DNA Replication Process

DNA replication is a vital process that ensures genetic information is accurately passed on during cell division. This involves several key players, including enzymes like DNA helicase and DNA polymerase, which work together to create new DNA strands efficiently and accurately.

1. Semi-conservative replication model

   • Each new DNA molecule consists of one original strand and one newly synthesized strand.
   • This model was confirmed by the Meselson-Stahl experiment using isotopes of nitrogen.
   • Ensures genetic continuity and fidelity during cell division.

2. DNA helicase and unwinding of the double helix

   • DNA helicase is an enzyme that unwinds the double helix by breaking hydrogen bonds between base pairs.
   • This process creates two single-stranded DNA templates for replication.
   • Unwinding occurs at the replication fork, allowing access for other enzymes.

3. DNA polymerase and its role in nucleotide addition

   • DNA polymerase is the enzyme responsible for adding nucleotides to the growing DNA strand.
   • It can only add nucleotides to the 3' end of a pre-existing strand, requiring a primer.
   • It also has proofreading capabilities to correct errors during DNA synthesis.

4. Leading strand synthesis

   • The leading strand is synthesized continuously in the 5' to 3' direction towards the replication fork.
   • It requires a single RNA primer to initiate synthesis.
   • DNA polymerase adds nucleotides rapidly as the fork opens.

5. Lagging strand synthesis and Okazaki fragments

   • The lagging strand is synthesized discontinuously in short segments called Okazaki fragments.
   • Each fragment requires a new RNA primer for initiation.
   • DNA polymerase synthesizes these fragments in the 5' to 3' direction, away from the replication fork.

6. Primase and RNA primers

   • Primase is an enzyme that synthesizes short RNA primers needed for DNA polymerase to start adding nucleotides.
   • RNA primers provide a free 3' hydroxyl group for DNA polymerase to extend.
   • Primers are later removed and replaced with DNA nucleotides.

7. DNA ligase and joining DNA fragments

   • DNA ligase is the enzyme that joins Okazaki fragments on the lagging strand.
   • It forms phosphodiester bonds between adjacent nucleotides, sealing gaps.
   • This ensures the integrity of the newly synthesized DNA strand.

8. Origin of replication

   • The origin of replication is the specific location on the DNA where replication begins.
   • It contains specific sequences recognized by initiator proteins that recruit helicase and other enzymes.
   • Multiple origins of replication can exist on eukaryotic chromosomes to speed up the process.

9. Replication fork

   • The replication fork is the Y-shaped region where the DNA double helix is unwound.
   • It is the site of active DNA synthesis, with leading and lagging strands being synthesized simultaneously.
   • The fork moves as replication progresses, with helicase continuously unwinding the DNA.

10. Proofreading and error correction mechanisms

    • DNA polymerase has a proofreading function that checks for base-pair mismatches during synthesis.
    • If an error is detected, the enzyme can remove the incorrect nucleotide and replace it with the correct one.
    • Additional repair mechanisms, such as mismatch repair, further ensure the accuracy of DNA replication.