DNA

• Meselon and Stahl Experiment:

  A) 100%

  B) 100%

  C) 50%

  D) 50%

  E) 25%

  F) 75%

  
• Summary of DNA replication

  A) DNA ligase

  B) RNA primer

  C) DNA polymerase

  D) Primase

  E) DNA polymerase

  F) Parental DNA

  
• DNA

  • 2 anti-parallel polynucleotide strands
  • One 3'-5' direction, other 5'- 3' direction
  • Complementary base pair:
  • Purine (double-ring) - pyrimidine (single-ring)
  • Adenine - Thymine (A-T, two H-bonds)
  • Guanine - Cytosine (G-C, three H-bonds)
  • Twists into double helix
  • One full turn; 10 base pairs
  • H-bonds
  • Individual weaker than phosphodiester
  • Collective
  • Protected by double helix
  • Long-term stable genetic store
  • Makes up chromosomes
  • Stores lots
  • Gene
  • DNA section
  • Codes for specific polypeptide
  • Codon (triplet code): Base sequence determining specific amino acid sequence
• DNA Replication

  • Replicates itself exactly
  • Occurs in S phase of interphase
  • Essential before mitosis (nuclear, not cell, division)
  • Ensures two daughter cells produced by parent have same chromosome number
  • Semiconservative
  • Each new DNA molecule has one old and one new strand
  • Original parent strand, it template for new daughter strand
  • Form two chromatids, joined by centromere, make chromosome visible in mitosis prophase
• Stages of DNA replication

  • Original molecule unwinds catalysed by DNA helicase
  • H bonds between nitrogenous bases break naturally
  • Two strands separate (like a zip), each now is template for new strand formation
  • Free nucleotides join exposed complementary bases
  • Two phosphate groups, each nucleotide removed by hydrolysis reactions; releasing energy form phosphodiester bonds
  • DNA polymerase catalyses bond between nucleotides, 5’ to 3’ direction only and checks new bases
  • 3’ to 5’ strand formed in short segments (5’ to 3’), Okazaki fragments, by DNA ligase
  • DNA winds into 2 double helices