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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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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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