MODULE 4

• Microbial Genetics is the study of the mechanisms of heritable information in microorganisms.
• Central Dogma of Molecular Biology mainly involves the conversion of DNA encoded information into RNA, that is then essential to form proteins.
• Gene transfer mechanisms in prokaryotes can either be via vertical gene transfer (movement of genetic material by descent) or horizontal gene transfer (movement of genes between cells that are not direct descendants of one another).
• Structural units of nucleic acids are the nucleotides.
• Nucleotides


  Structural units of nucleic acids
• Parts of a nucleotide

  • Nitrogen-containing base (adenine, thymine, cytosine, guanine, uracil)
  • Pentose (five-carbon) sugar (deoxyribose, ribose)
  • Phosphate group
• Purines


  Adenine (A) and guanine (G)
• Pyrimidines


  Cytosine (C), thymine (T), and uracil (U)
• DNA


  • Molecule consists of two strands that form a double helix structure
  • Each strand is composed of nucleotides
  • Sequences of nitrogenous bases on the two strands are complementary
  • Nitrogenous base pairs are joined by hydrogen bonds
  • Two strands are antiparallel
• RNA


  • Made up of nucleotides consisting of ribose, a phosphate group, and a nitrogenous base
  • 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 the complete genetic information that defines the structure and function of an organism
  • Proteins are formed using the genetic code of the DNA
  • Conversion of DNA encoded information to RNA is essential to form proteins
  • Genetic information flows from DNA → RNA → Protein
• Genotype


  The organism's genetic makeup - all its DNA
• Phenotype


  Refers to actual, expressed properties (proteins)
• Chromobacterium violaceum has the vioC gene, expression of which leads to the production of violacein pigment
• DNA Replication


  • Semi-conservative mode
  • Watson and Crick base pairing maintained
  • Synthesized in the 5' to 3' direction
  • Requires a primer
  • Complex process involving several enzymes and proteins
• Origin of Replication


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


  1. DNA gyrase and topoisomerases relax supercoiling ahead of the replication fork

  2. The two strands of parental DNA are unwound by helicase

  3. Primers signal the starting point of DNA replication, synthesized by primase
• DNA Replication Elongation


  1. Both parental strands serve as template

  2. DNA polymerase synthesizes only at the 5' to 3' direction

  3. DNA replication is biredirectional
• DNA 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


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


  1. RNA polymerase binds to the DNA at the promoter

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

  3. RNA synthesis continues until RNA polymerase reaches the 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 (Third Base Degeneracy / Wobble Hypothesis)
• In prokaryotic cells, the translation of mRNA into protein can begin even before transcription is complete (Co-transcription translation)
• Translation Steps


  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 new tRNA adds its amino acid 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


  • Spontaneous Mutation
  • Induced Mutation
• Spontaneous Mutation


  Occur without external intervention, most result from errors in base pairing by DNA polymerase during replication
• Induced Mutation


  Caused by agents in the environment, including natural radiation and chemicals that modify DNA
• Types of Base Substitution / Point Mutation

  • Transition - Purine to purine or pyrimidine to pyrimidine
  • 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, resulting in an amino acid substitution in the protein
• Spontaneous mutation


  Occur 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, result 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/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
• Types of base substitution/point mutation

  • Transition - Purine to purine or pyrimidine to pyrimidine
  • 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
• Deletion


  Mutation in which a region of the DNA has been eliminated