DNA replication is the process by which a cell makes an identical copy of its DNA prior to cell division. The process is essential for the accurate transmission of genetic information from parent cells to daughter cells.
The DNA replication process can be divided into three main stages: initiation, elongation, and termination.
Initiation: The replication process begins at specific sites on the DNA molecule called origins of replication. These sites are recognized by a protein complex called the origin recognition complex (ORC) in eukaryotic cells, or the DnaA protein in prokaryotic cells. The ORC or DnaA protein recruits other proteins, including helicases and single-stranded binding proteins, to the origin of replication. These proteins work together to unwind the double helix and separate the two strands of DNA, creating a replication bubble.
Elongation: Once the replication bubble is formed, two replication forks are created that move in opposite directions along the DNA molecule. At each fork, an enzyme called DNA polymerase binds to the template strand and synthesizes a new complementary strand of DNA by adding nucleotides to the 3′ end of the growing strand. DNA polymerase can only add nucleotides in the 5′ to 3′ direction, so the template strand is read in the 3′ to 5′ direction. To accommodate this, the new strand is synthesized in the 5′ to 3′ direction in short, discontinuous fragments known as Okazaki fragments on the lagging strand.
Primase, a type of RNA polymerase, adds short RNA primers to the lagging strand to initiate the synthesis of each Okazaki fragment. DNA polymerase then elongates the Okazaki fragments, removing the RNA primers and replacing them with DNA nucleotides. The newly synthesized DNA strands are antiparallel to the original template strands, with one strand being synthesized continuously (leading strand) and the other being synthesized discontinuously (lagging strand).
Termination: The DNA replication process continues until the replication forks meet at the opposite end of the DNA molecule or until they encounter another replication fork. At this point, the newly synthesized DNA strands are separated from the template strands and the replication process is complete.
Before the cell can divide, the newly synthesized DNA must be checked for errors and repaired by the cell’s DNA repair machinery to ensure that the genetic information is accurate. Once this is complete, the cell can proceed with cell division, ensuring that each daughter cell receives a complete and accurate copy of the genetic information.
