DNA structure

• 2 types of nucleic acids

  DNA: deoxyribonucleic acid

  RNA: ribonucleic acid
• what is DNA

  • genetic material that occurs in the chromosomes of the nucleus
  • macromules made by linking monomers(nucleotides) together
• What does a nucelotide consist of

  • sugar molecule (pentose / deoxyribose)
  • phosphate group
  • nitrogenous base (adenine, cytosine, gunaine, thymine/uracil)
• formation of a nucleotide

  • nitrogenous base + pentose sugar + phosphate
  • formed through condensation reactions, with formation of 2 water molecules
• Phosphodiester bond

  • covalent bond between the 5' - phosphate group of one nucleoyide and the 3' - OH group of another nucleotide
  • formed by a condensation reaction and a water molecule is lost in the process
• Sugar-phopsphate backbone

  • formed by alternating sugar and phosphate molecules with one of the bases attached to each of the sugar molecule along the strand (DNA stablity)
  • since the bases vary, theu represent a unqiue sequence that carries coded information held by the nucleic acid
• Rule of complementary base pairing(DNA stability)

  • Adenine bonds with thymine (2 hydrogen bonds)
  • Cytosine bonds with guanine (3 hydrogen bonds)
• DNA is anti-parallel

  • 2 strands of the DNA molecule run in opposite directions of each other
  • One chain runs from 5' to 3' while the other runs from 3' to 5'
• DNA replication

  • takes place in the nucleus during interphase: S-phase
• Part 1: separation of DNA strands

  1. DNA replication begins at the origin of replication
  2. Helicase unwinds the double helix by breaking the hydrogen bonds between the complementary base pairs in the parental strands
  3. Each parental strand acts as the template for the synthesis of a new DNA strand
• Part 2: synthesis of RNA primer


  4. Primase attaches to the unwond chain and catalyses the synthesis of RNA primer to provide free 3' OH ends for DNA polymerase III
• Part 3: Synthesis of new daugther strands

  5. DNA ploymerase III elongates the new daughter strands in the new 5' to 3' direction by catalysing phosphodiester bond formation between the incoming deoxyribonulceotides and the free 3' OH end of the daughter strand

  6. Free deoxyribonucleotides are incorporated by complemetary base pairing to the parental DNA strands, with adenine to thymine and guanine to cytosine
• Part 3

  7. In a replication fork, the leading strand is synthesized continuosly

  8. while the lagging strand is synthesized discontnously to form Okazaki fragments

  9. RNA primers are removed and replaced by deoxyribonucleotides by DNA polymerase I

  10. Nicks between the Okazaki fragments are filled in by DNA ligase by forming phoshphodiester bonds between Okazaki fragments