DNA replication

DNA replication is the process by which a cell duplicates its DNA before cell division, ensuring that each daughter cell receives a complete copy of the genetic information. It is a highly regulated and precise process involving multiple steps and enzymatic activities. Here's an overview of the mechanism of DNA replication in detail:

1. Initiation:

   • DNA replication begins at specific sites along the DNA molecule called origins of replication. In eukaryotic cells, multiple origins exist along each chromosome, whereas bacterial cells typically have a single origin.
   • Initiation of replication requires the assembly of a protein complex known as the pre-replication complex (pre-RC) at the origin. This complex includes initiator proteins, origin recognition complex (ORC) proteins, and other regulatory factors.
   • Once assembled, the pre-RC unwinds the DNA double helix and recruits additional proteins necessary for DNA synthesis.

2. Unwinding and Formation of Replication Forks:

   • Helicase enzymes bind to the unwound DNA strands and catalyze the unwinding of the double helix, separating the two strands to form a replication bubble.
   • As the DNA strands unwind, topoisomerase enzymes relieve the tension ahead of the replication fork by introducing transient breaks in the DNA strands and resealing them.

3. Priming and Synthesis of RNA Primers:

   • DNA polymerases require a free 3'-OH group to initiate DNA synthesis. However, they cannot initiate synthesis de novo and require a short RNA primer to start synthesis.
   • Primase enzymes synthesize RNA primers complementary to the single-stranded DNA template. These primers provide the starting point for DNA polymerases to begin synthesizing new DNA strands.

4. DNA Synthesis:

   • DNA polymerases catalyze the addition of deoxyribonucleotides (dNTPs) to the growing DNA strand in a 5' to 3' direction, using the RNA primers as starting points.
   • DNA polymerases extend the leading strand continuously in the direction of the replication fork, synthesizing a single, continuous DNA strand.
   • On the lagging strand, DNA polymerases synthesize short DNA fragments called Okazaki fragments in the opposite direction of the replication fork. These fragments are later joined together by DNA ligase.

5. Proofreading and Error Correction:

   • DNA polymerases possess intrinsic 3' to 5' exonuclease activity, allowing them to proofread newly synthesized DNA strands for errors.
   • If an incorrect nucleotide is added, the polymerase removes it through exonuclease activity and replaces it with the correct nucleotide before continuing DNA synthesis.

6. Termination:

   • DNA replication proceeds bidirectionally from each origin until the replication forks meet and the entire chromosome is replicated.
   • Termination of replication involves the completion of DNA synthesis, processing of any remaining RNA primers, and ligation of the Okazaki fragments on the lagging strand.
   • Once replication is complete, the two daughter DNA molecules separate from each other, and the replication machinery dissociates from the DNA.

Overall, DNA replication is a highly coordinated and accurate process that ensures faithful transmission of genetic information from one generation to the next. It involves the concerted action of numerous enzymes and protein factors, each performing specific functions to achieve accurate and efficient DNA synthesis.