Download Chapter 8 Gene Transfer in Bacteria Conjugation Hfr Cells

Gene Transfer in Bacteria
• Conjugation
– Bacterium to Bacterium gene transfer
Chapter 8
• Transformation
– Bacterium takes up naked DNA
Bacterial and Viral Genetic
Systems
• Transduction
– Bacterial virus carry DNA from one Bacterium
to another
Figure 8.9
Conjugation
Bacterium to Bacterium
(Lederberg and Tatum 1946)
Hfr Cells
• F Factor is integrated into the bacterial
chromosome
• Behave as F+ cells
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Figure 8.14
Figure 8.15
Should say “different alleles”
Mapping Through Interrupted
Conjugation
• Conjugation between Hfr and F- cells
Mapping Bacterial Genes
• Conjugation interrupted at selected
intervals
Conjugation Interuption
• Time required for gene transfer indicates
relative position
Figure 8.17
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Figure 8.18
Note the
different
orientations
of the F
Factor
Different Hfr strains
have different points of
integration of the F
Factor into the
bacterial chromosome.
Figure 8.18
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Problem Set 4B
• We can use interrupted conjugation data
to map bacterial genes.
Gene Transfer
and Antibiotic
Resistance
Gene Transfer and Antibiotic
Resistance
• Continuous use of antibiotics selects for
resistance
• Antibiotic resistance genes located on R
plasmids
• Some carry resistance to multiple antibiotics
• Can be transferred among bacterial species
Transformation
Transformation
• Cells capable of taking up DNA are called
Competent
Uptake of Naked DNA
• The DNA need not be bacterial.
• Cells that receive genetic material through
transformation are called Transformants
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Figure 8.21
Mapping Genes Through
Cotransformation
• Observe the rate at which two or more
genes are transferred together
(cotransformed)
• The rate of cotransformation is inversely
proportional to the distances between
genes.
Problem Set 4B
• We can use cotransformation data to map
bacterial genes.
Bacterial Genome Sequences
• The Institute for Genomic Research
(TIGR)
The DOE JGI can sequence
an entire bacterial genome
(8X coverage) in a day….
…assuming they wanted to.
– www.tigr.org
– Comprehensive Microbial Resource
• Contains:
– 168 complete genomes (18 Archaea, 150
Bacteria)
– 2 partial genomes
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Viral Genetics
A VIRUS is a simple
replicating structure
composed of nucleic acid
surrounded by a protein
coat.
T4 Bacteriophage
Viral Genomes
• DNA or RNA
• Circular or Linear
• Double or Single stranded
Phage Life Cycle
• Lytic cycle:
– Phage attaches to receptor on the cell
– Phage injects DNA into cell
– Phage DNA is replicated, transcribed, and
translated
– New phage particles assembled, cell lyses.
VIRULENT Phages
reproduce strictly
through the lytic
cycle and always kill
their host cells.
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Phage Life Cycle
Temperate Phage can
use either the lytic or the
lysogenic cycle
• Lysogenic cycle:
– Phage integrates into bacterial genome
(PROPHAGE)
– Prophage is replicated with bacterial genome
and passed along to daughter cells when
bacteria divide.
– Stimuli cause prophage to dissociate from
bacterial genome and enter the lytic cycle.
Figure 8.23
A Plaque
A lawn of bacterial cells
Gene Mapping in Phages
• Crosses between viruses with different
genotypes
• Recombinant progeny identified and
counted
• Example: Hershey and Rotman (1949),
T2 bacteriophage
Phage T2
Host Range
Plaque Morphology
• h+: infects Type B E.
coli, not B/2
• r+: small plaques
with fuzzy borders
• h-: infects both types
• r-: large abnormal
plaques with distinct
borders
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Figure 8.25
Transduction
Using Phage to Map
BACTERIAL Genes
Figure 8.26
Transduction
• Generalized Transduction:
– Any gene may be transferred
• Specialized Transduction:
– Only genes near particular sites on the
bacterial chromosome are transferred
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Figure 8.27
Figure 8.28: Mapping with transduction
Specialized Transduction
• Only genes near specific sites are
transferred.
• Requires lysogenic phage.
• See Figure 8.29 (If you are interested)
Complementation Test in Phage
• Heterozygotes can have the mutations in
TRANS (located on different
chromosomes) or in CIS (located on the
same chromosome).
• If the mutations are in TRANS and are in
the same locus (cistron):
– No protein is produced (mutant phenotype)
Figure 8.32
The problem set
frustrates and and
infuriates me!
E.coli strain K
Complementation
A functional gene
defined by the
complementation
test
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Viral Genomes
• First genome sequenced: ΦX174
– Overlapping genes!
Figure 8.33
Figure 8.33
RNA Viruses
•
•
•
•
Influenza
Colds
Polio
Aids
• Positive Strand RNA Viruses: genomic RNA
codes directly for protein
• Negative Strand RNA Viruses: complementary
copy of genomic RNA codes for protein
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