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The Genetics of Viruses and Bacteria. Chapter 18. Virus. Bacterium. Animal cell. 0.25 m. Animal cell nucleus. Figure 18.2. Microbial Model Systems. Recall that bacteria are prokaryotes With cells much smaller and more simply organized than those of eukaryotes Viruses

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microbial model systems

Virus

Bacterium

Animalcell

0.25 m

Animal cell nucleus

Figure 18.2

Microbial Model Systems
  • Recall that bacteria are prokaryotes
    • With cells much smaller and more simply organized than those of eukaryotes
  • Viruses
    • Are smaller and simpler still
viruses

0.5 m

Figure 18.1

Viruses
  • Viruses called bacteriophages
    • Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli
obligate intracellular parasites
Obligate Intracellular Parasites
  • A virus has a genome but can reproduce only within a host cell
the discovery of viruses scientific inquiry

Figure 18.3

The Discovery of Viruses: Scientific Inquiry
  • Tobacco mosaic disease
    • Stunts the growth of tobacco plants and gives their leaves a mosaic coloration
slide6
TMV
  • In the late 1800s
    • Researchers hypothesized that a particle smaller than bacteria caused tobacco mosaic disease
  • In 1935, Wendell Stanley
    • Confirmed this hypothesis when he crystallized the infectious particle, now known as tobacco mosaic virus (TMV)
viruses1
Viruses
  • Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope
  • Viral genomes may consist of
    • Double- or single-stranded DNA
    • Double- or single-stranded RNA
capsids and envelopes

RNA

DNA

Capsomere

Glycoprotein

70–90 nm (diameter)

18  250 mm

20 nm

50 nm

CAPSID

(b) Adenoviruses

Capsids and Envelopes
  • A capsid
    • Is the protein shell that encloses the viral genome

TMV & Adenovirus

capsids and envelopes1
Capsids and Envelopes

Envelopes

  • Membranous coverings derived from the membrane of the host cell
viral envelopes
Viral Envelopes
  • Many animal viruses
    • Have a membranous envelope
  • Viral glycoproteins on the envelope
    • Bind to specific receptor molecules on the surface of a host cell
bacteriophages
Bacteriophages
  • A.K.A. phages
    • Have the most complex capsids found among viruses
general features of viral reproductive cycles
General Features of Viral Reproductive Cycles
  • Viruses are obligate intracellular parasites
    • They can reproduce only within a host cell
  • Each virus has a host range
    • A limited number of host cells that it can infect
general features of viral reproductive cycles1

VIRUS

DNA

Capsid

HOST CELL

Viral DNA

mRNA

Viral DNA

Capsid proteins

General Features of Viral Reproductive Cycles
  • Viruses use enzymes, ribosomes, and small molecules of host cells to synthesize progeny viruses
viral reproductive mechanisms
Viral Reproductive Mechanisms
  • Lytic cycle

Is a phage reproductive cycle that culminates in the death (lysis) of the host

  • Lysogenic cycle

Replicates the phage genome without destroying the host

lytic cycle viral reproduction
Lytic Cycle (Viral Reproduction)

DOCKING with the host receptor protein

PENETRATION of the viral nucleic acid into the host cytoplasm (Restriction Endonucleases, A.K.A. restriction enzymes break up host DNA)

BIOSYNTHESIS of the viral components

Assembly (MATURATION) of the viral components into complete viral units

RELEASE of the completed virus from the host cell

slide18

Phage assembly

Head

Tail fibers

Figure 18.6

Tails

lysogenic cycle
Lysogenic Cycle

Lambda

  • Temperate phages

Are capable of using both the lytic & lysogenic cycles of reproduction

prophage
Prophage

When viral DNA is integrated into the bacterial chromosome (Plasmid)

slide21

Capsid

RNA

Envelope (with

glycoproteins)

HOST CELL

Viral genome (RNA)

Template

mRNA

Capsid

proteins

ER

Copy of

genome (RNA)

Glyco-

proteins

slide22

Glycoprotein

Viral envelope

Capsid

RNA(two identicalstrands)

Reversetranscriptase

HIV

  • Retroviruses, such as HIV, use the enzyme reverse transcriptase
    • To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirus (Integrates into host DNA)

2 of each

evolution of viruses
Evolution of Viruses
  • Viruses do not really fit our definition of living organisms since viruses can reproduce only within cells
    • They probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acid
viral diseases in animals
Viral Diseases in Animals
  • Viruses may damage or kill cells
    • By causing the release of hydrolytic enzymes from lysosomes
  • Some viruses cause infected cells
    • To produce toxins that lead to disease symptoms
viral diseases in animals1
Viral Diseases in Animals
  • Viruses may damage or kill cells (Amount of damage depends on the ability of infected tissue to regenerate by mitosis)
  • -Respiratory tract epithelium repairs quickly from adenovirus infection
  • - Nerve tracts affected by polio virus is permanent
  • Find host cells using “lock & key” fit with proteins on virus & host cell receptors
slide26

