The three most prerequisites for any living entity is Feeding, Locomotion and reproduction and so does the living cell. Each living cell needs foods. To get in search of food, it needs to locomote. To produce its next generation or offspring it needs energy. So it needs food.
The living cell divides and produces its offspring by mitotic division. During the division, the cytoplasm along with the nuclear content will be divided equally between the two daughter cells. The DNA content will be distributed equally between the daughter cells.
DNA replication takes place in both prokaryotes and eukaryotes. Though the entire mechanism of replication is same in both prokaryotes and eukaryotes, the machinery involving in the mechanism is different in prokaryotes and eukaryotes.
During the cell division, the DNA double helix uncoils and the two strands separate. Each such strand is used as a template for synthesizing new DNA molecules.
The entire cell division was divided into karyokinesis and Cytokinesis. During karyokinesis nuclear division takes place and during cytokinesis cytoplasm divides. And moreover the genetic information has to be passed on to the next generation with great precise. This process is called DNA replication.
DNA REPLICATION MECHANISM
What are the three steps of replication?
Mechanism of replication can be staged into three phases:
Initiation: The DNA strand is made up of two strands Leading and lagging. Initiation of replication begins on the Leading strand at the ori site. Ori site or origin of replication is a particular site on DNA strand with specific sequence that is responsible for initiation. This leads to formation of Preprimosome. There are about six proteins named as dnaA, dnaB, dnaC, gyrase, SSBP and H4 involved in this process. The dnaA protein binding occurs on the both sides of ori C region. DnaB and dnaC help in the primosome complex formation where, dnaB helps in recognizing single strand after the activity of helicase and also helpful in activating primase enzymes (dnaC—this is the gene that codes for enzymes Primase). The enzyme Gyrase allows the strand to rotate around the other whereas the SSBP (Single strand Binding protein) prevent the re-annealing of separated DNA strands.
Once initiation is done, elongation or extension of strand takes place.
Elongation: The main event occurring in the process of elongation is the formation of a replication fork which proceeds in the opposite direction from oriC region. This type of replication where two strands are separated and moves in opposite directions as such it is known as a Bi-directional Replication.
Replication fork is formed and it is defined as the point where, a pair of replicated segments come closer and joins with the non-replicated segments. The major enzymes taking part in this stage is DNA polymerase, primase, and ligase.
What is the role of DNA polymerase in DNA replication ?
RNA primer comes and joins at the region of initiation complex and the enzyme polymerase helps in the addition of nucleotides to the 3’-OH group of RNA primer. The daughter strand is synthesized in 5’-3’ direction which is anti-parallel to the parental strand. During the process of elongation DNA polymerase also helps in removing the mismatch or incorrect base pair. This activity of DNA polymerase is known as exonuclease activity. This process is important for the incorporation of correct, complementary base pair on the template strands.
On the leading strand since the extension takes place from 3’ to 5’, a continuous strand is formed, whereas on the lagging strand the extension takes place in the opposite direction. As such distinct fragments are formed which are called Okazaki fragments. Once the elongation is over, the strands need to be terminated.
Termination: This is the final stage of replication process. In this stage the leading + lagging strands moving in opposite direction are joined together following a path of anti-parallel direction. This process is being guided by a proper termination signal like Ter signals.
To conclude, DNA replication is one of the unique properties to make exact copies of itself which contributes to the cell growth and multiplication. If the replication is beyond the control of the cell, then it eventually leads to a tumour.
Figure 1: Image showing the mechanism of replication.
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