frederick griffith uncovered genetic role of dna transformation- change in genotype and phenotype...
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• 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
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