DNA Replication: A Comprehensive Guide with Illustrations, Schemes and Mind Maps of Genetics

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DNA REPLICATION

(a) Hypothesis 1: Semi-conservative replication (b) Hypothesis 2: Conservative replication Intermediate molecule (c) Hypothesis 3: Dispersive replication

MODELS OF DNA REPLICATION

Each “parent” strand must therefore act as a template for a new strand Nucleotides are successively added using deoxynucleoside triphosphosphates (dNTP’s)

Origin 5’ 3’ 3’ 5’ UNIDIRECTIONAL REPLICATION Origin 5’ 3’ 3’ 5’ BIDIRECTIONAL REPLICATION Is Replication Uni- or Bidirectional?

Evidence points to bidirectional replication Label at both replication forks

4. DNA Polymerase - DNA Polymerase I (Pol I) was the first enzyme discovered with polymerase activity, and it is the best characterized enzyme. Although this was the first enzyme to be discovered that had the required polymerase activities, it is not the primary enzyme involved with bacterial DNA replication. That enzyme is DNA Polymerase III (Pol III). Three activities are associated with DNA polymerase I;

• 5' to 3' elongation (polymerase activity)
• 3' to 5' exonuclease (proof-reading activity)
• 5' to 3' exonuclease (repair activity) The second two activities of DNA Pol I are important for replication, but DNA Polymerase III (Pol III) is the enzyme that performs the 5'-3' polymerase function. 5. Primase - The requirement for a free 3' hydroxyl group is fulfilled by the RNA primers that are synthesized at the initiation sites by these enzymes. 6. DNA Ligase - Nicks occur in the developing molecule because the RNA primer is removed and synthesis proceeds in a discontinuous manner on the lagging strand. The final replication product does not have any nicks because DNA ligase forms a covalent phosphodiester linkage between 3'-hydroxyl and 5'-phosphate groups.

A total of 5 different DNA POLs have been

reported in E. coli

• DNA Pol I: functions in repair and replication

• DNA Pol II: functions in DNA repair

• DNA Pol III: principal DNA replication enzyme

• DNA Pol IV: functions in DNA repair (discovered in

• DNA Pol V: functions in DNA repair (discovered in

The DNA Polymerase Family

Subsequent hydrolysis of PPi drives the reaction forward DNA polymerases add nucleotides to the 3’OH end of the growing strand. Strand elongation is therefore always 5’ to 3’