Replication

Replication means duplication of DNA molecules.

Purpose: DNA makes up chromosomes. Before a cell divides, it needs to double the number of chromosomes, meaning the DNA is duplicated. Replication occurs during the S (synthesis) phase of the cell cycle.

A. Helicase: Enyme that unwinds DNA helix and "unzips" DNA.

B. RNA Primase: Adds a short RNA primer to start replication (later removed).

C. DNA Polymerase III: Fits to RNA primer. Adds new DNA nucleotides to new strand in 5' ----> 3' direction.

D. DNA Ligase: Joins neighboring nucelotides together.

E. DNA Polymerase I: Removes RNA primer & replaces it with DNA nucleotides.

Replication Fork: Section of DNA double strand opened immediately behind the Helicase.

Leading Strand: Section of DNA that is being synthesized continuously toward the replication fork.

Lagging Strand: Section of DNA being synthesized away from the replication fork. It is synthesized in fragments (Okazaki Fragments).

Okazaki Fragments: Small sections of new DNA on lagging strand. They are small because replication moves away from replication fork on lagging strand.

Mutation: Mistakes in nucleotide code. Small mutations are naturally removed by enzymes as replication progresses.

Semi-conservative Replication: This process creates 2 new strands of DNA, each having one strand from the original DNA, each having a newly synthesized strand.

Polysome: Eukaryotic chromosomes replicate the same DNA at multiple sites (much faster). Prokaryotic chromosomes replicate from one site (slower!).

(See page 292 in your text)

A Site with a good beginning Animation of Replication (Needs Shockwave)

[Replication Diagram #2]

[Replication Diagram #3]