First, a quick review of DNA structure:

Two complementary strands form an antiparallel double helix. Each purine is hydrogen bonded to a pyrimidine.

5' --------------------------3'
3' --------------------------5'

Each strand can serve as a template for the synthesis of a 'daughter strand'. This process is termed replication [local]. After replication, each daughter strand is base paired to a complementary parent strand. This form of replication [local] is called 'semi-conservative'. This process needs to be coordinated with the cell cycle [local].

5' -------------------- 3'
3' ******************** 5'
5' ******************** 3'
3' -------------------- 5'

Helicase -- unwinds dsDNA one base at a time by hydrolyzing ATP. This unzipping causes tension preceeding the replication fork. The tension is relieved by DNA Gyrase, a topoisomerase, which nicks one of the strands, lets the tension spin out, and then rejoins the nicked ends.

DNA Helicase complexed with a DNA strand.

Single-Strand Binding Protein (SSB) -- binds to ssDNA to prevent reformation of the double helix.

Primase -- produces RNA on a DNA template in 5' -->3' direction. These short RNA molecules are necessary as primers for the DNA polymerase.

DNA Polymerase III -- polymerizes DNA on DNA template in 5'-->3' direction. Requires primer with 3'-OH. The nucleotide substrates for the polymerase reaction are the four dNTPs. The holoenzyme has two activities

5'-->3' polymerase

3'-->5' exonuclease. Corrects errors by backing up.

DNA Polymerase I -- this activity is used for repair of damaged DNA and to remove the RNA primer from nascent DNA molecules. It has the 5'--->3' polymerase activity and 3'--->5' exonuclease of DNA Polymerase III, but has 5'--->3' exonuclease activity (removes the short RNA sequence that primed the okazaki fragments).

DNA Polymerase I complexed with a DNA strand.

DNA Ligase -- joins adjacent 3'-OH and P-5' using energy from ATP or NAD⁺ to make a phosphodiester bond; creates covalently closed strand of DNA.

As the replisome moves along the DNA strand, both strands are replicated [local].

One strand (leading strand) is replicated continuously in the 5'-->3' direction. (Animations)

1. Helicase opens the dsDNA
2. SSB binds
3. Primase binds and produces a small RNA primer
4. DNA Polymerase III binds and extends the primer 5'-->3' using one parent strand as the template

The other strand (lagging strand) is replicated in small pieces. the discontinuous 3'-->5' synthesis of the lagging strand is catalyzed by DNA primase, DNA polymerase I, DNA ligase, and DNA polymerase III,

1. Primase produces small RNA fragments.
2. These primers are used by DNA Polymerase III to synthesize Okazaki fragments using the other parent strand as the template.
3. When DNA polymerase III meets the previous RNA primer, it stops and leaves.
4. DNA Polymerase I uses its 5'-->3' exonuclease activity to degrade the RNA primer and uses its 5'-->3' polymerase activity to extend the DNA strand. DNA polymerase I then leaves the lagging strand.
5. DNA ligase binds and closes the nick.