1. DNA REPLICATION

2. Review of DNA Structure

3. The chemical side groups of the nitrogen bases form hydrogen bonds, connecting the two strands.
Adenine forms two hydrogen bonds with thymine
Guanine forms three hydrogen bonds with cytosine
Fig. 16.6

4. Sugar-Phosphate Backbone
The phosphate group of one nucleotide is attached to the sugar of the next nucleotide in line.
The result is a “backbone” of alternating phosphates and sugars, from which the bases project.

5. The sugar-phosphate backbones of the double helix strands run antiparallel to each other
Each DNA strand has a 3’ end with a free hydroxyl group
and a 5’ end with a free phosphate group
One strand runs 5’ to 3’ and the other runs 3’ to 5’

6. DNA Replication Before a cell divides it must replicate its DNA
Semiconservative – the two parent strands serve as a template for the synthesis of the new complementary strands

7. DNA Replication
Old Strand
New Strand

8. Replication Origin
Replication begins at special sites called origins of replication
There may be hundreds or thousands of origin sites per chromosome.
Strands separate forming a replication “bubble” with replication forks at each end.
The replication bubbles elongate as the DNA is replicated and eventually fuse.

9. Enzymes Involved
Helicase – unwinds and unzips the DNA helix at the replication forks
Topoisomerases – relieves the torque from the unwinding DNA

11. Enzymes
Primase – constructs RNA primer complementary to the DNA templates
After formation of the primer, DNA polymerase III – elongates the new strand by adding nucleotides to the 3’end (~50 per second)
DNA polymerase I – later replaces RNA primers with DNA nucleotides

12. Elongation
When nucleotide triphosphates are linked to the sugar-phosphate backbone it loses two of its phosphates
Replication always occurs in the 5’ to 3’ direction

13. Elongation
Leading strand – elongates toward the replication fork, continuous
Lagging strand – elongates away from the replication fork
Okazaki fragments: discontinuous short segments

14. Ligase – enzyme that joins fragments into a single DNA strand

16. Proofreading and Repair
Replication is extremely accurate
1 error per billion nucleotides
DNA polymerase proofreads each nucleotide against the template and fixes any mismatches
Causes: replication errors, physical and chemical agents

17. When does DNA replicate?
DNA replication only occurs when the cell is preparing to divide
When the cell is NOT dividing DNA is used as a blueprint for protein synthesis