تكرار الحمض النووي DNA

DNA replication is semiconservative because each newly formed double helix contains one original template strand and one newly synthesized daughter strand. After replication, the two resulting DNA molecules each preserve one parental strand from the original double helix, ensuring genetic continuity while incorporating new genetic material.

Helicase unwinds the DNA double helix by breaking hydrogen bonds between the two strands. This separation creates a Y-shaped structure called the replication fork, which exposes the template strands so that other enzymes can access and copy them during DNA synthesis.

RNA primers are essential because DNA polymerase can only add nucleotides to an existing strand; it cannot initiate synthesis de novo. Primase adds short RNA fragments to each template strand, providing the 3'-OH group that DNA polymerase requires to begin extending the new DNA strand.

DNA polymerase synthesizes only in the 5' to 3' direction. The leading strand is synthesized continuously toward the replication fork. The lagging strand grows away from the fork, requiring DNA polymerase to repeatedly restart synthesis, producing short DNA fragments called Okazaki fragments separated by RNA primers.

Nucleotide triphosphates react with the 3'-OH group at the end of the growing strand. This reaction releases pyrophosphate, and the energy from breaking phosphate bonds drives the formation of a phosphodiester bond between nucleotides, linking them together in the DNA backbone.

After RNase H or DNA polymerase variants remove RNA primers and fill the gaps with DNA nucleotides, the enzyme DNA ligase seals the remaining nicks. DNA ligase catalyzes the formation of phosphodiester bonds between the 3'-OH end of one DNA fragment and the 5' phosphate end of the adjacent fragment.

Eukaryotic genomes are larger and more complex, with multiple origins of replication per chromosome, whereas prokaryotes have single origins. Eukaryotic replication occurs at approximately 100 nucleotides per second—ten times slower than prokaryotic replication. Despite these differences, the essential enzymatic steps and base pairing and its significance in DNA replication remain fundamentally similar.