Bacterial Genetics - PowerPoint

							Bacterial Genetics (Ch18)
   Genetic diversity caused by
         Rapid reproduction
         Mutation
         Recombination


Bacteria
   – One of the simplest genetic model systems to study the
     mechanisms of molecular genetics


Escherichia coli (E. coli)
   – Intestinal flora
   – “lab rat” of molecular biology
Replication of bacterial genome

• One circular DNA genome
  – Single origin of replication (Ori)
  – Bidirectional DNA replication


• May have plasmids
  – smaller circular DNA molecules
     • Autonomously replicated (contain ori)
     •
• Bacteria divide by binary fission
• asexual reproduction
• Progeny are genetically identical to parent (clones)
     LE 18-14


 Bacterial DNA                                   Replication fork

 replication
                  Origin of
                  replication


Both genome
& plasmid replicate
in this manner




                                Termination
                                of replication
Bacterial DNA Mutation

Caused spontaneously (mistakes in DNA
 synthesis)
  – Physicochemical forces (UV, X rays,
    chemical mutagens, etc.)

•Since reproduction is quick e.g.doubling time=20
min


      New mutations spread quickly
   Calculation of incidence of mutations



 If doubling time= 20 min, then 23cells/hr
  Over 12 hr, 236 cells (~1010)produced from a single cell


If spontaneous mutation rate = 1 x 10-7 / gene,

then in 12 hr (day) (1010) (10-7)= 103 mutations/gene/day

If bacteria have ~4000 genes
        then (4x103genes)(103)= 4 x 109 mutations/day
Take home message:
 bacterial mutations rare per gene

But due to rapid cell division, become frequent




Major contribution to genetic diversity and
             ability to adapt
Another source of bacterial genetic diversity:

          Genetic Recombination

  • Three processes bring bacterial DNA
    from different individuals together:
     – Transduction
     – Transformation
     – Conjugation
Transduction

• Bacteriophages (bacterial viruses) transfer
  bacterial genes from one host cell to another
LE 18-16

                      Phage DNA

                      A+ B+




                   A+ B+


                           Donor
                           cell




                              A+


           Crossing
           over
                      A+
                  A- B–

                           Recipient
                           cell




                      A+ B–

             Recombinant cell
Transformation
 • Alteration of a bacterial cell’s genotype and
   phenotype
    – by the uptake of naked, foreign DNA from the
      surrounding environment


 •For example, harmless Streptococcus pneumoniae
 bacteria can be transformed to pneumonia-causing
 cells

 •Uptake of ampicillin resistant/ GFP-carrying plasmid
 (done in lab) (pGLO)
Conjugation


  • Direct transfer of DNA between live bacterial
    cells that are temporarily joined

  • Transfer one-way:
     – “Male” donates DNA
     – “Female” receives DNA
• “Maleness,”
  – Contains F (fertility) genes on plasmid or in genome

  – Encode sex pilus
     • Forms passage way for DNA from donor to recipient
LE 18-17




           Sex pilus   5 µm
• Donor cells containing the F plasmid: F+

• Recipient cells: F-

• Cells with F factor integrated into genome: Hfr cell
           (high frequency of recombination)

• Hfr cells
  – Transfer some genomic DNA to recipient cell
 LE 18-18_4
      F plasmid     Bacterial chromosome

    F+cell                                                         F+ cell
Mating
bridge
    F– cell                                                        F+ cell
                        Bacterial
                        chromosome
       Conjugation and transfer of F plasmid from and F+ donor to an F– recipient



              F+ cell                                                    Hfr cell

                                                      F factor


  Hfr cell


    Formation of Hfr (high frequency of recombination) cell
   F– cell

                    Temporary
                    partial                                      Recombinant F–
                    diploid                                      bacterium
      Conjugation and transfer of part of the bacterial chromosome from an
      Hfr donor to an F– recipient, resulting in recombiination
Questions???