DNA Replication Chapter 12 Section 2

History of DNA Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA

Transformation Fred Griffith worked with virulent S and nonvirulent R strain Pneumoccocus bacteria He found that R strain could become virulent when it took in DNA from heat-killed S strain Study suggested that DNA was probably the genetic material (not proteins as previously thought)

Griffith Experiment

History of DNA Chromosomes are made of both DNA and protein Experiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material Radioactive 32P was injected into bacteria!

Discovery of DNA Structure Chargaff’s rules: Adenine must pair with Thymine Guanine must pair with Cytosine The bases form weak hydrogen bonds G C T A

DNA Structure Rosalind Franklin took diffraction x-ray photographs of DNA crystals. This showed the shape of DNA

DNA Structure In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-ray diffraction.

DNA Deoxyribonucleic acid Made up of subunits (monomers) called nucleotides Nucleotide made of: 1. Phosphate group 2. 5-carbon sugar 3. Nitrogenous base

DNA Double Helix “Rungs of ladder” Nitrogenous Base (A,T,G or C) “Legs of ladder” Phosphate & Sugar Backbone

DNA Replication

Replication Facts DNA has to be copied before a cell divides DNA is copied during the synthesis phase (S) of interphase New cells need identical DNA strands

DNA Replication Step 1: DNA unwinds (helicase) Step 2: DNA unzips Two strands open to form Replication Forks New strands grow at the forks (DNA polymerase) Replication Fork Parental DNA Molecule 3’ 5’

Origins of Replication Replication begins at special sites called origins of replication, where the two DNA strands separate and open up a replication “bubble” A eukaryotic chromosome may have many origins of replication, prokaryotes only have one. Replication occurs in both directions from each origin At the end of each replication bubble is a replication fork, a Y-shaped region

Prokaryote Replication

Eukaryote Replication

Semiconservative Replication Parent strands separate and serve as a template They produce new DNA molecules that have one strand of parent DNA and one strand of new DNA DNA Template New DNA Parental DNA

Antiparallel Structure of DNA Two strands oriented in opposite directions. DNA polymerases add nucleotides only to the free 3end of a growing strand; a new DNA strand can elongate only in the 5 to 3direction

Leading & Lagging Strands Leading strand is built continuously, towards the replication fork Lagging strand built discontinuously (in fragments) away from the replication fork. fragments called Okazaki fragments DNA ligase joins the fragments together Replication Video

Steps of DNA Replication Unwinding the helix DNA helicase Base Pairing (add complimentary nucleotides) DNA Polymerase Joining of Okazaki Fragments DNA Ligase

Proofreading New DNA Enzymes are involved in many steps of DNA replication Their final role in replication is to proofread and correct mistakes.

DNA Damage & Repair Chemicals & ultraviolet radiation damage the DNA in our body cells. Cells must continuously repair damaged DNA. Excision repair occurs when a repair enzyme removes damaged parts of DNA Enzymes replace and bond the new nucleotides together