BIOL1102 Week 4

• DNA is a double helix
• Watson and Crick's model of DNA structure

  • Based on earlier work:
  • Knowledge of basic building blocks of DNA (Deoxyribose Nucleic Acids)
  • Chargaff's rules of base composition
  • X-ray diffraction analysis of DNA

  
• Nucleotide


  Phosphate + sugar + base

  
• Chargaff's rules: A + G = T + C (total amount purine = total amount pyrimidine)
• 1. ray diffraction analysis showed DNA has two strands, parallel, helical
• Watson and Crick proposed the double helix structure of DNA in 1953

  
• In the average human cell there is around 2 meters of DNA
• Nucleosome


  DNA wrapped around histones
• DNA is packaged into the nucleus which is 10 μm in diameter
• Prokaryotes and eukaryotes are the two structural types of cells
• Mitochondria and chloroplasts are prokaryotic in origin
• The three domains of cellular life are prokaryotes, eukaryotes, and the oldest fossils are 3.5 billion years old
• Watson and Crick


  Proposed the DNA double helix model in 1953

  
• Watson and Crick's model was based on

  • Knowledge of basic building blocks of DNA (Deoxyribose Nucleic Acids)
  • Chargaff's rules of base composition
  • X-ray diffraction analysis of DNA

  
• Chargaff's rules of base composition


  A + G = T + C (total amount purine = total amount pyrimidine)

  
• DNA has a double helix structure with two parallel, helical strands
• Rosalind Franklin


  Produced X-ray diffraction pattern of DNA that contributed to Watson and Crick's model
• 5' end


  DNA strand has phosphate group attached to 5' carbon

  
• 3' end


  DNA strand has hydroxyl group attached to 3' carbon

  
• DNA polymerase works in the 5' to 3' direction adding new nucleotides to the 3' end
• Chargaff's rules of base composition

  • A = T
  • G = C
  • A + G = T + C
• Chargaff's work established empirical rules about the amount of each type of nucleotide found in DNA

  
• The total amount of pyrimidine nucleotides (T+C) always equals the total amount of purine nucleotides (A+G)
• DNA is replicated to create two identical copies of the original molecule
• The Meselson-Stahl experiment demonstrated that DNA replication is semi-conservative
• DNA replication


  1. Parental molecule has two complementary strands

  2. Strands separate and each is used as a template to synthesise a complementary strand

  3. Result is two identical double helices
• DNA replication


  1. Parental molecule has two complementary strands of DNA

  2. Two strands of helix separate and each strand used as template to synthesise complementary strand

  3. Result: two identical double helicases for distribution to progeny cells during mitosis

  
• Basic processes of DNA replication are the same in prokaryotes and eukaryotes, but simpler in prokaryotes

  
• Bacterial chromosomes


  • Have a single origin of replication
  • Have a single replication bubble
• Eukaryotic chromosomes


  • Have multiple replication bubbles
• DNA replication


  1. Replication fork

  2. Leading strand

  3. Lagging strand

  4. Helicase

  5. Primase

  6. DNA polymerase

  7. Topoisomerase

  8. dNTPs (Deoxyribonucleoside triphosphates)

  
• There are differences in DNA replication in prokaryotes and eukaryotes

  
• Differences in eukaryotic DNA replication

  • DNA molecule is bigger
  • Smaller Okazaki fragments
  • Replication fork moves slower than prokaryotes
  • Chromosomes have many origins of replication (many replication bubbles)
  • Different DNA polymerases are involved

  
• Humans and dogs share 84% of their DNA
• Humans and chimpanzees share 98.8% of their DNA
• Dog faeces as evidence to solve a homicide