Summary
Highlights
DNA replication is the process of creating a duplicate copy of genetic material before cell division. While there are slight differences between bacteria and eukaryotes, this animation focuses on bacterial replication for simplicity. The process is divided into three phases: initiation, elongation, and termination.
To begin, the double-helical DNA must open at the origin of replication, regulated by initiation regulatory proteins. A DNA helicase ring is loaded onto one strand at each side of the replication bubble, creating two replication forks. DNA polymerases, responsible for DNA synthesis, cannot start from scratch; they need a primer. An enzyme called primase synthesizes a small RNA primer, which DNA polymerase will later elongate.
The sliding clamp, a ring-shaped structure, is loaded onto the three-prime end of the RNA primer by a clamp loader complex. ATP binding opens the clamp, allowing it to encircle the template-primer junction. ATP hydrolysis and dissociation of the clamp loader then allow the replicative DNA polymerase to bind, positioning the three-prime end of the primer in its active site for nucleotide addition.
During elongation, the replication bubble expands as helicases move, creating two replication forks. DNA polymerases synthesize DNA only in the five-prime to three-prime direction. The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously in short segments called Okazaki fragments. Priming and elongation occur repeatedly on the lagging strand.
Okazaki fragments are stitched together. When a lagging strand polymerase encounters a previously made fragment, DNA polymerase 3 is replaced by DNA polymerase 1, which degrades the RNA primer and synthesizes DNA. The DNA sections are then joined by DNA ligase. In reality, leading and lagging polymerases are coupled and move with the helicase, requiring the lagging strand to loop around for synchronous synthesis.
Less is known about termination, but once the entire chromosome is copied, the two replication forks meet and are dismantled. The ends of the new DNA strands are joined by DNA ligase. Each new DNA molecule consists of one parental strand and one newly synthesized strand, making DNA replication a semiconservative process.