DNA replication

• Nucleic Acids


  Large biomolecules made up of long chains of repeating nucleotides (monomers) - POLYMERS
• Nucleic Acids


  • Concerned with storage, transmission and use of information
  • Make up genetic material of cells and viruses
  • Information coded in nucleic acid and controls cell activity
• DNA

  Deoxyribonucleic acid
• RNA


  Ribonucleic acid
• Components of Nucleotides

  • Phosphate group
  • Pentose sugar
  • Nitrogenous base
• DNA Nucleotides

  Deoxyribose sugar
• RNA Nucleotides


  Ribose sugar
• Nitrogenous Bases

  • Purines (Adenine, Guanine)
  • Pyrimidines (Cytosine, Thymine/Uracil)
• Nucleoside

  Nitrogenous base + pentose sugar
• Nucleotide


  Nucleoside + phosphate group
• Nucleotides as parts of other biomolecules

  • Coenzymes (e.g. CoA)
  • Flavine Nucleotides (e.g. FAD)
  • Nicotinic Acid Nucleotides (e.g. NAD, NADP)
  • ATP
• Phosphodiester Bonds

  Nucleotides join together by condensation reactions, forming phosphodiester bonds that link C3 of one sugar to C5 of another
• Polynucleotides


  Long chain of nucleotides
• Nucleic Acids

  Polynucleotides with a sugar-phosphate backbone and bases projecting out
• Nucleic Acids

  • 5' end has free phosphate group, 3' end has free hydroxyl group
  • Have polarity
• RNA Structure

  • Nucleotides have ribose sugar
  • Bases: Adenine, Uracil, Guanine, Cytosine
  • Single-stranded, can form internal base pairing
  • Ratio of purines to pyrimidines is not 1:1 (no Chargaff's rule)
  • Joined by phosphodiester bonds
• Types of RNA

  • Messenger RNA (mRNA)
  • Ribosomal RNA (rRNA)
  • Transfer RNA (tRNA)
  • Small Nuclear RNA
• mRNA


  Long RNA molecules that transcribe information from DNA in nucleus and transport it to ribosome in cytoplasm for translation
• rRNA


  RNA in ribosomes that catalyses formation of peptide bonds during translation
• tRNA


  Small molecules that carry amino acids to ribosome during translation, with an anticodon sequence complementary to mRNA codon
• Small Nuclear RNA


  Associated with proteins forming ribonucleoproteins involved in splicing to excise introns and join exons
• DNA Structure

  • Double-stranded molecule with two polynucleotide chains held together by hydrogen bonds between complementary base pairs (A-T, G-C)
  • Strands run anti-parallel (3' to 5' and 5' to 3')
  • Forms a stable double helix structure
• DNA Structure

  • Chargaff's rule: number of purines = number of pyrimidines, and amounts of A+T and G+C vary between species
  • Complementary base pairing ensures consistent diameter of double helix
  • Genetic information stored in sequence of bases
• Prokaryotic Chromosomes


  1 circular chromosome and small circular plasmids in nucleoid
• Eukaryotic Chromosomes


  • Linear chromosomes consisting of DNA wound around histone proteins, existing in euchromatin and heterochromatin forms
  • Condense and decondense during cell division
• Models of DNA Replication

  • Conservative Model
  • Semi-Conservative Model
  • Dispersive Model
• Meselson & Stahl Experiment

  1. Experimental group supplied with heavy nitrogen (N15) nucleotides, control group with N14

  2. Standards: experimental DNA has high density, control has low density

  3. Another sample grown for multiple generation in N15 nucleotides, then transferred into N14 solution.

  4. After first generation, extracted DNA has medium density, eliminating conservative model.

  5. After second generation, DNA has both medium and low density, confirming semi-conservative model
• DNA Replication

  1. Separating DNA strands using helicase and single-stranded DNA-binding proteins

  2. Formation of RNA primer by primase

  3. Synthesis of new DNA strands by DNA polymerase III, leading strand continuously, lagging strand discontinuously

  4. Removal of RNA primer and nick translation by DNA polymerase I

  5. Joining of Okazaki fragments by DNA ligase

  6. Proofreading by DNA polymerase III
• DNA replication results in two identical double-stranded DNA molecules, each with one original parent strand and one newly synthesized complementary daughter strand (semi-conservative model)