DNA replication is the process by which a cell makes an identical copy of its DNA, ensuring that genetic information is faithfully passed on to daughter cells during cell division. The process involves several steps and enzymes and occurs in the nucleus of eukaryotic cells and the cytoplasm of prokaryotic cells. Here's an overview of the process of DNA replication and how the fidelity of replication is maintained:
1. Initiation:
DNA replication begins at specific sites called origins of replication, where the double-stranded DNA molecule is unwound and separated into two single strands by an enzyme called helicase.
Helicase breaks the hydrogen bonds between complementary nucleotide bases, creating a replication fork—a Y-shaped structure with two single-stranded DNA templates.
2. Primer Synthesis:
Once the DNA strands are unwound, another enzyme called primase synthesizes short RNA primers on each template strand. These primers provide a starting point for DNA synthesis by DNA polymerase.
3. Elongation:
DNA polymerase enzymes, such as DNA polymerase III in prokaryotes and DNA polymerase δ and ε in eukaryotes, catalyze the addition of complementary nucleotides to the growing DNA strands.
DNA polymerase can only add nucleotides to the 3' end of a growing DNA strand, using the template strand as a guide for base pairing.
The leading strand is synthesized continuously in the 5' to 3' direction toward the replication fork, while the lagging strand is synthesized discontinuously in short segments called Okazaki fragments, also in the 5' to 3' direction but away from the replication fork.
4. Termination:
DNA replication proceeds bidirectionally from each origin of replication until the entire DNA molecule is replicated.
When the replication forks from adjacent origins meet, the replication process is terminated, and the newly synthesized DNA strands are ligated together by DNA ligase, forming two complete double-stranded DNA molecules.
Maintenance of DNA Replication Fidelity:
1. Proofreading Activity of DNA Polymerase:
DNA polymerase possesses intrinsic proofreading activity, which allows it to recognize and correct errors in nucleotide base pairing.
If an incorrect nucleotide is added during DNA synthesis, DNA polymerase can remove the mismatched nucleotide and replace it with the correct one, thereby reducing the error rate of DNA replication.
2. Mismatch Repair Mechanisms:
Cells have specialized mismatch repair mechanisms that identify and correct errors that escape the proofreading activity of DNA polymerase.
These repair systems recognize distortions in the DNA helix caused by mismatched base pairs and excise the incorrect nucleotide and replace it with the correct one.
3. DNA Damage Response Pathways:
Cells have mechanisms to detect and repair various types of DNA damage, including chemical modifications, UV-induced lesions, and strand breaks, which can compromise the fidelity of DNA replication.
These DNA damage response pathways activate repair enzymes that correct the damage and restore the integrity of the DNA molecule.
In summary, DNA replication is a highly accurate process due to the combined actions of DNA polymerase proofreading activity, mismatch repair mechanisms, and DNA damage response pathways. These mechanisms work together to ensure the fidelity of DNA replication and maintain the integrity of the genetic information passed on to future generations.
.png)
.png)
Social Plugin