7. Microbial Genetics

• Nucleotides

  Structural units of nucleic acids

  
• Parts of a nucleotide

  • A nitrogenous base
  • A pentose (five-carbon) sugar (ribose or deoxyribose)
  • A phosphate group (phosphoric acid)

  
• Nitrogenous bases

  • Purines: Adenine (A) and Guanine (G)
  • Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U)
• DNA

  • A molecule of DNA consists of two strands that form a double helix structure
  • Each DNA strand is composed of nucleotides
  • The sequences of nitrogenous bases on the two strands of a DNA molecule are complementary
  • The nitrogenous base pairs are joined by hydrogen bonds
  • The two strands of DNA are antiparallel

  
• RNA


  • RNA is also made up of nucleotide consisting of a 5-carbon sugar ribose, a phosphate group, and a nitrogenous base
  • RNA uses the sugar ribose instead of deoxyribose
  • RNA is generally single-stranded
  • RNA contains uracil in place of thymine

  
• Types of RNA


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

  
• mRNA

  Type of RNA generated from transcribing DNA, carries information for the translation of a particular protein

  
• rRNA

  Structural component of ribosomes
• tRNA

  Carries amino acids to the ribosome during translation to help build an amino acid chain

  
• Genotype

  The organism's genetic makeup – all its DNA – the information that codes for all the particular characteristics of the organism
• Phenotype

  Refers to actual, expressed properties (proteins)
• Origin of Replication


  Sequence of DNA at which replication is initiated on a chromosome, plasmid or virus

  
• Stages of Replication: Elongation

  1. Leading strand – continuous, one primer, DNA polymerase ; 5' to 3'

  2. Lagging strand –synthesized opposite to the fork movement; discontinuous, several primers, DNA polymerase; 5' to 3', Okazaki fragments

  
• Stages of Replication: Termination

  1. Forks converge until all intervening DNA is unwound

  2. Any remaining gaps are filled and ligated (DNA ligase)

  3. Replication proteins are unloaded

  
• Transcription

  1. RNA polymerase binds to the DNA at a site called the promoter

  2. RNA polymerase synthesize mRNA in the 5' – 3' direction

  3. RNA synthesis continues until RNA polymerase reaches a site on the DNA called the terminator

  
• Codon


  Groups of 3 nucleotides on mRNA that determine the sequence of amino acids in the protein being synthesized

  
• There are 61 possible codons but only 20 amino acids
• Third Base Degeneracy or Wobble Hypothesis

  Codons for same amino acid usually differ in 3rd position only

  
• Mutation


  Any heritable alteration in the base sequence of the genetic material

  
• Types of Mutation

  • Spontaneous mutation
  • Induced mutation

  
• Spontaneous mutation

  Occurs without external intervention, and most result from occasional errors in the pairing of bases by DNA polymerase during DNA replication

  
• Induced mutation


  Caused by agents in the environment and include mutations made deliberately by humans, results from exposure to natural radiation that alters the structure of bases in the DNA, or from a variety of chemicals that chemically modify DNA

  
• Base Substitution


  A single base at one point in the DNA sequence is replaced with a different base during replication, can either be (1) transition – purine to purine (A↔G) or pyrimidine to pyrimidine (C↔T) – or (2) transversion – purine to pyrimidine or vice versa

  
• Mutation


  The initial and final states of the organisms
• When established, mutation may be permanently present whether or not the conditions of development of the mutated organism allow their detection
• Mutation


  Can either be spontaneous or induced
• Induced mutation


  Caused by agents in the environment and include mutations made deliberately by humans
• Induced mutation results from exposure to natural radiation that alters the structure of bases in the DNA, or from a variety of chemicals that chemically modify DNA

  
• Base Substitution

  Also called as point mutation or base pair changes
• Base Substitution


  A single base at one point in the DNA sequence is replaced with a different base during replication

  
• Transition


  Purine to purine (A→G) or pyrimidine to pyrimidine (C→T)
• 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 in the protein product

  
• Nonsense mutation

  Changes a codon for an amino acid with a codon for chain termination (UAG, UAA, UGA)

  
• 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
• Deletion


  A mutation in which a region of the DNA has been eliminated

  
• Insertion


  Occurs when new bases are added to the DNA
• Mutagens


  Physical or chemical agents that changes the genetic material
• Physical mutagens


  High energy radiations that penetrate living cells