Frederick Griffith uncovered genetic role of DNA Transformation- change in genotype and phenotype due to assimilation of external DNA by a cell Pathogenicity was inherited by all descendants of transformed bacteria Oswald Avery determined DNA was the transforming agent

1.Incorporate radioactive sulfur on the protein surface of viruses 3. Mix bacteria and viruses in blender and centrifuge 4. Bacteria forms a pellet on bottom and viruses remain in the liquid 2. Viruses infect bacteria

Same process as before except radioactive phosphorus was incorporated into phage DNA Experiment showed DNA is phage’s genetic material

A pairs with TG pairs with C Erwin Chargaff found that the four nitrogenous bases were present in specific ratios in all organisms. Could not explain his discovery A= 30.9% G= 19.9% T= 29.4% C=19.8%

Watson saw an image of DNA produced by x-ray crystallography and deduced DNA has a helical shape. Watson and Crick built a model of DNA with a double helix

Conservative Semiconservative Dispersive Grow first in heavy isotope culture then light isotope culture

Bacterial DNA Origin of Replication Daughter Strand Parent Strand Replication Fork Replication Fork- y shaped region where new strands of DNA elongate

DNA Polymerase- an enzyme that catalyzes elongation of new DNA Energy for polymerization of DNA comes from nucleotide triphosphate As each monomer joins the growing strand, the nucleotide triphosphate loses 2 phosphate groups (pyrophosphate)

3’ end- hydroxyl group 5’ end- phosphate group

DNA elongates in the 5’ to 3’ direction Leading strand Lagging strand Okazaki fragment nucleotides long DNA Polymerase- adds new nucleotides to new DNA strand DNA Ligase- joins the Okazaki fragments

DNA polymerase can not initiate synthesis of polynucleotide Primer- existing RNA chain bound to template DNA to which nucleotides are added during DNA synthesis Primase- an enzyme that joins RNA nucleotides to make the primer Only one primer required for leading strand; each Okazaki fragment of lagging strand must be primed

Initiation of Replication Helicase- enzyme that untwists the double helix, separating the two strands Single-Strand Binding Protein- holds the two template strands apart while new strands are synthesized Synthesis of Leading StrandSynthesis of Lagging Strand Primase- priming DNA Polymerase- elongation DNA Polymerase- replacement of RNA primer with DNA Primase- priming for Okazaki fragments DNA Polymerase- elongation of fragment DNA Polymerase- replacement of RNA primer with DNA Ligase- joining of fragments

Nuclease- DNA cutting enzyme During DNA replication, DNA polymerase itself proofreads each nucleotide Nucleotide Excision repair- replaces damaged DNA using undamaged strand as guide

DNA polymerase can only add nucleotides to the 3’ end of a preexisting polynucleotide (elongation occurs from 5’ to 3’)

Telomere- multiple repetition of one short nucleotide sequence at the end of eukaryotic DNA; in humans the repeated nucleotide sequence is TTAGGG Telomerase- enzyme that catalyzes the lengthening of telomeres; includes a molecule of RNA that serves as a template for new telomere segments Telomeres are not present in most cells of multicellular organisms