DNA REPLICATION

Review of DNA Structure

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

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.

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’

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

DNA Replication Old Strand New Strand

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.

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

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

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

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

Ligase – enzyme that joins fragments into a single DNA strand

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

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