DNA structure

• RNA is ribonucleic acid
• DNA is deoxyribonucleic acid

  • in chromosomes of nucleus
  • consists of 2 strands twisted around each other to form a double helix
  • monomers of DNA: nucleotide
• nucleotide

  • made up of sugar, phosphate group and nitrogenous base => adenine, cytosine, guanine and thymine/uracil
  • DNA: ACGT, RNA: ACGU
  • formed through condensation reaction with formation of 2 water molecules
• DNA characteristics

  1. anti-parallel (2 strands of DNA molecule run in opposite directions of each other)
  2. complementary base pairing (A:T 2, C:G 3)
• DNA replication

  1. Separation of DNA strands

  2. Synthesis of RNA primer

  3. Synthesis of new daughter strands
• DNA replication takes place

  In the nucleus during interphase before nuclear division
• Separation of DNA strands

  1. Begins at the origin of replication (ori)

  2. Helicase unwinds the double helix by breaking the hydrogen bonds between complementary base pairs in parental strands

  3. Each parental strand acts as template for synthesis of new DNA strand
• Synthesis of RNA primer

  Primase attaches to the unwound chain and catalyses synthesis of RNA primer to provide free 3 OH ends for DNA polymerase III
• Synthesis of new daughter strands

  1. DNA polymerase III elongates new daughter strand in 5' to 3' direction by catalysing phosphodiester bond formation between incoming deoxyribonucleotides and free 3'OH end of daughter strand

  2. Free deoxyribonucleotides are incorporated by complementary base pairing to parental DNA strands

  3. In replication fork, leading strand is synthesized continuously while lagging strand is synthesized discontinuously to form Okazaki fragments

  4. Rna primers are removed and replaced by deoxyribonucleotides by DNA polymerase I

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