genetics ch 7 and 12

• Nucleoid


  A region that contains the genetic material which lacks clear boundaries
• Escherichia coli


  • Found in lower intestines of warm-blooded animals
  • Produces vitamin K2 which reduces risk of cancer & heart disease, supports immune system & bone health
• Viruses

  • Inert particles if no host
  • Capsid (protective coat) encloses viral genetic material
  • Only "living" when their genetic material directs their own multiplication
• Bacteriophages


  Viruses infecting bacterial cells
• Lytic Cycle of a Bacteriophage

  1. Phage particle attaches to host bacterium

  2. Injects nucleic acid into bacterial cell
• Useful bacterial mutants

  • Antibiotic-resistant Mutants
  • Nutritional Mutants
  • Carbon-source Mutants
• Antibiotic-resistant Mutants

  • Grow in streptomycin (Str) and tetracycline (Tet)
  • Wildtype (prototroph)
  • Str-r = resistant ; Str-s = sensitive
• Nutritional Mutants

  • Unable to utilize nutrients unless the required nutrient is supplied in the medium
  • Arginine auxotroph (Arg-) only grows in a medium with Arg
  • Adenine prototrophs (Ad+)
• Carbon-source Mutants


  • Cannot utilize sources of carbon (energy)
  • Lac- cannot utilize lactose for growth
• Phenotype in Bacteria

  • 3 letters, first is capitalized
  • (+) : the presence of a designated character
  • (-) : absence
  • s : sensitivity
  • r : resistance
• Genotype in Bacteria

  3 letters, lowercase, italicized
• Genetic transfer in bacteria

  • Bacterial Transformation
  • Conjugation
  • Transduction
• Bacterial Transformation

  DNA molecule is taken up from an external environment and incorporated into the genome
• Free DNA can become available in the soil by lysis (spontaneous breakage) of donor cells
• Frequency of transformation

  • 1 transformed cell per 103 cells = 10-3
  • 2 genes far in the chromosome (contained in 2 DNA fragments) - 10-3 x 10-3 = 1 transformant per 106 cells
  • 2 genes near each other (contained in 1 DNA fragment) - 10-3 = 1 cotransformant per 103 cells
• Cotransformation of two genes at a frequency substantially greater than the product of the single-gene transformation frequencies implies that the two genes are close together in the bacterial chromosome
• Conjugation (Bacterial Mating)

  • Plasmids - non-essential, DNA that exists inside bacteria and replicates independent of bacterial genome
  • F Plasmid - mediates conjugation between E. coli, F+ is donor with F ("fertility") factor, F- is recipient with no F factor, F factor is 100 kb and exists as free in a cell or integrated in the chromosome (episome)
• Joshua Lederberg (1951) discovered conjugation in E. coli
• Conjugation

  1. Physical contact between donor cell and recipient cell

  2. F plasmid DNA moves through a pore in the membrane from donor to recipient

  3. Transfer is always accompanied by replication of the plasmid

  4. Contact between F1 and F2 cell initiates rolling-circle replication that results in the transfer of a single-stranded linear branch of the rolling circle to the recipient cell

  5. Transfer of 1 strand of F factor to F- cell

  6. DNA is synthesized in both F+ & F- cells

  7. Circularization of new DNA strand in F- cell

  8. Connecting bridge breaks apart

  9. Both cells contain F factor and can act as donors
• High frequency of recombination (Hfr) Cells

  Integration of F factor by recombination between a nucleotide sequence in F plasmid & a homologous sequence in the bacterial chromosome
• Hfr x F- mating

  1. Coming together between donor (Hfr) and F- cells

  2. Rolling-circle replication in donor cell follows

  3. Transfer of ss-DNA into the recipient (Part of F factor -> Chromosomal genes -> final part of F factor)

  4. ss-DNA is converted into ds-DNA in the recipient

  5. The conjugating cells break apart long before the entire bacterial chromosome is transferred, the last part of the F factor is not transferred

  6. Recombination between Hfr fragment & the F- chromosome follows
• Interrupted-mating Technique


  • In an Hfr x F- cross, a technique by which donor & recipient cells are broken apart at specific times, allowing only a particular length of DNA to be transferred
  • Done by violent agitation of the suspension of mating cells
  • The number of recombinants of any particular allele increases with the time during which the cells are in contact
• Interrupted-mating Technique

  1. Mixing of cells

  2. Agitating of samples violently

  3. Plating on a series of media with streptomycin & different combinations of the 5 substances A through E
• Time-of-Entry Mapping

  • The number of recombinants in each curve increases with length of time of mating
  • For each marker, there is a time (the time of entry) before which no recombinants are detected
  • Each curve has a linear region that can be extrapolated back to the time axis, defining the time of entry of each gene
  • The number of recombinants of each type reaches a maximum, the value of which decreases with successive times of entry
• Due to circularity of the E. coli chromosome in F- cells and the many points of integration of the F plasmid, both the DNA molecule and the genetic map are circular
• The entire chromosome requires 100 minutes to be transferred (it usually breaks first), so the total map length is 100 minutes
• Generalized Transduction

  Bacterial DNA is transferred from one bacterial cell to another by a phage particle (transducing phage) containing the DNA
• Generalized Transduction

  1. E. coli phage P1 infects a bacterium

  2. P1 DNA directs the host cell to make more DNA

  3. P1 phage makes nuclease that cuts the bacterial DNA into fragments

  4. New P1 phages are formed with phage DNA

  5. Occasional phage carries bacterial DNA (arg+)

  6. Transducing phage infects new host cell & injects the bacterial DNA (arg+) into a recipient bacterium (arg-)

  7. Recombination occurs & yields a transductant bacterium (arg+), Arg- fragment is digested by nucleases
• Cotransduction

  Depends on how close the genes are
• Two types of transducing phages: Specialized transducing phages (can transduce only certain bacterial genes) and Generalized transducing phages (transduce any gene)
• Formation of an F' lac Plasmid (merodiploids / partial diploids)


  • Breakage and reunion are between nonhomologous regions
  • They render any recipient bacteria diploid for the region of the chromosome they carry
  • Allow dominance tests and gene-dosage tests
• Transduction

  1. Specialized transducing phages can transduce only certain bacterial genes

  2. Generalized transducing phages transduce any gene
• Formation of an F' lac Plasmid (merodiploids / partial diploids)

  1. Aberrant excision of F from an Hfr chromosome

  2. Breakage and reunion are between nonhomologous regions
• F' plasmids

  They render any recipient bacteria diploid for the region of the chromosome they carry
• Importance of diploid regions

  • Allow dominance tests
  • Allow studies on gene-dosage tests
• Lysogenic Life Cycle of E. coli phage λ

  1. Phage injects its nucleic acid into a bacterium

  2. Ends of λ ss-DNA are cohesive & complementary in sequence so they can pair forming a circular molecule

  3. Circular λ DNA & the E. coli DNA undergo site-specific recombination

  4. Integrase catalyzes the recombination event

  5. Integration of the λ DNA (prophage) into the bacterial DNA (lysogen)
• Phage types

  • Lytic phages
  • Temperate phages (undergo lysogenic cycle)
  • Virulent phages (undergo lytic cycle only)
• Transposable Elements (TEs)


  DNA sequences capable of moving from one location to another in a genome
• Transposon


  A TE that has bacterial genes, e.g. antibiotic resistance genes
• Transposons (bacterial TEs)

  • Can insert randomly at potential target sites in the genome
  • Many transposons code for their own transposase