• 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 T G 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 StrandParent 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- 100-200 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 Strand Synthesis 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