DNA Structure

• Chargaff discovered significant DNA variation between species, thus DNA is the genetic material.
• First rule base pairing

  Adenine [A] to Thymine [T]

  Guanine [G] TO Cytosine [C]
• Second Rule is the composition of DNA varies between species
• The DNA structure is a double stranded helix molecule with particular features.
• Components of the DNA:

  1. Phosphate
  2. Deoxyribose sugar
  3. Nitrogenous base
• Phosphate provides negative charge and links adjacent nucleotides
• Deoxyribose sugar is a 5 carbon sugar that forms the backbone of DNA molecule
• Nitrogenous base is [A], [T], [G] and [C]
• Monomeric units is oriented in antiparallel fashion. One strand runs 5' to 3' direction. the antiparallel fashion ensures nitrogenous bases on one strand pair.
• The nucleotide monomers are joined together with phosphodiester bonds to form polynucleotide - nucleic acid.
• Phosphodiester bonds are formed by a hydroxyl group (OH-) on the 3rd carbon on one of the nucleotides and reacts with the phosphate group attached to the 5th carbon on another nucleotide.
• The antiparallel strands from the double stranded helix, which is important for:

  • storage of genetic information
  • replication and transmission of genetic information
  • structural stability
• Watson Crick model

  Pairs of complementary bases form hydrogen bonds that hold the two strands of DNA double helix together. T - A pairs have two hydrogen bonds. C - G pairs have three hydrogen bonds.
• In the Watson Crick model, sugar phosphate backbone is on the outside, bases are on the inside. It is stabilised by hydrogen bonds and 2 polynucleotide strands are orientated in opposite directions.
• In the semi-conservative model of a replication, each of the DNA strands used is a template strand for synthesis of new strands because the body needs to make more cells.