Download DNA Structure

III.
DNA AND CHROMOSOMES
DNA Structure and Replication
• 18th C: Friedrich Miescher (nuclei from WBC in pus…Material
(nitrogen/phosphorus = “nuclein”)
• 1928: Frederick Griffith: transformation experiments with bacteria
(Genetic material = DNA)
DNA (Deoxyribonucleic Acid)
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The “blueprint of life”
Unique and unrepeatable
Genes: bits of DNA
Composed of nucleotides
 Sugar (deoxyribose) + phosphate + bases
 Covalent Bonds between S-P
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Complementary Base Pairs (AT, CG)
Purines (A, G) Pyrimidines (C, T)
A-T (2 hydrogen bonds) C-G (3 hydrogen bonds)
Sugar (deoxyribose) + base = nucleoside
• 1953: Rosalind Franklin (under the
supervision of Maurice Wilkins): X ray
diffraction
Photo 51
• James Watson and Francis Crick
 Double Helix Model
 1962 Nobel Prize (with Wilkins)
• DNA coils around proteins called
histones
• Chromatin: DNA + 30% histones
• Central Dogma of Molecular Genetics:
• DNA makes RNA which makes protein
• To be a genetic molecule, DNA must: replicate,
store info, express info, vary by mutation
The Race for the DNA Model
King’s College
London
Cavendish Lab
London
Rosalind Franklin
James Watson
Francis Crick
(X ray Diffraction Studies)
Photo
#51
Maurice Wilkins
Nobel Prize
1962
(Double Helix Model)
Semi-Conservative Replication
• 1957: Meselson and Stahl labeled bacterial DNA with
15N
2 new DNA (1 old strand/1 new strand)
Steps of DNA Replication
• Helicases (enzymes) unwind DNA
 Repair errors in replicated DNA
• Replication fork: where DNA opens
 Hydrogen bonds between base pairs break
 Replicon: length of DNA to be replicated
• Primase (enzyme) attracts complementary bases to build
a short piece called a primer
• DNA polymerase (enzyme) adds more bases to primer
• Ligase (enzyme) seals S-P backbone, building the new
strand (Okazaki fragments)
• Technology called Polymerase Chain Reaction (PCR):
used DNA polymerase to rapidly produce millions of
copies of a specific DNA sequence of interest
 Forensics: establish blood relationships, identify remains, convict
criminals, evaluate those falsely accused
 Weakness of PCR: sensitivity
 Strength of PCR: works on crude samples of rare, old, minute
sequences