PPT2.1: DNA Structure

• DNA

  Deoxyribonucleic acid, the genetic material of all living things and viruses

  
• Gene

  Definition not provided
• Chromosome

  Definition not provided
• Nucleic acids are the information molecules of cells and the genetic code found in all organisms
• Types of nucleic acids

  • DNA (deoxyribonucleic acid)
  • RNA (ribonucleic acid)

  
• Monomer

  Smaller building blocks that make up macromolecules (polymers)
• Nucleotide

  Consists of a sugar molecule, a phosphate group, and one of four different nitrogenous bases: adenine (A), cytosine (C), guanine (G), and thymine (T) or uracil (U)

  
• Formation of a nucleotide

  1. Nitrogenous base + pentose sugar + phosphate = Nucleotide

  2. Formed through condensation reaction, with formation of 2 water molecules

  
• Deoxyribonucleotides are the building blocks of DNA

  
• Phosphodiester bond

  A covalent bond between the 5'-phosphate group of one nucleotide and the 3'OH group of another nucleotide, formed by a condensation reaction with loss of a water molecule
• Formation of a dinucleotide

  Phosphodiester bond forms between two nucleotides

  
• Nucleic acid (polynucleotide)

  Macromolecules formed by the combination of nucleotides through condensation reactions, with a backbone of alternating sugar and phosphate molecules and bases attached to the sugars

  
• The unique sequence of bases in nucleic acids carries coded information

  
• Chargaff's ratio

  • The ratio of A:T and G:C in DNA samples is always close to 1
  • No. of G ≈ No. of C
  • No. of A ≈ No. of T
  • No of bases differed from species to species

  
• Complementary base pairing

  • Adenine (A) bonds with Thymine (T) (2 hydrogen bonds)
  • Guanine (G) bonds with Cytosine (C) (3 hydrogen bonds)

  
• Anti-parallel

  DNA strands are arranged with one chain running from 5' to 3' and the other from 3' to 5'
• DNA replication

  1. Separation of DNA strands

  2. Synthesis of RNA primer

  3. Synthesis of new daughter strand(s)

  4. Removal of RNA primers and replacement with deoxyribonucleotides

  5. Filling in of nicks between Okazaki fragments

  
• DNA replication

  • It is an extremely accurate process as DNA carries the genetic message
  • Replication occurs in the nucleus during interphase, before nuclear division
  • Strands of the DNA double helix are built up individually from free nucleotides
  • DNA replication is bidirectional

  
• The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously to form Okazaki fragments