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DNA. Searching for Genetic Material. Mendel: modes of heredity in pea plants (1850’s) Morgan: genes located on chromosomes (early 1900’s) Griffith: bacterial work (1920’s) transformation - change in genotype and phenotype due to assimilation of external substance (DNA) by a cell

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Presentation Transcript
searching for genetic material
Searching for Genetic Material
  • Mendel: modes of heredity in pea plants (1850’s)
  • Morgan: genes located on chromosomes (early 1900’s)
  • Griffith: bacterial work (1920’s)
    • transformation- change in genotype and phenotype due to assimilation of external substance (DNA) by a cell
  • Avery: transformation agent was DNA (1944)
hershey chase experiment
Hershey-Chase Experiment
  • 1952 Experiment with bacteriophages
    • viruses that infect bacteria
  • Tested if DNA or protein was the hereditary material
    • in T2 (a bacteriophage that infects E. coli)
  • Sulfur (S) is in protein, phosphorus (P) is in DNA
  • Only P was found in host cell
dna structure
DNA Structure
  • Chargaff
    • ratio of nucleotide bases (A=T; C=G)
  • Structure of DNA researched by Pauling and Wilkins/Franklin
  • Watson & Crick (1953)
    • Proposed the structure of DNA after viewing an X-ray diffraction photo by Rosalind Franklin
the double helix
The Double Helix
  • Nucleotides: nitrogenous base (thymine, adenine, cytosine, guanine); sugar (deoxyribose); phosphate group
base pairing rules
Base Pairing Rules
  • Purines: A & G
  • Pyrimidines: C & T (Chargaff rules)
  • A’s H+ bonds (2x) with T and C’s H+ bonds (3x) with G
  • Van der Waals attractions between the stacked pairs
dna replication
DNA Replication
  • Watson & Crick strands are complementary; nucleotides line up on template according to base pair rules
  • Meselson & Stahlreplication is semiconservative; Expt: varying densities of radioactive nitrogen
3 replication models
3 Replication Models
  • Meselson and Stahl concluded that DNA follows the semiconservative model
dna replication in action
DNA Replication in Action
  • Origin of replication (“bubbles”)= beginning of replication
  • Replication fork: Y-shaped region where new strands of DNA are elongating
  • Helicase: catalyzes the untwisting of the DNA at the replication fork
  • DNA polymerase: catalyzes the elongation of new DNA
antiparallel nature
Antiparallel Nature
  • Sugar/phosphate backbone runs in opposite directions (Crick)
  • One strand runs 5’ to 3’, while the other runs 3’ to 5’
  • DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA strands in the 5’ to 3’ direction only
leading and lagging
Leading and Lagging
  • Leading strand:
    • synthesis toward the replication fork (only in a 5’ to 3’ direction from the 3’ to 5’ master strand)
  • Lagging strand:
    • synthesis away from the replication fork in small pieces (Okazaki fragments)
    • joined by DNA ligase
the lagging strand
The Lagging Strand
  • Primase enzyme attaches a primer
    • a short sequence of RNA (sometimes DNA)
  • Small segments replicated 5’  3’
  • After strand replicates, primer falls off
proofreading
Proofreading
  • Mismatch repair: DNA polymerase
  • Excision repair: nuclease
shortening ends
Shortening Ends
  • Because lagging strands need 3’ end, replication cannot extend to the very end of a DNA strand
  • Chromosomes contain telomeres
    • Repeating sequences of DNA
    • Don’t contain genes
  • Telomerase lengthens telomeres so that they don’t continually get shorter
    • Chromosomes can divide without losing genes
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