the genetics of bacteria: bacterial reproduction

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