genetics

• Five key properties required of the genetic material:

  • Information carrier
  • Information must be deciphered by the organism to control its traits
  • Can be copied
  • Stable, but rare changes possible
  • Needs to be compact enough to be stored within cells but also accessible
• Historical background for identifying the genetic material:

  • 1869: Meischer discovers DNA
  • 1883: Weisman and Nageli identify 'genetic substance' as a chemical
  • 1884-5: Hertwig et al. identify chromosomes as carriers
  • 1928: Griffiths demonstrates transformation of bacteria with substance
  • 1944: Avery et al. confirm DNA as the genetic material
  • 1950: Chargaff establishes rules for base composition of DNA
  • 1953: Watson and Crick propose the double helix structure
• Griffith's experiment:

  • Injected live non-virulent bacteria and dead 'virulent' bacteria into mice
  • Recovered live virulent bacteria with genotype of the dead
  • Smooth colonies (S) with capsule present are lethal infections
  • Rough colonies (R) without capsule are harmless
  • Smooth bacteria make polysaccharide capsule, while rough bacteria lack it
  • Conclusion: Genetic material is not restricted to living things and is likely a chemical
• Avery, MacLeod & McCarty experiment:

  • Provided direct evidence for DNA as the genetic material by transforming bacteria
  • Different phenotypes observed in rough (R) and smooth (S) bacteria
  • Conclusion: DNA is the genetic material transferred from S to R bacteria
• DNA structure:

  • Primary structure: nucleotides composed of sugar, phosphate, and base
  • Secondary structure: double helix with antiparallel strands
  • Nucleotides have 3 components: sugar, phosphate, and base
  • Bases include purines (G, A) and pyrimidines (C, T in DNA, U in RNA)
  • Nucleotides linked by phosphodiester bonds in polynucleotide chains
  • Polynucleotide chains have a sugar-phosphate backbone with 5' to 3' polarity
• Determining DNA secondary structure:

  • Influenced by Chargaff's rules on base composition, X-ray diffraction studies, and model building
  • Watson and Crick proposed the double helix model in 1953 based on antiparallel strands
• 1. ray diffraction revealed the double helical structure of DNA and the physical dimensions of the helix
• Key experiments by Rosalind Franklin and Maurice Wilkins:
• Directed X-ray beam at DNA molecules oriented perpendicular to the beam
• 1. rays are short wavelength and are diffracted by atoms in the DNA
• Intense spots on a film reveal where constructive interference has occurred due to diffraction by regular repeating units in a crystal of DNA
• Photo 51, May 2nd, 1952, by Franklin and her PhD student, Raymond Gosling
• Watson & Crick used Franklin's data, along with other data, to propose the double helical structure of DNA
• Watson and Crick built models based upon Franklin's X-ray diffraction patterns, Pauling's data, revised structures of the bases, and Chargaff's rules
• Properties of the double helix:
• Two polynucleotide chains twisted around a common axis to form a right-handed helix
• Sugar-phosphate backbone on the outside, bases in the center
• Bases are flat and stack on top of each other
• Two strands are antiparallel, one runs 5' - 3', the other 3' - 5'
• Hydrogen bonds between bases: G-C has 3 H bonds, A-T has 2 H bonds
• Conventions for writing DNA sequences:
• Write sequence using initials for the nucleotides
• If you know the sequence of one strand, you can infer the other
• Write from 5' to 3' (assume 5' to 3' unless labeled otherwise)
• RNA structure:
• Similar to DNA but has a different sugar - ribose
• RNA has uracil instead of thymine
• RNA is usually single-stranded but base pairing within a single strand can form complex secondary structures
• Ribonucleotides can base pair with other nucleotides within the same strand
• Ribonucleotides can pair with deoxyribonucleotides
• Cytosine (C) in DNA can be deaminated to uracil (U), which is less damaging in RNA
• RNA has a 2' OH group on the sugar, making it less stable than DNA
• RNA structure is important for processes like transcription
• DNA structure immediately suggests how it is replicated
• Relationship between the template and the newly synthesized DNA:

  • Conservative
  • Semi-conservative
  • Dispersive
• E. coli is the model system for studying replication
• Complementarity provides the basis for replication
• Requires separation of the two strands
• Following the rules of base-pairing, one strand can act as the template for the synthesis of a new complementary strand
• 3 possible models for DNA replication: semiconservative, conservative, or dispersive