Microbial Genetics

• Nucleic Acids


  Structural units are nucleotides
• Parts of a nucleotide

  • Nitrogen-containing base
  • Pentose (five-carbon) sugar
  • Phosphate group
• Nitrogen-containing bases


  • Adenine (A)
  • Thymine (T)
  • Cytosine (C)
  • Guanine (G)
  • Uracil (U)

  
• Purines


  A and G
• Pyrimidines


  C, T, and U
• DNA


  Deoxyribose sugar
• RNA


  Ribose sugar
• Structure of DNA


  • Two strands forming a double helix
  • Nucleotides with complementary nitrogenous base pairs joined by hydrogen bonds
  • Antiparallel strands

  
• Structure of RNA


  • Generally single-stranded
  • Contains uracil instead of thymine

  
• Types of RNA


  • mRNA (messenger RNA)
  • rRNA (ribosomal RNA)
  • tRNA (transfer RNA)

  
• Central Dogma of Molecular Biology

  • DNA contains genetic information
  • Proteins are formed using the genetic code of DNA
  • Conversion of DNA to RNA is essential to form proteins
  • Genetic information flows from DNA → RNA → Protein

  
• Genotype


  Organism's genetic makeup - all its DNA

  
• Phenotype


  Actual, expressed properties (proteins)
• Universal Features of DNA Replication

  • Semi-conservative mode
  • Watson and Crick base pairing maintained
  • DNA synthesized in 5' to 3' direction
  • Primer needed for initiation
  • Complex process involving several enzymes and proteins

  
• Origin of Replication


  Sequence of DNA at which replication is initiated

  
• Replication Fork


  The point at which replication actively occurs
• DNA Replication


  1. Initiation

  2. Elongation

  3. Termination

  
• Transcription


  Synthesis of a complementary strand of RNA from a DNA template

  
• Transcription


  1. RNA polymerase binds to DNA at promoter

  2. RNA polymerase synthesizes mRNA in 5' to 3' direction

  3. Transcription continues until reaching terminator

  
• Translation


  Also known as protein synthesis, from mRNA to protein

  
• Genetic Code


  61 possible codons but only 20 amino acids, leading to codon redundancy and the Wobble Hypothesis

  
• Translation


  1. Ribosome binds to mRNA

  2. Ribosome matches tRNA anticodon to mRNA codon

  3. Amino acids added to polypeptide chain

  4. Ribosome releases polypeptide at stop sequence

  5. Polypeptide folds into native shape

  
• Mutation


  Any heritable alteration in the base sequence of genetic material

  
• Types of Mutation


  • Spontaneous Mutation
  • Induced Mutation
• Spontaneous Mutation


  Occur without external intervention, mostly from errors in DNA replication
• Induced Mutation


  Caused by environmental agents that alter DNA structure

  
• Types of Base Substitution/Point Mutation

  • Transition (purine to purine or pyrimidine to pyrimidine)
  • Transversion (purine to pyrimidine or vice versa)

  
• Missense Mutation


  Changes a codon for one amino acid to a codon for another amino acid, resulting in an amino acid substitution

  
• Mutation


  Physical or chemical agents that change the genetic material
• Types of mutation


  • Base substitution / point mutation
  • Frameshift mutation
  • Deletion
  • Insertion

  
• Base substitution / point mutation


  • A single base at one point in the DNA sequence is replaced with a different base during replication
  • Can be transition (purine to purine or pyrimidine to pyrimidine) or transversion (purine to pyrimidine or vice versa)

  
• Consequences of base pair changes

  • Missense mutation (changes a codon for one amino acid to a codon for another amino acid)
  • Nonsense mutation (changes a codon for an amino acid with a codon for chain termination)
  • Silent mutation (a change in codon composition that has no effect on the resulting polypeptide)

  
• Frameshift mutation


  Adds or deletes one or two bases (or any non-multiple of 3) from a coding sequence in a DNA, so that the genetic code is read out-of-phase

  
• Frameshift mutation results in incorrect amino acid or premature termination and severe phenotypic effects

  
• Deletion


  Mutation in which a region of the DNA has been eliminated

  
• Insertion


  Occurs when new bases are added to the DNA
• Physical mutagens


  • Electromagnetic radiations (gamma rays, X-rays, ultraviolet rays)
  • Particulate radiations (alpha particles, beta particles, neutrons)
• Electromagnetic radiations


  • Gamma rays and X-rays (ionizing radiation) can cause direct effect of single or double stranded breaks in the DNA molecules, and indirect effect of free radicals creating harmful chemical reactions
  • Ultraviolet rays (non-ionizing radiation) can form pyrimidine dimers, some of which escape repair and inhibit replication and transcription
• Particulate radiations


  • Beta particles have greater penetrating power than alpha particles due to smaller size
  • Neutrons are extremely penetrant and can cause severe damage to living tissues and genetic material
• Chemical mutagens


  • Deaminating agents (e.g. nitric oxide, nitrous acid)
  • Base analogs
  • Alkylating agents (e.g. MMS, EMS, MNNG)
  • Intercalating agents (e.g. acridine derivatives, ethidium bromide)