8
The Genetics of
Bacteria and Their
Viruses
Plasmids
• Plasmids are circular DNA molecules
which replicate independently of the
bacterial chromosome
• Plasmids often carry antibiotic
resistance genes transferred to
recipient cells by transformation
• Plasmids are used in genetic
engineering as gene transfer vectors
Conjugation
• Conjugation = process in
which DNA is transferred
from bacterial donor cell (F+) to a
recipient cell (F-) by cell-to-cell contact
• F (fertility) factor = plasmid transferred by
conjugation
• F factor = episome = genetic element that
can insert into chromosome or replicate
as circular plasmid
Transposable Elements
• Transposable elements = DNA sequences
present in multiple copies which are
capable of movement within the genome
• Insertion (IS) elements = mobile elements
contain transposase = enzyme catalyzes
IS element transfer
• Transposons = IS elements which contain
bacterial genes
Transposable Elements
• Transposons can insert into plasmids which
can be transferred to recipient cells by
conjugation
Transposable elements are flanked by inverted
repeats and often contain multiple antibiotic
resistance
genes= R
plasmids
Bacterial Genetics
Types of bacterial mutants:
• Antibiotic-resistant mutants
• Nutritional mutants: wildtype=prototroph
mutant=auxotroph which cannot grow in
minimal media providing basic nutrients
only
• carbon-source mutants=cannot use some
carbon sources
Bacterial Transformation
• Recipient cells acquire genes from DNA
outside the cell
• DNA is taken up by cell and often
recombines with genes on bacterial
chromosome
• Bacterial transformation showed that DNA
is the genetic material
• Transformation may alter phenotype of
recipient cells
Cotransformation of Linked
Genes
• Donor DNA which contains genes located
close together are often transferred as a unit
to recipient cell = cotransformation
• The greater the
distance between
genes the less likely
they will be transferred
as a unit to recipient cell
• Cotransformation is
used to map gene order
Hfr
• Hfr (high frequency
recombination) exchange
between donor cells F+ and
few cells where F factor
integrated into bacterial
chromosomes
• Recombination inserts
Hfr genes into
chromosome
Chromosome Mapping
• Time-of-Entry mapping =
method of mapping genes
by Hfr X F- matings using
interrupted mating
technique
• A plot of time (minutes) versus # of
recombinants is used to map genes as
transfer order map
• Circular genetic map of E. coli shows map
distances of genes in minutes
Transduction
• Transduction = bacterial DNA fragment is
transferred from one
bacterial cell to another
by a virus (phage)
containing bacterial DNA
= transducing phage
• Generalized transducing
phage = transfers DNA
derived from any part of
the bacterial chromosome
Transduction
• Phage P1 cuts bacterial chromosome into
pieces and can package bacterial DNA
into phage particles
• Transducing particle will insert
‘transduced” bacterial genes into recipient
cell by infection
• Transduced genes may be inserted into
recipient chromosome by homologous
recombination
Transduction
• Specialized transducing phage = particles
contain phage and bacterial genes from a
specific point of bacterial chromosome
• Cotransduction can be
used to demonstrate
linkage between
bacterial genes
• Frequency of
cotransduction is a
measurement of linkage
Transduction
• Specialized transducing phages transduce
bacterial genes at the site of prophage
insertion into the bacterial chromosome
• Transduction of bacterial genes occurs by
aberrant excision of viral DNA which
results in the incorporation of bacterial
genes into phage chromosome
Temperate Bacteriophages
• Temperate bacteriophages have two life
cycles: lytic cycle=infection which results
in production of progeny phage and
bacterial cell lysis and lysogeny = nonproductive viral infection results in
insertion of viral DNA into bacterial
chromosome
• Viral DNA integration= site-specific
insertion into bacterial chromosome
Lysogenic Bacteriohages
• In the lysogenic cycle, the viral DNA
integrated into the bacterial chromosome is
called a prophage
• Lysogen=bacterial cell containing integrated
prophage
• Integration is catalyzed by a viral enzyme=
integrase which
carries out sitespecific recombination
between the virus and
bacterial cell
Lysogenic Bacteria
• Prophage induction=
excision of prophage from
bacterial chromosome and
entry to lytic cycle
• Prophage induction results
from damage to the
bacterial chromosome
by chemicals or radiation
• Excisionase=viral enzyme which removes
prophage by site-specific recombination