1. DNA Replication

2. 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.

3. 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.

4. 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.

5. DNA Replication in Action
Animation 1:

6. 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.

7. Drawing of the Initiation Step

8. 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.

9. 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.

10. Drawing of the Elongation Step

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

12. Drawing of the Termination Step

13. The Leading Strand
DNA Polymerase

14. The Lagging Strand

15. 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!!

16. 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.

17. 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.