DNA Replication

Helicase

• Function: Enzyme that unwinds the DNA double helix.

• Significance: the double helix needs to be unwound before replication can occur

Origin of Replication

• Function: A segment on the DNA double helix strand that signals the start of replication.

• Significance: helps to initiate DNA replication

SSBs

• Function: proteins that keep the double helix open (after helicase unwinds it).

• Significance: Without SSBs, the double helix will have the tendency to re-coil into a double helix (via hydrogen bonding)

Replication Fork

• Function: The area (direction) in which the DNA double helix is being unwound.

• Significance: called a “Fork” because the Y-shape (direction of helix opening) resembles a fork

Topoisomerase

• Function: Relieves tensional forces.

• Significance: allows the DNA double helix to be continually unwound during replication

DNA Polymerase 3

• Function: Enzyme that replicates DNA by adding nucleotides in a 5’ to 3’ direction.

• Significance: allows DNA replication to occur

Primase

• Function: Enzyme that builds an RNA primer on theleading and lagging strands.

• Significance: Primers are necessary for DNA Polymerase III to add new nucleotides (DNA Pol III cannot initiate its own strand, it can only add to an existing one)

RNA Primer

• Function: A small segment of RNA which is required for replication.

• Significance: DNA Polymerase III will add nucleotides to the primer, thus elongating the leading & lagging strands

Leading Strand

• Function & Significance: the replicated DNA strand that is built from the 5’ to 3’ direction. It uses the anti-parallel 3’ to 5’ parental/template strand. It is built continuously by DNA Polymerase I

Lagging Strand

• Function & Significance: the replicated DNA strand that is build discontinuously in smaller Okazaki Fragments. Smaller 5’ to 3’ fragments are then glued together using DNA ligase. The lagging strand uses the 5’ to 3’ parental DNA strand as a template

Okazaki Fragments

• Function & Significance: small fragments of 5’ to 3’ DNA that are built discontinuously on the lagging stand.

DNA Polymerase 1

• Function: Replaces RNA Primers with DNA.

• Significance: the newly replicated strands of DNA must replace RNA nucleotides with DNA.

DNA Ligase

• Function: enzyme that “glues” together the sugar-phosphate backbone of DNA using phosphodiester bonds.

• Significance: this is required to connect the Okazaki Fragments together on the lagging strand

Exonuclease

• Function & Significance: enzyme with proofreading functions. Able to cut & paste (remove incorrect nucleotides and replace them with the correct ones). DNA Polymerase III is also an example of an exonuclease.