DNA structure & replication

• Formation of a nucleotide

  1. Pentose sugar / deoxyribose
  2. Phosphate group
  3. Nitrogenous base - can be adenine, cytosine, guanine, or thymine / uracil
  4. 2 water molecules are formed

  Bonds involved

  1. Hydrogen bonds between nitrogenous bases (A=T, C≡G)
  2. Phosphodiester bond between 3' OH deoxyribose and 5' phosphate group - loss of 1 water molecule
• Sugar-phosphate backbone provides stability to the DNA
• DNA replication - Part 1

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

  4. Primase attaches to the unwound chain and catalyses the synthesis of RNA primer to provide free 3’OH ends for DNA polymerase III
• DNA replication - Part 3

  5. DNA polymerase III elongates the new daughter strand in the 5' to 3' direction

  6. Free deoxyribonucleotides are incorporated by complementary base pairing to the parental DNA strands
• DNA replication - leading and lagging strand

  7. Leading strand is synthesised continuously while lagging strand is synthesised discontinuously to form Okazaki fragments

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

  9. Nicks between the Okazaki fragments are filled in by DNA ligase by forming phosphodiester bonds between Okazaki fragments