DNA Replication pt1

• Nucleic acids


  Molecules that play a general role in organisms
• DNA


  Deoxyribonucleic acid
• RNA


  Ribonucleic acid
• Nucleic acids as information molecules


  They carry genetic information
• DNA structure


  • Double helix
  • Anti-parallel
  • Complementary base pairing
  • Base pair hydrogen bonds
  • Phosphodiester bonds
• Griffith's experiment showed that a non-virulent bacterium could be transformed into a virulent form

  
• Avery, MacLeod & McCarty's experiment pointed to DNA, not protein, as the hereditary material
• Hershey & Chase's experiment using radioactive labeling showed that DNA is the hereditary material
• Chargaff determined that the DNA proportions are always A = T, G = C
• Franklin, Watson & Crick used X-ray diffraction and model building to determine the structure of DNA
• Meselson & Stahl's experiment using density labeling showed how DNA replicates
• DNA replication in prokaryotes

  1. Single-stranded binding proteins

  2. Helicase

  3. DNA polymerase III

  4. DNA polymerase I

  5. Primase

  6. DNA ligase
• Semi-discontinuous replication


  Leading and lagging strands, Okazaki fragments, proofreading
• Prokaryotic and eukaryotic DNA replication have general similarities and differences
• In 1856-1863, the molecule that transmits hereditary information was unknown at the time of Mendel
• DNA discovered by Friedrich Miescher


  1869
• Miescher's study revealed that DNA is made up of a phosphate group, a pentose sugar, and a nitrogenous base

  
• Griffith's experiment showed that traits (virulence) could be passed from dead to live bacteria, but the transmitting molecule was unknown

  
• Avery, MacLeod & McCarty's experiment determined that DNA, not protein, was the hereditary material
• Hershey & Chase's experiment using radioactive labeling proved that DNA, not protein, is the genetic material in bacteriophages
• Chargaff determined the consistent ratios of DNA nucleotides (A=T, G=C)

  
• Bacteriophages


  Viruses that infect bacteria
• Chase use bacteriophages to prove that DNA is the hereditary molecule
• Protein


  Molecules that have sulfur, but no phosphorus
• DNA


  Molecules that have phosphorus, but no sulfur
• Hershey and Chase use bacteriophages to prove that DNA is the hereditary molecule


  1952
• Protein use a radioactive isotope (35S)
• DNA use a radioactive isotope (32P)
• When the virus protein is tagged with a radioactive element, the tag doesn't transfer to the bacteria
• When the virus DNA is tagged with a radioactive element, the tag transfers to the bacteria
• Erwin Chargaff found the matching proportion of nucleotides: 30% Adenine, 30% Thymine, 20% Cytosine, 20% Guanine
• Chargaff's rules


  The proportion of A = the proportion of T, The proportion of C = the proportion of G
• Chargaff discovered the nitrogenous base pairing, but did not figure out the structure of DNA
• Rosalind Franklin


  • She was a researcher in the Wilkins Lab and produced the first good DNA X-ray crystallography
  • X-Ray diffraction technique needs a solid with regular, repeating units of atoms (crystal)
• DNA X-Ray crystallography


  • Purine + Purine diameter > 2 nm
  • Pyrimidine + Pyrimidine diameter < 2 nm
  • Purine + Pyrimidine diameter = 2 nm

  
• Watson and Crick


  • They solved the structure of DNA based on Franklin's patterns and Chargaff's rules
  • They developed the first molecular model of DNA (1953)
  • Rosalind Franklin did not receive a Nobel prize, since she died of cancer in 1958 (possibly caused by her exposure to X-ray)
• Complementarity of bases


  Adenine (A) can form two hydrogen bonds with Thymine (T), Guanine (G) can form three hydrogen bonds with Cytosine (C)
• DNA replication


  1. Semiconservative model

  2. Conservative model

  3. Dispersive model
• The experiment of Meselson and Stahl proved that the semiconservative model is the correct model of DNA replication
• Based on the structure of DNA, Watson and Crick suggested a semi-conservative DNA replication mechanism