LEC #6

• Microbial Genetics


  The study of the mechanisms of heritable information in microorganisms (bacteria, archaea, viruses and some protozoa and fungi)
• Genotype


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


  The actual, expressed properties (proteins)
• Structural units of nucleic acids

  • Nucleotides
• Parts of a nucleotide


  • Nitrogenous base
  • Pentose (five-carbon) sugar (ribose or deoxyribose)
  • Phosphate group (phosphoric acid)
• Nitrogenous bases


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


  Made up of nucleotides consisting of a 5-carbon sugar ribose, a phosphate group, and a nitrogenous base
• Differences of RNA from DNA

  • 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)
• DNA


  A molecule consisting of two strands that form a double helix structure, composed of nucleotides with a sugar (deoxyribose), a phosphate group, and a nitrogenous base
• DNA structure


  • The sequences of nitrogenous bases on the two strands are complementary
  • The nitrogenous base pairs are joined by hydrogen bonds
  • The two strands of DNA are antiparallel
• Central Dogma of Molecular Biology


  The genetic information flows from DNA → RNA → Proteins
• DNA replication


  The process by which DNA makes a copy of itself during cell division
• Universal features of DNA replication


  • Semi-conservative mode
  • Watson and Crick base pairing maintained
  • DNA is synthesized in the 5' to 3' method
  • A primer is needed for initiation
  • A complex process involving several enzymes and proteins
• Stages of DNA replication

  1. Initiation

  2. Elongation

  3. Termination

  
• Transcription


  The synthesis of a complementary strand of RNA from a DNA template
• Steps in transcription


  1. RNA polymerase binds to the DNA at a promoter

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

  3. RNA synthesis continues until RNA polymerase reaches a terminator

  
• Translation


  Also known as protein synthesis, from mRNA to protein

  
• Genetic code


  There are 61 possible codons but only 20 amino acids, so most amino acids are signaled by several alternative codons. The third base degeneracy or wobble hypothesis states that codons for the same amino acid usually differ in the 3rd position only.
• Steps in translation


  1. The ribosome binds to mRNA at a specific area

  2. The ribosome starts matching tRNA anticodon sequences to the mRNA codon sequence

  3. Each time a new tRNA comes into the ribosome, the amino acid it was carrying gets added to the elongating polypeptide chain

  4. The ribosome continues until it hits a stop sequence, then it releases the polypeptide and the mRNA

  5. The polypeptide forms into its native shape and starts acting as a functional protein in the cell
• Mutation


  Any heritable alteration in the base sequence of the genetic material
• Types of mutation


  • Base substitution
  • Insertion
  • Deletion
  • Frameshift
• Base substitution


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


  • Missense mutation
  • Nonsense mutation
  • Silent 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

  
• Types of Mutation


  • Base Substitution
  • Frameshift mutation
  • Deletion
  • Insertion
• Base Substitution


  • Also called as point mutation or base pair changes
  • 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
• Consequences of Base Substitutions

  • Missense mutation
  • Nonsense mutation
  • Silent mutation

  
• Missense mutation


  • Changes a codon for one amino acid to a codon for another amino acid
  • Results 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
  • Consequences: (a) incorrect amino acid or premature termination and/or (b) severe phenotypic effects
• 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


  • Electromagnetic radiations
  • Particulate radiations
• Electromagnetic radiations


  • Gamma rays, X-rays and ultraviolet rays
  • Penetration power is inversely proportional to their wavelength

  
• Ionizing radiations (Gamma rays and X-rays)


  • Direct effect: Single or double-stranded breaks in the DNA molecules
  • Indirect effect: Free radicals created; form compounds, HO2 – initiate harmful chemical reactions within the cells; can lead to cell death
• Non-ionizing radiations (UV rays)


  Formation of pyrimidine dimers – most are immediately repaired, but some escape repair and inhibit replication and transcription