Prions

  • Protein infectious particles
  • Contain no RNA or DNA
  • Long incubation period (~10 years)
prions

Originalprion

Prion

Many prions

Normalprotein

Newprion

Prions

Prions
  • Are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
  • Propagate by converting normal proteins into the prion version
emerging viruses
Emerging Viruses
  • Are those that appear suddenly or suddenly come to the attention of medical scientists
  • 3 processes contribute to emerging viruses
  • Mutation of existing viruses as RNA is not corrected by proofreading e.g. SARS
  • Spread from one host species to another e.g. Hanta Virus
  • Dissemination from a small isolated population e.g. HIV
sars severe acute respiratory syndrome

(b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.

(a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

Figure 18.11 A, B

SARS – Severe Acute Respiratory Syndrome
emerging viruses are not new
Emerging Viruses are NOT new
  • They are existing viruses that
  • Mutate
  • Spread to new host species
  • Disseminate more widely in the host species
slide34

Polio

Polio

other viruses that affect humans
Other Viruses that affect humans
  • Influenza Virus
  • Rubella
  • Parvo-virus
  • Epstein Barr Virus
  • Hanta Virus
  • (HPV) Human Papilloma Virus
  • (RSV) Respiratory Syncytial Virus
  • Rabies
  • Rhinovirus
  • Rotavirus
  • West Nile Virus
viral diseases in plants
Viral Diseases in Plants
  • More than 2,000 types of viral diseases of plants are known
  • Common symptoms of viral infection include
    • Spots on leaves and fruits, stunted growth, and damaged flowers or roots
viral diseases in plants1
Viral Diseases in Plants
  • Plant viruses spread disease in two major modes
    • Horizontal transmission, entering through damaged cell walls
    • Vertical transmission, inheriting the virus from a parent
viroids the simplest infectious agent
Viroids -The Simplest Infectious Agent
  • Are circular RNA molecules that infect plants and disrupt their growth
bacteria
Bacteria
  • Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria
  • Bacteria allow researchers
    • To investigate molecular genetics in the simplest true organisms
the bacterial genome and its replication
The Bacterial Genome and Its Replication
  • The bacterial chromosome
    • Is usually a circular DNA molecule with few associated proteins
  • In addition to the chromosome
    • Many bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome
the bacterial genome and its replication1

Replicationfork

Origin of replication

Termination of replication

Binary Fission

The Bacterial Genome and Its Replication
  • Bacterial cells divide by binary fission
    • Which is preceded by replication of the bacterial chromosome
mutation and genetic recombination as sources of genetic variation
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 recombination of the DNA from two different bacterial cells
    • Remember that prokaryotes don’t undergo meiosis or fertilization
recombination in bacteria
Recombination in Bacteria
  • Three processes bring bacterial DNA from different individuals together
    • Transformation
    • Transduction
    • Conjugation
transformation
Transformation

Is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment

transduction
Transduction

Phages carry bacterial genes from one host cell to another

conjugation and plasmids

Sex pilus

1 m

Figure 18.17

Conjugation and Plasmids
  • Conjugation
    • Is the direct transfer of genetic material between bacterial cells that are temporarily joined

DNA transfer is one way

the f plasmid and conjugation
The F Plasmid and Conjugation
  • Cells containing the F plasmid, designated F+ cells
    • Function as DNA donors during conjugation
    • Transfer plasmid DNA to an F recipient cell
slide53

3

2

4

1

F Plasmid

Bacterial chromosome

F+ cell

F+ cell

Mating bridge

F+ cell

Bacterial chromosome

F– cell

A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid.

DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands.

The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+.

A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow).

