DNA Replication

Replication Replication is the process in a cell where DNA copies itself. To accomplish this feat, DNA must unwind, split into two chains, and each chain serves as a template for two new strands of DNA. In Eukaryotic cells, replication occurs during the Synthesis (S) phase of Interphase during the Cell Cycle.

Structure of the DNA Template *DNA chains are labeled with a 3’ and a 5’ end. *DNA always copies in the 5’ to 3’ direction.

Structure of the DNA Template The outside strands are the Template Strands The side of new DNA that copies toward the replication fork is the Leading Strand. The side of new DNA that copies away from the replication fork is the Lagging Strand. This strand is copied in segments called Okazaki Fragments.

DNA Replication in Action Animation 1: http://highered.mcgraw-hill.com/olc/dl/120076/micro04.swf

The Replication Process Step 1. Initiation The two nucleotide chains separate. The place where the chains separate forms the replication fork. DNA Helicase is an enzyme that breaks the Hydrogen Bonds between the chains.

Drawing of the Initiation Step

The Replication Process Step 2. Elongation Molecules of DNA Polymerase bind to one strand of the DNA and begin moving along it, using it as a template for assembling a leading strand of DNA. DNA Polymerase is the enzyme responsible for bonding new bases and making new DNA. DNA Polymerase also proofreads DNA for errors.

The Replication Process Step 3. Elongation A DNA Ligase stitches up the new chains of DNA Ligase bonds the DNA backbones of the Okazaki fragments.

Drawing of the Elongation Step

The Replication Process Step 4: Termination Two exact copies of the DNA have been created and the cell may now undergo division.

Drawing of the Termination Step

The Leading Strand DNA Polymerase

The Lagging Strand

A Semi Conservative Process DNA Replication is a semi-conservative process because half of each new DNA molecule is old and half is new. Replication Song!!

Accuracy and Repair DNA replication is very accurate. In every 10,000 base pairs there may be one error. A mismatched, skipped, or changed nucleotide can create problems called mutations. There are several types of mutations, and these will be discussed at a later time. DNA Polymerase corrects base pairing mistakes Other mechanisms exist to catch base pairing mistakes. Generally speaking, polymerase’s proofreading ability keeps errors down to about one in every one-billion base pairs.

Accuracy and Repair Despite proofreading capabilities following replication, mistakes (mutations) still occur in base pairing. These mistakes are amplified by environmental factors. DNA can be damaged by chemicals (called carcinogens) and it can be damaged by the sun. For this reason, a cell is constantly proofreading its DNA, even when it is not replicating.