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Genetics of Viruses and Bacteria. Bacterium. Virus. Animal cell. 0.25 µm. Animal cell nucleus. Virus Size. Bacteria are prokaryotes w ith cells much smaller and more simply organized than those of eukaryotes Viruses a re smaller and simpler than bacteria.

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virus size

Bacterium

Virus

Animal

cell

0.25 µm

Animal cell nucleus

Virus Size
  • Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes
  • Viruses are smaller and simpler than bacteria
the discovery of viruses scientific inquiry
The Discovery of Viruses: Scientific Inquiry
  • Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration
  • In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease
  • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV)
viruses non living entities
Viruses: “Non-living” Entities
  • Small packages of nucleic acids in a protein coat
  • Are NOT cells—no cytoplasm and do not perform metabolic reactions
  • Obligate intracellular parasites—dependent upon other cells for replication
types of viruses
Types of Viruses
  • DNA viruses
    • Genome is DNA
  • RNA viruses
    • Genome is RNA
    • Smaller than DNA viruses
    • Lack of proofreading—leads to  evolution rate
viruses have a specific host range
Viruses have a Specific Host Range
  • Recognition by complementary fit between external viral protein and specific cell surface receptor sites

http://pathmicro.med.sc.edu/mhunt/rep1.jpg

bacteriophages
Bacteriophages
  • Viruses that infect bacteria
  • Set in motion a genetic takeover of bacteria, such as Escherichia coli
viruses replicate inside living cells
Viruses Replicate Inside Living Cells
  • Obligate intracellular parasites
    • Viruses lack enzymes needed for metabolism and have no structures to make proteins
    • Use cells own machinery to replicate viruses
  • Pathogen: agent that causes disease
    • Viruses damage cells during replication
reproductive cycles of phages
Reproductive Cycles of Phages
  • Phages are the best understood of all viruses
  • Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle
lytic cycle
Lytic Cycle

Virulent Viruses-reproduce only by lytic cycle

reproductive cycles of animal viruses
Reproductive Cycles of Animal Viruses
  • Two key variables in classifying viruses that infect animals:
    • DNA or RNA?
    • Single-stranded or double-stranded?
animal viruses
Animal Viruses
  • Glycoproteins on viral envelope recognize/bind specific receptors on host cell
  • Viral envelope fises with cell’s plasma membrane, and the capsid and viral genome enter the cell
  • Cellular enzymes remove capsid
  • Viral genome serves as template for replication of RNA strands
    • Templates for new RNA
    • Serve as mRNA for protein synthesis
  • Vesicles transport glycoproteins to cell’s plasma membrane
  • Capsid forms around viral genome
  • Virus buds from the cell
slide18
After entering the cell, viral DNA uses host nucleotides and enzymes to replicate itself

It uses host materials and machinery to produce capsid proteins

Viral DNA and capsid proteins assemble into new virus particles, which leave the cell

plant viruses

RNA

Protein

Figure 10.19

Plant Viruses
  • Plant viruses are serious agricultural pests
    • Most plant viruses
      • Have RNA genomes
      • Enter their hosts via wounds in the plant’s outer layers
        • Injuries, insects feeding, contaminated farming tools
      • Once infected, virus spreads through plasmodesmata
rna as viral genetic material
RNA as Viral Genetic Material
  • The broadest variety of RNA genomes is found in viruses that infect animals
  • Retroviruses use reverse transcriptase to copy their RNA genome into DNA
  • HIV is the retrovirus that causes AIDS
slide21

Genetic flow :

  • RNA  DNA
  • 2 identical strands ofRNA
  • Infects white blood cells
vaccinations
Vaccinations
  • Antibiotics don’t work—no metabolic reactions to interfere with
  • Vaccines—harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen
    • Parts of viruses, modified or killed viruses are injected into the body
    • Allows immune system to make antibodies against specific markers on the viral coat
      • HIV mutates too fast for immune system to keep up with
influenza vaccine
Influenza Vaccine
  • Influenza, also known as the flu, is a contagious disease that is caused by the influenza virus.  It attacks the respiratory tract in humans (nose, throat, and lungs).  The flu is different from a cold.  Influenza usually comes on suddenly and may include these symptoms:
    • Fever
    • Headache
    • Tiredness (can be extreme)
    • Dry cough
    • Sore throat
    • Nasal congestion
    • Body aches
emerging viruses
Emerging Viruses
  • Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists
  • Severe acute respiratory syndrome (SARS) recently appeared in China
  • Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory
le 18 11

LE 18-11

The SARS-causing agent is a

coronavirus like this one

(colorized TEM), so named for

the “corona” of glyco-protein

spikes protruding form the

envelope.

