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Advanced DNA Replication

This animation shows DNA replication in more detail than that seen in the introductory section of this web page. As you recall, DNA replication occurs during the S phase of the cell cycle and is the process by which a DNA helix gets copied, generating two new daughter strands.

Animal DNA replication begins when a section of the double helix, called the origin of replication, unwinds and the two strands separate, breaking the hydrogen bonds that normally hold the complimentary purine and pyrimidine bases together. Helicase and single-stranded binding proteins help in this process. As the DNA unwinds, its structure resembles a Y-shaped fork in the road, with the unwound strands forming the arms of the Y and the helical portion of the molecule forming the base. Thus, the portion of the DNA helix that is being actively replicated is called the replication fork.

In the next step, a protein complex called a primeosome synthesizes an RNA primer (a short, single strand of RNA that is complimentary to the DNA bases) that binds to the parent DNA strand and serves as the starting point for DNA synthesis. DNA polymerase, the enzyme that replicates DNA, cannot start without this primer, which has a required hydroxyl (OH) group on its 3' end.

DNA polymerase then uses the original parent strand as a template for daughter strand DNA synthesis. Basically, it brings in a base that is complimentary to the one on the parent strand and forms a phosphodiester bond between the 5' phosphate on the incoming base and the 3' hyroxyl on the previous one. Since each new base further extends the 3' end of the daughter strand, DNA replication is said to occur in a 5' to 3' direction.

As you can see, the replication fork moves in one direction while DNA replication only goes in the 5' to 3' direction. This means that one daughter strand can be made continuously while the other must be made in pieces as the replication fork opens and reveals more template DNA. The strand made continuously is known as the leading strand. The strand made in pieces, discontinuously, is known as the lagging strand. Each of the pieces made on the lagging strand is called an Okazaki fragment and has its own RNA primer.

Once DNA replication has been completed in a section, DNA polymerase removes the RNA primers from the DNA strand.

A second DNA polymerase next adds new nucleotides to replace those that were removed.

Finally, DNA ligase finishes up the process of replication by forming phosphodiester bonds between adjacent pieces of DNA.