DNA replication

DNA replication is the biological process by which a cell makes an identical copy of its DNA before cell division, ensuring the accurate transmission of genetic information from one generation to the next.

Overview of the process

DNA replication occurs during the S phase of the cell cycle in eukaryotic organisms and follows a semi-conservative mechanism, where each of the two original DNA strands serves as a template for the synthesis of a new complementary strand. This process results in two identical DNA molecules, each consisting of one original strand and one newly synthesized strand. The replication process is highly accurate, with error rates as low as one mistake per billion nucleotides, thanks to multiple proofreading and repair mechanisms.

Molecular machinery and key enzymes

The replication process involves numerous enzymes and proteins working in coordination. DNA helicase unwinds the double helix by breaking hydrogen bonds between base pairs, creating a replication fork. Single-strand binding proteins stabilize the separated strands, while topoisomerase relieves torsional stress ahead of the fork by cutting and rejoining DNA strands. DNA primase synthesizes short RNA primers that provide starting points for DNA synthesis. DNA polymerase III (in prokaryotes) or DNA polymerase δ and ε (in eukaryotes) then add nucleotides to the growing DNA chain in the 5' to 3' direction, using the existing strands as templates.

Mechanism of strand synthesis

DNA replication proceeds bidirectionally from specific origins of replication. The leading strand is synthesized continuously in the 5' to 3' direction toward the replication fork. The lagging strand is synthesized discontinuously away from the replication fork as short fragments called Okazaki fragments. Each Okazaki fragment requires its own RNA primer. After synthesis, DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides. DNA ligase then seals the gaps between Okazaki fragments, creating a continuous DNA strand.

Fidelity and proofreading

The accuracy of DNA replication is maintained through several mechanisms. DNA polymerases possess 3' to 5' exonuclease activity that proofreads newly synthesized strands, removing mismatched nucleotides. Mismatch repair systems identify and correct errors that escape proofreading. Additional repair pathways, including nucleotide excision repair and base excision repair, address various types of DNA damage that may occur during or after replication.

Termination and completion

In prokaryotes with circular chromosomes, replication terminates when two replication forks meet at specific termination sites. In eukaryotes with linear chromosomes, replication continues until forks from adjacent origins meet. The ends of eukaryotic chromosomes, called telomeres, present a special challenge as DNA polymerase cannot fully replicate them. The enzyme telomerase adds repetitive nucleotide sequences to telomeres, preventing the gradual shortening of chromosomes with each replication cycle.