Figure 18.18a

f plasmid recombination
F Plasmid recombination
  • Chromosomal genes can be transferred during conjugation when the donor cell’s F factor is integrated into the chromosome

Hfr cell

  • A cell with the F factor built into its chromosome
  • The F factor of an Hfr cell
    • Brings some chromosomal DNA along with it when it is transferred to an F– cell
r plasmids and antibiotic resistance
R plasmids and Antibiotic Resistance

Confer resistance to various antibiotics

transposition of genetic elements
Transposition of Genetic Elements
  • Transposable elements
    • Can move around within a cell’s genome
    • Are often called “jumping genes”
    • Contribute to genetic shuffling in bacteria by folding the DNA
insertion sequences
Insertion Sequences
  • An insertion sequence contains a single gene for transposase
    • An enzyme that catalyzes movement of the insertion sequence from one site to another within the genome
slide59

Insertion sequence

A T C C G G T…

A C C G G A T…

3

5

3

5

T A G G C C A …

T G G C C T A …

Transposase gene

Inverted

repeat

Inverted

repeat

(a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that encodes transposase, which catalyzes movement within the genome. The inverted repeats are backward, upside-down versions of each other; only a portion is shown. The inverted repeat sequence varies from one type of insertion sequence to another.

Figure 18.19a

transposons
Transposons
  • Bacterial transposons
    • Also move about within the bacterial genome
    • Have additional genes, such as those for antibiotic resistance
slide61

Transposon

Antibioticresistance gene

Insertion sequence

Insertion sequence

5

3

5

3

Inverted repeats

Transposase gene

(b) Transposons contain one or more genes in addition to the transposase gene. In the transposon shown here, a gene for resistance to an antibiotic is located between twin insertion sequences. The gene for antibiotic resistance is carried along as part of the transposon when the transposon is inserted at a new site in the genome.

Figure 18.19b

prokaryotic gene expression
Prokaryotic Gene Expression
  • Individual bacteria respond to environmental change by regulating their gene expression
  • E. coli, a type of bacteria that lives in the human colon
    • Can tune its metabolism to the changing environment and food sources
response to the environment
Response to the environment
  • This metabolic control occurs on two levels
    • Adjusting the activity of metabolic enzymes already present
    • Regulating the genes encoding the metabolic enzymes
slide64

(a) Regulation of enzyme activity

(b) Regulation of enzyme production

Precursor

Feedback

inhibition

Enzyme 1

Gene 1

Enzyme 2

Gene 2

Regulation

of gene

expression

Gene 3

Enzyme 3

-–

Gene 4

Enzyme 4

Gene 5

Enzyme 5

Tryptophan

Figure 18.20a, b

Feedback Inhibition

operons the basic concept
Operons: The Basic Concept
  • In bacteria, genes are often clustered into operons, composed of
    • An operator, an “on-off” switch
    • A promoter
    • Genes for metabolic enzymes
  • An operon
    • Is usually turned “on”
    • Can be switched off by a protein called a repressor
operon parts
Operon Parts
  • The regulatory gene codes for the repressor protein.
  • The promoter site is the attachment site for RNA polymerates.
  • 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.
operon parts1
Operon Parts
  • 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.
the trp operon regulated synthesis of repressible enzymes

trp operon

Promoter

Promoter

Genes of operon

RNA polymerase

Start codon Stop codon

trpR

trpD

trpC

trpB

trpE

trpA

DNA

Operator

Regulatory

gene

3

mRNA 5

mRNA

5

C

Polypeptides that make up

enzymes for tryptophan synthesis

E

D

B

A

Inactiverepressor

Protein

(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.

Figure 18.21a

The trp operon: regulated synthesis of repressible enzymes
slide69

DNA

No RNA made

mRNA

Active

repressor

Protein

Tryptophan

(corepressor)

(b)

Tryptophan present, repressor active, operon off. As tryptophan

accumulates, it inhibits its own production by activating the repressor protein.

Figure 18.21b

trp operon
Trp Operon

http://bcs.whfreeman.com/thelifewire/content/chp13/1302002.html

repressible and inducible operons two types of negative gene regulation
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
  • In a repressible operon
    • Binding of a specific repressor protein to the operator shuts off transcription (Found in anabolic pathways)
  • In an inducible operon
    • Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription (Found in catabolic pathways)

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120080/bio26.swf::The%20Tryptophan%20Repressor

lac operon
Lac Operon

The lac operon: regulated synthesis of inducible enzymes

http://www.sumanasinc.com/webcontent/animations/content/lacoperon.html

slide73

Promoter

Regulatorygene

Operator

DNA

lacl

lacZ

NoRNAmade

3

RNApolymerase

mRNA

5

Activerepressor

Protein

Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.

(a)

Figure 18.22a

slide75

lac operon

DNA

lacz

lacY

lacA

lacl

RNApolymerase

3

mRNA 5

mRNA 5'

mRNA

5

-Galactosidase

Permease

Transacetylase

Protein

Inactiverepressor

Allolactose(inducer)

(b)

Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.

Figure 18.22b

types of operons
Types of Operons
  • Inducible enzymes
    • Usually function in catabolic pathways
  • Repressible enzymes
    • Usually function in anabolic pathways