the genetics of bacteria: bacterial reproduction
DESCRIPTION
Did you know..? human gut holds about 1,000 different bacterial species & some 10 trillion bacterial cellsTRANSCRIPT
Ms. Gaynor
The Genetics of Bacteria: Bacterial
Reproduction
Did you know..?• human gut holds
about 1,000 different bacterial species & some 10 trillion bacterial cells
What is Bacteria?• Single celled prokaryote• No nucleus (nucleoid
region instead) • Very few (“no”) organelles• Has a cell wall, cell
membrane, ribosomes, cytoplasm
• Has circular chromosome and plasmid(s)
• http://media.pearsoncmg.com/bc/bc_campbell_biology_7/media/interactivemedia/activities/load.html?6&B
Prokaryote vs. Eukaryote
It is important to note that most bacteria do not have histones, and yet they do have SIR2-like proteins with similar activity
2 Types (domains) of Bacteria• Eubacteria (can be harmful)
– “regular” bacteria–They live in most environments– their cell-wall DOES contain peptidoglycan
• Archaea Bacteria (not harmful)– “older” bacteria; live in EXTREME habitats–more similar to eukaryotes than to bacteria in several
ways: • their cell-wall does NOT contain peptidoglycan
Eubacteria Cell Wall
Antibiotics kill these
type!
3 Bacterial Shapes
Genetic Diversity of Bacteria
• Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria
• The well-studied intestinal bacterium Escherichia coli (E. coli) is “the laboratory rat of molecular biology”
Bacterial Genetics• Nucleoid region
densely packed with DNA (no membrane)• The bacterial chromosome–usually a circular DNA
molecule with few associated proteins
• Reproduction binary fission (asexual)
Mutation and Genetic Recombination as Sources of Genetic Variation
•Since bacteria can reproduce rapidly
–New mutations can quickly increase a population’s genetic diversity
•Genetic diversity
–Can also arise by genetic recombination of the DNA from 2 different bacterial cells
***Remember that prokaryotes don’t undergo meiosis (crossing over) or
fertilization
Plasmids
• In addition to the chromosome, some bacteria (and plants) have plasmids –smaller circular DNA molecules that can replicate independently of
the bacterial chromosome–Extra chromosomal DNA
• Does not code for genes that aid in cell replication• Do not transcribe/translate their DNA into protein
Since asexual reproduction is used, how does Bacteria
Transfer DNA?• Conjugation
– direct transfer of genetic material; forms cytoplasmic bridges; sex pili used
• Transduction– phages that carry bacterial genes from 1 host cell to
another – generalized~ random transfer from host #1 to host #2– specialized~ incorporation of prophage DNA into host
chromosome• Transformation
– gene alteration by the uptake of naked, foreign DNA (plasmid) from the environment
#1) Conjugation• Conjugation –the direct transfer of genetic material
between bacterial cells that are temporarily joined
• Sex pili are used in the transfer of DNA
LE 18-17
Sex pilus 5 µm
Bacterial Plasmids• Small, circular, self-replicating DNA separate from the
bacterial chromosome• F (fertility) Plasmid: codes for the production of
sex pili (bacteria are either F+ or F-)• R (resistance) Plasmid: codes for antibiotic drug
resistance• Transposons (transposable genetic elements):
piece of DNA that can move from location to another in a cell’s genome – chromosome to plasmid, plasmid to plasmid, etc. – They are commonly referred to as “jumping
genes”
F plasmid Bacterial chromosome
F+ cellMatingbridge
F+ cell
F+ cellBacterial chromosome
F– cell
Conjunction and transfer of an F plasmid From and F+ donor to an F– recipient
R plasmids and Antibiotic Resistance
• R plasmids resist various antibiotics• When a bacterial population is exposed to an
antibiotic, individuals with the R plasmid will survive and increase in the overall population
Conjugation Animations of F Plasmid
• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter13/animation_quiz_3.html
Conjugation Animations of R Plasmid•http://www.hhmi.org/biointeractive/animations/conjugation/conj_frames.htm
Rolling Circle Plasmid Transfer Mechanisms Animations
• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter13/animation_quiz_6.html
#2) Transduction• Transduction
– Phages (viruses) that carry bacterial genes from 1 host cell to another
– generalized~ random transfer from host #1 to host #2
– specialized~ incorporation of prophage DNA into host chromosome
Generalized Transduction Animations
• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter13/animation_quiz_2.html
Specialized Transduction Animations
• http://highered.mcgraw-hill.com/sites/0072552980/student_view0/chapter9/animation_quiz_4.html
#3) Transformation• Transformation
– “Naked” Plasmids (present in environment) are taken up by certain bacteria
– Viruses are NOT used in this method!
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter13/animation_quiz_1.html
Bacterial Transformation Animations
Transposon Animations• http://highered.mcgraw-hill.com/sites/00725
56781/student_view0/chapter13/animation_quiz_5.html
Ms. Gaynor
Chapter 18 (PART 3)
The Genetics of Bacteria: Operons
REVIEW
• How do bacteria exchange DNA or acquire NEW genes? 1. Transformation2. Trandsduction (both generalized and
specialized)3. Conjugation4. Insertion sequences and Transposons
OPERONSUsed by bacteria for
gene regulation
How do Bacteria Control Gene Expression?
