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Bacterial Genetics
March 2015
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Terminology
Genetics:
The study of what genes are, how they
carry information, how information is expressed, and
how genes are replicated
Gene: A segment
of DNA that encodes a functional
product, usually a protein
Chromosome:
Structure containing DNA that
physically carries hereditary information; the
chromosomes contain the genes
Genome: All
the genetic information in a cell
– small circular DNA molecule that
replicates independently of the chromosome
Plasmid
+ DNA
Polymer
of nucleotides:
Adenine, thymine,
cytosine, and guanine
Double
helix associated
with proteins
"Backbone"
is
deoxyribose-phosphate
Strands
are held together
by hydrogen bonds
between AT and CG
Strands
are antiparallel
Figure 8.3b
+ DNA Synthesis
Figure 8.5
+ Replication of Bacterial DNA
Figure 8.6
+ Transcription
DNA is
transcribed to make RNA (mRNA,
tRNA, and rRNA)
Transcription
begins when RNA polymerase
binds to the promoter sequence
Transcription
proceeds in the 5' 3'
direction
Transcription
stops when it reaches the
terminator sequence
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The Process of Transcription
Figure 8.7
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Translation
mRNA
is translated in
codons (three
nucleotides)
Translation
of mRNA
begins at the start
codon: AUG
Translation
ends at
nonsense codons:
UAA, UAG, UGA
Figure 8.2
+ Simultaneous Transcription &
Translation
Figure 8.10
+ Genetic Transformation –
Frederick Griffith (1920s)
Figure 8.24
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Recombinant DNA Technology
Recombinant
DNA: DNA made from two
different sources.
In the 1960s, Paul Berg inserted animal DNA into bacterial
DNA, and the bacteria produced an animal protein
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Recombinant DNA Technology
1941:
George Beadle and Edward Tatum
showed that genes encode a cell’s enzymes
1944:
Oswald Avery, Colin MacLeod, and
Maclyn McCarty showed that DNA was the
hereditary material
1961:
Francois Jacob and Jacques Monod
discovered the role of mRNA in protein
synthesis
+ The Flow of Genetic Information
Figure 8.2
+ Genetic
Recombination
Vertical
gene transfer:
Occurs during
reproduction between
generations of cells.
Horizontal
gene
transfer: The transfer of
genes between cells of
the same generation.
+ Genetic Recombination
Figure 8.25
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Transmission of Genetic Material in
Bacterial Cells
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Conjugation
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Transformation
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Transduction
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3 main categories of genes
Structural
genes
• Code for
proteins
Regulatory
genes
• Control
gene
expression
RNA coding
genes
+ Operons: The Basic Concept
Operator
-the regulatory “switch”, a segment
of DNA
usually positioned within the promoter
– (DNA) includes the operator, the
promoter, and the genes that they control
Operon
In
coordinate control, a cluster of functionally
related genes can be controlled by a single onoff “switch”
+ Operons: Activation
Activator – (protein) when stimulated,
changes shape and binds to the promoter
to increase the affinity of RNA
polymerase to the promoter (increasing
the rate of transcription).
- The lac operon is stimulated by a small
organic molecule known as cAMP that is
produced in response to low cellular
glucose.
+ Operon: Lactose Metabolism
+ Lac Operon: Sensing Glucose vs.
Lactose
Bacterial will always prefer glucose over lactose
if it is available.
Thus, how does E. coli sense when glucose
levels are low?
How does it relay that information to the
genome?
+ Operons: Activation of Lac Operon
Lactose
present
No glucose
Lactose
and
Glucose present
Figure 8.15
+ Operons: Repression
– (protein) switches the operon off
Prevents transcription by binding to the
operator and blocking RNA polymerase
the product of a separate regulatory gene
can be in an active or inactive form, depending
on the presence of other molecules
Corepressor – cooperates with a repressor
protein to switch an operon off
Repressor
+ Operons: Relieving Repression
When
a repressor is bound to the operator,
RNA poly cannot bind the promoter and
transcription is turned “off”
When
the repressor is not bound to the
operator, RNA poly can bind the promoter and
transcription is turned “on”.
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Biotechnology
Recombinant
DNA technology, a new
technique for biotechnology, enables
bacteria and fungi to produce a variety of
proteins including vaccines and enzymes
Missing or defective genes in human cells
can be replaced in gene therapy
Genetically modified bacteria are used to
protect crops from insects and from
freezing
Figure 9.11
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pGLO Plasmid DNA
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Lac Operon vs Ara Operon
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Replacing Ara operon with GFP
gene
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Bacterial Conjugation
Figure 8.26
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Plasmids
Conjugative
plasmid: Carries genes
for sex pili and transfer of the plasmid
Dissimilation
plasmids: Encode
enzymes for catabolism of unusual
compounds
R
factors: Encode antibiotic resistance
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R Factor, a Type of Plasmid
Figure 8.29
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Conjugation in E. coli
Figure 8.27a
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Conjugation in E. coli
Figure 8.27b
+ Conjugation in E. coli
+ Transduction by a Bacteriophage
Figure 8.28