Concept 6 Review

Glossary of Terms

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Another DNA polymerase

This DNA polymerase replaces the RNA primer with DNA. This is a different type of DNA polymerase from the main DNA polymerase which synthesises DNA on a DNA template. In E. coli the main enzyme is DNA polymerase III and the enzyme that replaces the RNA primer with DNA is DNA polymerase I. When the RNA primer has been replaced with DNA, there is a gap between the two Okazaki fragments and this is sealed by DNA ligase.

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DNA ligase

DNA ligase seals the gap left between Okazaki fragments after the primer is removed. As the Okazaki fragments are joined, the new lagging strand becomes longer and longer.

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DNA polymerase

Location: On the template strands.

Function: Synthesizes new DNA in the 5' to 3' direction using the base information on the template strand to specify the nucleotide to insert on the new chain. Also does some proofreading; that is, it checks that the new nucleotide being added to the chain carries the correct base as specified by the template DNA. If an incorrect base pair is formed, DNA polymerase can delete the new nucleotide and try again. In E. coli the enzyme used for all new DNA synthesis except for the replacement of the RNA primers is DNA polymerase III. DNA polymerase I replaces the primers.

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Helicase

Location: At the replication fork.

Function: Unwinds the DNA double helix.

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Lagging Strand

The new DNA strand made discontinuously in the direction opposite to the direction in which the replication fork is moving.

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Leading strand

The new DNA strand made continuously in the same direction as movement of the replication fork.

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Okazaki fragment

Location: On the template strand which dictates new DNA synthesis away from the direction of replication fork movement.

Function: A building block for DNA synthesis of the lagging strand. On one template strand, DNA polymerase synthesizes new DNA in a direction away from the replication fork movement. Because of this, the new DNA synthesized on that template is made in a discontinuous fashion; each segment is called an Okazaki fragment.

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Overall direction of replication (movement of replication fork)

The direction of replication i.e., the direction in which the replication fork moves as the DNA double helix unwinds.

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Parent DNA

The parental DNA double helix that will be unwound and used as the template for new DNA synthesis.

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Primase

Location: Wherever the synthesis of a new DNA fragment is to commence.

Function: DNA polymerase cannot start the synthesis of a new DNA chain, it can only extend a nucleotide chain primer. Primase synthesizes a short RNA chain that is used as the primer for DNA synthesis by DNA polymerase.

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Single-strand binding (SSB) proteins

Location: On single-stranded DNA near the replication fork.

Function: Binds to single-stranded DNA to make it stable.