DNA structure and replication

• DNA is how we inherit our genetic information
• how were nucleic acids discovered?


  purifying cell with large nuclei
• method to discover nuclei acid

  • white blood cells purified to obtain their nuclei
  • nuclei found to have precipitate rich in phosphorous and nitrogen (nuclein)
  • nitrogen rich fraction is protein
  • acidic phosphorous fraction is nucleic acid
• nucleic acid consists of

  • 5 carbon sugar
  • nitrogenous bases
  • phosphate
• 2 types of nucleic acid

  RNA and DNA
• RNA found in the nucleoplasm but not chromosomes
• DNA found in chromosomes
• Griffith's experiment used 2 strains one that was benign and one that was virulent
• Griffith's experiment determined that the heat killed virulent strain transforms the benign form into a virulent form that is disease causing because


  genetic material is capable of reprogramming benign form into virulent form
• Oswald Avery experiment was used to identify the transforming principle
• Oswald Avery experiment method

  • resistant to proteases- so not protein
  • resistant to lipases- so not lipid
  • resistant to ribonucleases- so not RNA
  • ethanol insoluble- so not carbohydrate
  • high molecular weight (like DNA)
  • positive reaction to the Dische test for the deoxyribose of DNA
• Oswald Avery conclusion 


  transforming principle is DNA
• Hershey-Chase experiment was used to determine what is the genetic material
• Hershey-Chase experiment method:

  • phage protein labelled with 35 S
  • phage DNA labelled with 32 P
  • protein with radioactive sulfur label was found in the supernatant
  • DNA with radioactive phosphorous was found in the pellet, containing the intact cells)
• Hershey-Chase experiment conclusion


  DNA is injected into the cell, not protein
• what is Chargaff's rule

  • base composition always obey strict rule of A=T and G=C
• • ratio of the 4 bases is not 1:1:1:1
  • ratio is species specific
• DNA nucleotide

  labels

  A) phosphate

  B) base

  C) pentose sugar

  
• what are the bases in DNA?

  • adenine
  • guanine
  • cytosine
  • thymine
• which bases are purines

  adenine and guanine
• which bases are pyrimidines 


  cytosine and thymine
• sugar-phosphate backbone

  labels

  A) phosphodiester bond

  B) 5' phosphate

  C) 3' hydroxyl

  
• X-ray diffraction conclusions

  • DNA is a helix
  • DNA is 2 nm wide
  • length of each turn is 3.4 nm
  • distance between repeating units is o.34 nm
  • there are 10 nucleotide pairs per turn
• Watson and Crick concluded that bases would only fit in DNA helix if purine was paired with pyrimidine
• A binds to T with 2 hydrogen bonds
• A binding to T is weaker than G binding to C
• G binds to C with 3 hydrogen bonds
• 2 strands of DNA are anti-parallel
• what does anti-parallel mean

  they run in opposite directions
• the transforming principle is 


  DNA
• haploid human genome contains ~3 billion base pairs, the length of dsDNA in 1 diploid cell is

  2m
• Watson and Crick suggested that each strand of DNA can act as a template for the synthesis of a new complementary stand
• Meselson-Stahl experiment

  • bacteria cultured in medial with N 15 which is a heavy isotope
  • bacteria transferred to medium with N 14 the lighter isotope
  • DNA sample is centrifuged after the first replication
• Meselson-Stahl experiment concluded


  all DNA of an intermediate density (one heavy strand and one light strand)
• DNA synthesis

  labels

  A) DNA polymerase

  B) pyrophosphate

  C) nucleoside triphosphate

  
• for DNA synthesis to occur need

  • single stranded template DNA
  • all four nucleoside triphosphates (dNTPs)
  • free 3' hydroxyl (primer)
• DNA polymerase function

  • synthesises DNA in 5' to 3' direction
  • inserts complementary nucleosides
  • uses energy from breaking phosphate bonds
  • has proof reading ability to remove incorrectly inserted nucleotides
• replication begins at origins of replication
• each replication fork has a lagging and leading strand
• strands of replication

  labels

  A) primer

  B) leading strand

  C) lagging strand

  D) origin of replication