Increase in body size, height, and weight of every organism start from the body cells, and the whole activities of the cells are determined by the nucleic acid (Deoxyribonucleic acid) that is found in their nucleus and nucleoid of eukaryotic and prokaryotic cells respectively.
Human growth starts with cell division. This starts with the division of the DNA at the S-phase of meiosis and mitosis for gamete and soma cells.
The process of by which DNA divides is called replication.
Hypothesis on methods of DNA replication
- Semi-conservative method of replication
- Conservative method of replication
- Dispersive method of replication
Semi-conservative method of replication
This posits that the newly replicated strands of DNA strands contain both an old strand from the parent DNA and a newly synthesized DNA
Conservative method of replication
This hypothesis posits that during replication the old strands separate from the new strands. The old strands stay together while the new strands stand together.
Dispersive method of replication
It is assumed that in the strands of copies of DNA. Each strand is has a distinct region which is composed of either old strand or new strand only.
DNA replication process occurs in three stages:
- Initiation process
- Elongation process
- Termination process
Replication starts from a point on the DNA called origin (oriC) that is recognized by an enzyme called helicase which unwinds the double strands of DNA to form a Y-shaped structure called replication fork, followed by the binding of single-stranded binding (SSB) proteins to the unwinded strands of DNA. This help to keep the strands separate from each other.
The enzyme primase adds primer (a short single-stranded RNA strand) to the 5’ and 3’ ends of the DNA strand which is recognized by DNA polymerase III in prokaryotes that add nucleotides during complimentary strand production. The strand that starts at the 5’ end is called the leading strand while the strand that starts from the 3’ ends is called the lagging strand. However, DNA polymerase delta and epsilon are responsible for this activity in eukaryotes.
At the end of the synthesis of the new DNA strands, fragments of DNA strands known as Okazaki fragment is left on the lagging strands, thus leaving gaps between the fragments on the lagging strand. The gaps are filled with nucleotides and glued by an enzyme called DNA ligase.
This type of replication is known as semiconservative replication, where each strand contains a copy of old strand and new strand.
Just as proofreading is an essentially stage in every good writing, DNA polymerase also proof reads the strand to ensure the no error is found and correct errors where necessary. In prokaryotes proofreading can be performed by either DNA polymerase I, II or III while this activity is performed in prokaryotes by DNA polymerase delta and epsilon carry out this activity.
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