Download Bacterial Genetics

L9 4-15
Bacterial Genetics
Bacteria’s genome contains:
1. One chromosome: circular with few associated proteins
2. Plasmids: smaller rings of circular DNA; not part of
chromosome, so they are called accessory genes.
- Self-replicating
- Not necessary for the survival of the bacteria under
normal conditions.
- Can help bacteria if living in stressful conditions.
Recombination of bacteria:
[Note: In Eukaryotes = meiosis – fertilization – zygote =
recombinant DNA]
In Prokaryotes:
 Transformation: uptake of foreign DNA from surroundings
- Bacteria have cell surface receptors that recognize and
transport DNA from closely related species.
- Result = recombinant; chromosome from two different
cells.
- Can alter bacteria’s genotype and phenotype.
NOTE: THIS IS WHAT OUR NEXT LAB IS BASED
ON…KNOW THIS!
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 Transduction: transfer of DNA of one bacteria to another
by bacteriophages (phages) as a result of phage’s life cycle
Part of host DNA packaged into new “virus” capsid
- phage injects its DNA into bacteria (host #1)
- Host DNA is fragmented
- Phage uses host DNA to make its own proteins and
replicate its DNA
- Host DNA is packaged into new viral capsid (not on
purpose)
- Lysis of host cell releasing many phages with host’s (#1)
DNA
- Phage with host’s #1 DNA infects bacterial host #2
- Recombination of host #1 and #2 DNA
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 Conjugation (bacterial “sex”): transfer of DNA through two
bacteria temporarily joined via sex pilli (like a grappling
hook)
- “F” = “Fertility”
- F+ cells = have F+ factor/gene carried on plasmid to
form sex pilus and transfer DNA.
- When “mating” of F+ cells and F- cells occurs, only F
plasmid DNA is transferred and (not the chromosome)
and the F- cell becomes an F+ cell.
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Control of Protein Production (in Eukaryotes)
How much protein do we want synthesized? Don’t want to be
wasteful!
Modes of Regulation
1. Transcription
- transcription factors (proteins) = “glue dot”
- processing (cutting & pasting exons)
- stability of mRNA in the cytoplasm
2. Translation
- protein initiation factors = “glue dot”
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Let’s focus on the Transcriptional level of regulation:
o Transcriptional control of mRNA production; happens on two
levels:
1. (Pro. & Euk) Adjust the activity of enzymes already present
by feedback inhibition. In general, feedback mechanisms
operate by a(an)
- Lack of substance turns on genes that code for enzymes
to synthesize that substance. (positive feedback)
- Abundance of substance will turn off genes that code
for enzymes to build substance. (negative feedback)
(see page 353 fig. 18.20)
2. (Pro. & Euk) Adjust the amount being made of certain
enzymes by regulating expression of genes coding for
enzymes
Prokaryotes only
- structural genes ("SG"): code for enzymes, structural
proteins
- regulatory genes: control the output of structural genes
Regulatory examples – using Operon Models
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 Operons
a group of genes associated with other
segments of DNA regulated as a whole by regulatory gene (not
part of operon)
 The regulatory gene controls a part of the operon know as the
operator gene = single “on/off” switch for the SG’s collectively
Operons = operator + promoter + (few to many) SG
 The lac operon of E. coli:
Regulates the hydrolysis of lactose (disaccharide) - catabolic
 Prokaryote example of INDUCIBLE OPERON = usually
turned off by a repressor protein and needs to be induced
(turned on) by specific molecule (lactose).
 E. coli in human’s digestive track; humans drink milk = E. coli
goes to work = lac operon is induced
- SG's for related enzymes adjacent on chromosome are
activated == enzymes made
- lactose metabolism uses these three enzymes
• operator gene next to 3 SG's for lactose metabolism
• promoter genes adjacent to operator
• the operator and promoter do not code for mRNA
Op
 This is the lac operon [ P + Op + SG1 + SG2 + SG3 ]
 In the presence of lactose:
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- lactose binds to the repressor protein
- preventing repressor protein from binding to the operator
- so transcription can continue unblocked
 This is an inducible operon:
- it is induced to transcribe
- by the presence of a molecule (lactose)
- requiring enzymes for its metabolism
o RNA Pol II attaches to promoter
o synthesis of all three SG's starts
 a regulatory gene is located close by
o this produces a repressor protein
o which attaches to the operator
o blocking RNA Pol II, preventing transcription
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 The tryp operon of E. coli :
 Example of REPRESSIBLE OPERON = usually turned on until
repressed by a specific molecule (ex. tryptophan) binds to
regulatory protein.
- if tryptophan is present, no need for synthesis
o tryptophan (the co-repressor) binds to the repressor
protein
o which is then able to bind to the operator
o blocking RNA Pol II, preventing transcription
 in absence of tryptophan
o the repressor protein alone is unable to bind to operator
o transcription is free to proceed
 this is a repressible operon
- when a needed substance appears in environment
- structural genes for its synthesis are no longer
transcribed
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- saving the cell energy and materials
So,…
Lac operon and tryp operon both operate by negative feedback;
stops expression of SG’s
 both are turned off by the active form of a repressor
protein
Lac operon also operates by positive feedback; induces expression
of SG’s
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