for - DNA replication accuracy - 1 in 10,000 - too high for successful replication - another higher level mechanism to correct for these errors - need a whole body for that - Denis Noble

Can broadly split the process into three parts. Initiation, elongation and termination

Primase and DNA polymerase A work in close association.

This is the S-phase CDK.

• Replication origin licensing.
• All of this occurs in G1 phase

Orc1-5 complex.

Cdc6 is only present in G1 phase.

DNA replication pathway in humans. Orc complex binds the origin of replication.

The initial sample consisted of 1 band. F = 0. 1st generation = The sample overall less dense, still one band. Intermediate density. dsDNA made of one strand heavy and one light. After 2 generations, there was a band for intermediate density and for strands of just light, N-14, dsDNA.

Three methods of replication were initially hypothesized: * Conservative * Semi-conservative * Dispersive

Semi-conservative method was eventually determined to be correct based on empirical evidence from Messelson & Stahl, in 1958.

Parental strands consist of N-15 isotopes. Replicated daughter strands consist of N-14 isotopes. The CsCl ultracentrifugation process creates density gradient. This allows DNA fragments of different densities to migrate and form a band at the point at which their buoyant density equals that of the salt.

1. E.coli grown in an heavy nitrogen, N-15, enriched medium
2. Then transferred to N-14 based media to reproduce
3. As several points in the experiment, cells were lysed and underwent ultracentrifugation through a CsCl concentration gradient