prokaryotes:
To understand the duplication in higher organisms ( higher Cells) it is congent to study this process in prokaryotes such as bacteria.
Taking E.coli as a representative of bacteria which contain DNA molecule having the length of about 1.1mm. The chromosomes of E.coli is made up of single molecule DNA, interestingly without associated histones. ( histones are absent in E.coli )
As mentioned above DNA replicates only once in the course of cell cycle and the first step or sign of DNA synthesis is decoiling of double helix of DNA. The mechanism of synthesis of new DNA strand is set into action by formation of polynucleotides chain along the parent or template molecule. All this is carried out by a large complex enzyme called DNA polymerase. It is not a single enzyme rather it is a complex molecule containing all the enzymes required for different steps of DNA Duplication.
As far as the replication of DNA of E.coli is concerned it shows ( as can be visualized ) an advancing fork forming Y with the parent strand. In the chromosome of E.coli there aretwo replication forks, whereas in eukaryotic cells there may be several thousand forks.
In Bacteria, for example, E.coli the process of duplication may take place in 20 to 30 minutes. therefore it can be concluded that the helix in the molecule should unwind at the extraordinary rate of ten thousand revolutions per minute.
Here is a short summary of process of replication from helix unwinding to formation of new double helix i.e. from parent DNA to daughter DNA.
Step 1:
The point at which DNA replication starts is Known as O-Point. the point at which DNA replication ends is called T-Point. In this account the O-Point and T-Point would be taken as the references. In the first step enzyme known as Helicase unwind short segment of DNA just ahead of the replicating fork. Energy is provided by ATP which hydrolyze to ATP and phosphate.
Step 2:
As the unwinding proceeds the double helical region ahead of replicating fork is prevented from twisting by topoisomerase enzyme called DNA gyrase.
Step 3:
Once the helical has opened up into protien called single strand binding (SSB) protien readily associated or bind tightly to each of the separate strand, preventing them from incoming into base pairing again. SSB-protein is long enough to cover 8-nucleotides of DNA
Step 4:
In this step short strand of RNA synthesized by an enzyme called Primase and the RNA strand called Primer.
Step 5:
In this step the primer is then elongated by DNA polymerase III. Nucleotides are added to primer strand in accordance with base sequence of the template strand. A polynucleotide chain is synthesized in 5' to 3' end. As the DNA molecules are double stranded and anti-parallel. Since the replication DNA occurs on both strands simultaneously at replication fork. So, on one strand with 3' to 5' end only one primer is formed while on the other strand with 5' to 3' end DNA is synthesized discontinuosly in the form of short pieces called Okazaki pieces.
Step 6:
RNA pieces are removed by the action of exonuclease activity of DNA polymerase I . The filling of Okazaki pieces or fragments takes place by the action of DNA ligase.
Helix is formed after this joining of DNA molecule it may either be dispersive, conservative or semi-conservative type of replication depending upon the need and species of organism or even cell.
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