Question

1. In what ways can a primer for DNA synthesis by a DNA polymerase be provided?

2. In E.coli, how are the Okazaki fragments of the lagging strands joined to create a complete DNA duplex?

3. What are the three eukaryotic nuclear DNA replicases and their functions? How is DNA replication of a circular bacterial chromosome terminated?

Answer 1

Answer:

1. In what ways can a primer for DNA synthesis by a DNA polymerase be provided?

• Primase enzyme synthesizes a Primer that is used by DNA polymerase to extend the DNA synthesis.
• Primase is an RNA polymerase that adds short primers (with free 3'-OH group) to the template DNA strand. Once the DNA replication occurs, these strands are removed and the gaps are sealed. For the leading strand, a single RNA primer is required and RNA primer for the lagging strand depends on the number of okazaki fragments.

2. In E.coli, how are the Okazaki fragments of the lagging strands joined to create a complete DNA duplex?

• Okazaki fragments are short segments of DNA that are synthesized in the lagging strand as polymerase works in the direction away from replication fork.
• DNA ligase enzyme joins these nicks in the okazaki fragments together resulting in a single joined and continous strand.

3. What are the three eukaryotic nuclear DNA replicases and their functions? How is DNA replication of a circular bacterial chromosome terminated?

Three eukaryotic nuclear DNA replicases are:

• Polymerase a/primase complex (Pol a)
  • Involved in initiation of DNA replication.
• Polymerase d (Pol d) or (Pol δ)
  • Synthesizes the leading strand and fill gap after the primer is removed
  • major replicative DNA polymerases in eukaryotes
• Polymerase e (Pol e)
  • Involved in repair of DNA

DNA replication of a circular bacterial chromosome is terminated by:

• Due to circular shape of bacterial chromosome, the replication proceeds bidirectionally and the two replication forks must meet at opposite ends for termination to occur.
• Prokaryotic bacterial cells contain termination sequences which upon binding to the Tus protein (Terminal utilization protein - a DNA binding protein) enables only single direction of replication fork to pass through.Tus protein binds to Ter sites leading to the formation of Tus-Ter complex that stops the replicating fork and forms catenated circular chromosome.