DNA polymerase I is the only polymerase that has 5'  3' exonuclease activity. This means that it can remove nucleotides in the 5'  3' direction. It also has 3'  5' exonuclease activity, as does DNA polymerase III; this is like a "backspace" for nucleotides that have just been added and need to be removed. DNA polymerase II's functions are largely unknown, DNA polymerase V plays a complex role in DNA repair, not replication.

Replication of DNA is both continuous and discontinuous, each form of replication occurring simultaneously. Continuous DNA synthesis occurs in the 3’  5’ direction on the parent strand. This is often referred to as the leading strand with new nucleotides being added to the 3’ end. Discontinuous DNA synthesis occurs in the 5’  3’ direction on the parent strand. This strand is often referred to as the lagging strand. It is completed in short sequences of nucleotides called Okazaki fragments. Replication on the lagging strand begins with the addition of an RNA primer by the enzyme primase. Primase adds the RNA primers ahead of the 5’ end of the lagging. This allows DNA polymerase III to add the Okazaki fragments to fill in the space between primers. This process repeats itself until the entire strand has been replicated. DNA polymerase I then comes to exchange the RNA primer with DNA nucleotides, then DNA ligase reinforces the bonding between the fragments and the DNA nucleotides that replaced the RNA primer. Note that in both leading and lagging strand synthesis, nucleotides are added to the 3' end of the growing chain, thus synthesis always occurs in the 5'  3' direction of the growing strand.

Replication of DNA is both continuous and discontinuous, each form of replication occurring simultaneously. Continuous DNA synthesis occurs from the 3’ end to the 5’ end of the parent strand. This is often referred to as the leading strand with new nucleotides being added to the 3’ end. Discontinuous DNA synthesis occurs from the 5’ end to the 3’ end of the parent strand. This strand is often referred to as the lagging strand. It is completed in short sequences of nucleotides called Okazaki fragments. Replication on the lagging strand begins with the addition of an RNA primer by the enzyme primase. Primase adds the RNA primers ahead of the 5’ end of the lagging. This allows DNA polymerase III to add the Okazaki fragments to fill in the space between primers. This process repeats itself until the entire strand has been replicated. DNA polymerase I then comes to exchange the RNA primer with DNA nucleotides, then DNA ligase reinforces the bonding between the fragments and the DNA nucleotides that replaced the RNA primer. Once both the leading and lagging stranded have completed replication, the result is two identical strands of the original DNA molecule.

The enzyme helicase unzips the two strands of the double helix. Once unzipped, single stranded binding (SSB) proteins stabilize the newly single strands. The enzyme DNA gyrase ensure the double stranded areas beyond the replication fork do not supercoil onto one another. After stabilization of the replication fork, an enzyme complex known as DNA polymerase III commences the addition of nucleotides to the new strand. Proteins such as the beta clamp and clamp loader assist in keeping DNA polymerase III in its place on the strand of DNA, The enzyme primase adds sequences of RNA primers to the DNA strand to begin replication. DNA Polymerase III cannot begin replication without this primer. DNA ligase reinforces the bonding between the Okazaki fragments and the DNA nucleotides that replace the RNA primer.

DNA synthesis always occurs in the 5' to 3' direction, so one new strand is synthesized continuously towards the replication fork, producing the leading strand. The other strand, known as the lagging strand, forms away from the replication fork in small fragments.

DNA replication occurs both continuously and discontinuously at the same time. Nucleotides can only be added to a new strand of DNA on the 3' end, so the process has to start with the 5' end. As DNA continues to be split apart, the leading strand (growing in the direction towards the replication fork) can continuously add new nucleotides. However, for the lagging strand, the 5' to 3' direction is away from the replication fork, so new nucleotides are added in small chunks called Okazaki fragments as the DNA strand continues to separate.

The enzyme helicase opens the double helix of DNA at points called replication forks.

The unwinding of the double helix of DNA is caused by an enzyme called helicase, which breaks the hydrogen bonds holding the complementary base pairs together, creating two template strands of DNA ready to begin the next step of replication. The place where this enzyme 'unzips' the DNA is called the replication fork.