• Individual bacteria respond to environmental change by regulating their gene expression
• A bacterium can ADJUST its metabolism to the changing environment and food sources
• This metabolic control occurs on 2 levels:–Adjusting activity of enzymes–Regulating genes that encode enzymes
LE 18-20
Regulation of enzymeactivity
Regulation of enzymeproduction
Enzyme 1
Regulation of gene expression
Enzyme 2
Enzyme 3
Enzyme 4
Enzyme 5
Gene 2
Gene 1
Gene 3
Gene 4
Gene 5
Tryptophan
Precursor
Feedbackinhibition
Operons: The Basic Concept• Mostly in bacteria genes are
often clustered (grouped) into operons– INCLUDES:
• An operator, an “on-off” switch• A promoter with a TATA box• Genes for metabolic enzymes
• An operon can be switched off by a protein called a repressor
• A corepressor is a small molecule that cooperates with a repressor to switch operon off
Operon Parts•The regulatory gene codes for the repressor protein.•The promoter site is the attachment site for RNA polymerase (proceeded by TATA box)•The operator site is the attachment site for the repressor protein.•The structural genes code for the proteins.•The repressor protein is different for each operon and is custom fit to the regulatory metabolite.
•Whether or not the repressor protein can bind to the operator site is determined by the type of operon.
•The regulatory metabolite is either the product of the reaction or the reactant depending on the type of operon.•What is a regulatory protein? •http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_3.html
•
Operon- Example #1• trp operon- a repressible operon
– Used to MAKE tryptophan (amino acid) – Promoter (and TATA box): RNA polymerase binding site;
begins transcription – operator: controls access of RNA polymerase to genes
(EMPTY when tryptophan NOT present)
– repressor: protein that binds to operator and prevents attachment of RNA polymerase • coded from a regulatory gene (when tryptophan is present ~
acts as a corepressor)
• transcription is repressed when tryptophan binds to a regulatory protein
NO Tryptophan present repressor Inactive operon ON
DNA
Regulatorygene
mRNA
Protein
TATA Box and Promoter
trpR
RNApolymerase3
5
Inactiverepressor
mRNA 5
trpE trpD trpC trpB trpA
Operator
Start codon
Stop codon
trp operon
Structural Genes of operon
E
Polypeptides that make upEnzymes needed to make tryptophan
D C B A
LE 18-21b_1
DNA
Protein
Tryptophan(corepressor)
mRNA
Activerepressor
Tryptophan present repressor active operon OFF
LE 18-21b_2
DNA
Protein
Tryptophan(corepressor)
Tryptophan present, repressor active, operon off
mRNA
Activerepressor
No RNA made
Tryptophan Repressor Operon
• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html#– Animation #1
Operon- Example #2• lac operon- an inducible operon • lactose metabolism (assume NO glucose in habitat)
– When lactose not present: • repressor active• operon off• no transcription for lactose enzymes
– When lactose present: • repressor inactive• operon on• inducer molecule inactivates protein repressor (allolactose)
• transcription is stimulated when inducer binds to a regulatory protein• http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html
Recall…What are Glucose and Lactose?
• Glucose– Monosaccharide– Needed for bacterial glycolysis and
proton gradient formation…why? • To make their ATP (remember no
cellular respiration b/c NO mitochondria)
• Lactose– Disaccharide
• made of glucose and galactose–
How do bacteria make ATP?• Glucose is needed!
– Needed for bacterial glycolysis to make 2 ATP via substrate level phosphorylation (just like eukaryotes)
– Electrons from glucose needed to create H+ gradient so ATP synathase can function• …WAIT!!! Bacteria have NO mitochondria cristae so where
does this proton gradient/ATP synthase complex take place– IT THE CELL MEMBRANE OF THE BACTERIAL CELL!!!
LE 18-22a
DNA lacl
Regulatorygene
mRNA5
3
RNApolymerase
ProteinActiverepressor
NoRNAmade
lacZ
Promoter
Operator
Lactose absent repressor active operon OFF
LE 18-22b
DNA lacl
mRNA5
3
lac operon
lacZ lacY lacA
RNApolymerase
mRNA 5
Protein
Allolactose(inducer)
Inactiverepressor
-Galactosidase Permease Transacetylase
Lactose present repressor inactive operon ON
Enzymes needed for lactose metabolism
Repressible and Inducible Operons: 2 Types of Negative Gene
Regulation• A repressible operon is usually on– binding of a repressor to operator shuts off
transcription– The trp operon is a repressible operon
• An inducible operon is usually off– a molecule called an inducer inactivates the
repressor and turns on transcription– Example of an inducible operon is the
lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose
• Inducible enzymes usually function in catabolic pathways
• Repressible enzymes usually function in anabolic pathways
• Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor
Positive Gene Regulation• Some operons are also subject to positive
control through a stimulatory activator protein, such as catabolite activator protein (CAP)
• When glucose (a preferred food source of E. coli ) is scarce, the lac operon is activated by the binding of CAP
• When glucose levels increase, CAP detaches from the lac operon, turning it off
REVIEW OF ATP• ATP vs. ADP
• Low levels of glucose (AMP/cAMP)
LE 18-23a
DNA
cAMP
lacl
CAP-binding site
Promoter
ActiveCAP
InactiveCAP
RNApolymerasecan bindand transcribe
Operator
lacZ
Inactive lacrepressor
Lactose present, glucose scarce (cAMP level high): abundant lac mRNA
synthesized
DNA lacl
CAP-binding site
Promoter
RNApolymerasecan’t bind
Operator
lacZ
Inactive lacrepressor
InactiveCAP
Lactose present, glucose present (cAMP level low): little lac mRNA
synthesized
Lac Operon (with and without lactose/ glucose)
• http://wps.prenhall.com/wps/media/objects/487/499061/CDA14_1/CDA14_1b/CDA14_1b.htm
• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter18/animations.html#
• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_4.html
• http://www.dartmouth.edu/~cbbc/courses/movies/LacOperon.html
• http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_3.html