DNA: The Genetic Material • Search for genetic material---is it composed of protein/DNA or RNA? • Griffith’s Transformation Experiment • Avery’s Transformation Experiment • Hershey-Chase Bacteriophage Experiment • Tobacco Mosaic Virus (TMV) Experiment • Lederberg and Tautum conjugation expt. • Chemistry of DNA: composition and structure • Double-helix model of DNA - Watson & Crick
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DNA: The Genetic Material
• Search for genetic material---is it composed of protein/DNA or RNA?
1. Biologically useful and stable source of information2. Ability to replicate accurately and transmitted across generations3. Capable of change4. Express itself to make other biomolecules
Timeline of events:• 1900 Chromosomes shown to contain hereditary information,
later shown to be composed of protein & nucleic acids.
RNA (not protein) is genetic material of some viruses, but no known prokaryotes or eukaryotes use RNA as their genetic material.
Alfred HersheyNobel Prize in Physiology or Medicine1969
Chemistry of DNA1869: Fred Miescher isolated nuclein(acidic with high phosphorous content)1880: Emil Fischer identified purines and pyrimidines1910: Kossel identified nucleotides (base+sugar+phosphate)Got the Nobel Prize1950: Alexander Todd discovered that 2 nucleotides are linkedby 3’ to 5’ phosphodiester bond1953: Watson and Crick propose double-helical model of DNA
Maurice WilkinsMaurice Wilkins Rosalind FranklinRosalind FranklinXX--Ray diffraction image of DNA Ray diffraction image of DNA
taken by Franklin in 1951taken by Franklin in 1951
Conclusion-DNA is a helical structure with distinctive regularities, 0.34 nm & 3.4 nm.
X-ray diffraction studies by Rosalind Franklin & Maurice Wilkins
The Race to Discover DNA’s Structure
19501950
ChargaffChargaff’’s Rule: s Rule: Equal amounts of Equal amounts of AAdenine and denine and TThymine, hymine, and equal amounts of and equal amounts of GGuanine and uanine and CCytosineytosine
Why do you think Why do you think the bases match up the bases match up
this way?this way?
PurinePurine + + PurinePurine = Too wide= Too wide
PyrimidinePyrimidine + + PyrimidinePyrimidine = Too Narrow= Too Narrow
PurinePurine + + PyrimidinePyrimidine = Perfect Fit from X= Perfect Fit from X--ray dataray data
James Watson and Francis Crick propose double-helical model of DNA
Double Helix Model of DNA: Six main features
1. Two polynucleotide chains wound in a right-handed (clockwise) double-helix.
2. Nucleotide chains are anti-parallel: 5’
3’3’
5’
3. Sugar-phosphate backbones are on the outside of the double helix, and the bases are oriented towards the central axis.
4. Complementary base pairs from opposite strands are bound together by weak hydrogen bonds.
A pairs with T (2 H-bonds), and G pairs with C (3 H-bonds).
5’-TATTCCGA-3’3’-ATAAGGCT-5’
5. Base pairs are 0.34 nm apart. One complete turn of the helix requires 3.4 nm (10 bases/turn).
6. Sugar-phosphate backbones are not equally-spaced, resulting in major and minor grooves.
Nucleotide = monomers that make up DNA and RNA (Figs. 2.8)