Young ballet students in Hong

Kong wear face masks to

protect themselves from the

virus causing SARS.

emerging viruses1
Emerging Viruses
  • RNA viruses have unusually high mutation rate
  • Spread of virus from one host species to another
  • Dissemination of a virus from a small, isolated populations to widespread epidemics
global view of hiv epidemic as of 2008
Global View of HIV epidemicas of 2008

http://www.who.int/hiv/facts/en/hiv_global2003sm.jpg

viruses and cancer
Viruses and Cancer
  • Tumor viruses can transform cells into cancerous cells
viruses and cancer1
Viruses and Cancer
  • Virus inserts viral nucleic acids into host cell DNA
    • Insertion is permanent-provirus never excises
    • Insertion for DNA tumor viruses straightforward
  • Oncogenes-genes found in viruses or as part of normal eukaryotic genome; trigger transformation of a cell to a cancerous state
    • Usually more than one must be activated to transform a cell
viroids
Viroids
  • Smaller and simpler than viruses
  • Small, naked, circular RNA molecules that do not code for proteins
  • Disrupt normal plant metabolism, development, and growth by causing errors in gene regulation
  • Affect many commercial plants—tomatoes, potatoes, chrysanthemums
  • Thought to have originated from escaped introns—sequences similar to self-splicing introns
slide32

Chrysanthemum with chrysanthemum chlorotic mottle viroid

Green tomato infected with tomato spotted wilt virus

prions
Prions
  • Pathogens that are proteins
  • Cause several degenerative brain diseases (Scrapie in sheep, “Mad Cow” disease, Creutzfeldt-Jakob disease)
slide35

INFECTIOUS PRION PROTEINS have a different shape, which they impose on normal prion proteins in a chain reaction that ends in sickness and death.

slide36

A hypothesis of how infectious protein particles, or prions, cause disease: PrPSc - an abnormal protein - communicates with its normal twin - PrPc - creating an abnormal form, that will eventually harm neurons.

(Adapted by Leigh Coriale Design and Illustration, with permission, Science [July 12], 1996, American Association for the Advancement of Science.)

slide37

Normal Brain

Kuru Infected Brain

It exists only among a single tribe in Papua New Guinea. The afflicted tribe - the Fore Highlanders - describe it as the "laughing death", because it leads to loss of coordination accompanied by dementia.

the bacterial chromosome
The Bacterial Chromosome
  • One double-stranded, circular molecule of DNA
  • Located in nucleoid region, so transcription and translation can occur simultaneously
  • Many also contain extrachromosomal DNA in plasmids
genetic recombination produces new bacterial strain
Genetic Recombination Produces New Bacterial Strain
  • Transformation
  • Transduction
  • Conjugation

Gene transfer occurs separately from bacterial reproduction

transformation
Transformation
  • Alteration of bacterial cell’s genotype by uptake of naked, foreign DNA from the environment
transformation1
Transformation
  • Biotech companies use this technique to artificially introduce foreign genes into bacterial genomes (human insulin, human growth hormone)
transduction
Transduction
  • Gene transfer from one bacterium to another by a bacteriophage
plasmids
Plasmids
  • Short, circular DNA molecules outside the chromosome
  • Carry genes that are beneficial but not essential
  • Replicate independently of chromosome
  • Episomes—plasmids that can be incorporated into chromosome
conjugation

“female”

“male”

F-

F+

Sex pili

Conjugation
  • Direct transfer of genetic material between bacterial cells that are temporarily joined (bacterial sex)

“Maleness” results from presence of F factor—segment of DNA in chromosome or in F plasmid

r plasmids
R Plasmids
  • Contain genes that confer antibiotic resistance
  • Medical consequences:resistant strains of pathogens due to overuse of antibiotics
transposition of genetic elements
Transposition of Genetic Elements
  • The DNA of a cell can also undergo recombination due to movement of transposable elements within the cell’s genome
  • Transposable elements, often called “jumping genes,” contribute to genetic shuffling in bacteria
insertion sequences

Insertion sequence

Inverted

repeat

Inverted

repeat

Transposase gene

Insertion Sequences
  • The simplest transposable elements, called insertion sequences, exist only in bacteria
  • An insertion sequence has a single gene for transposase, an enzyme catalyzing movement of the insertion sequence from one site to another within the genome
transposons
Transposons
  • Transposable elements called transposons are longer and more complex than insertion sequences
    • Discovered by Barbara McClintock
  • In addition to DNA required for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance
le 18 19b

LE 18-19b

Transposon

Insertion

sequence

Insertion

sequence

Antibiotic

resistance gene

Transposase gene

Inverted repeat

individual bacteria respond to environmental change by regulating their gene expression
Individual bacteria respond to environmental change by regulating their gene expression
  • A bacterium can tune its metabolism to the changing environment and food sources
  • This metabolic control occurs on two levels:
    • Adjusting activity of metabolic enzymes
    • Regulating genes that encode metabolic enzymes
le 18 20

Regulation of enzyme

production

Regulation of enzyme

activity

Precursor

LE 18-20

Feedback

inhibition

Enzyme 1

Gene 1

Gene 2

Enzyme 2

Regulation

of gene

expression

Gene 3

Enzyme 3

Enzyme 4

Gene 4

Gene 5

Enzyme 5

Tryptophan

operons the basic concept
Operons: The Basic Concept
  • In bacteria, genes are often clustered into operons, composed of
    • Regulatory gene—makes repressor protein that blocks RNA polymerase
    • Promoter region—DNA sequence that RNA polymerase binds to start transcription
    • Operator region—an “on-off” switch; can block RNA polymerase if region is blocked by repressor protein
    • Structural genes—DNA sequences that code for several related metabolic enzymes that direct production of some end product
  • 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 an operon off
repressible and inducible operons two types of negative gene regulation
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
  • A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription
  • The trp operon is a repressible operon
  • An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription
  • The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose
slide60
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
le 18 21a

trp operon

Promoter

Promoter

Genes of operon

DNA

trpB

trpA

trpE

trpC

trpD

trpR

Operator

Stop codon

RNA

polymerase

Regulatory

gene

Start codon

mRNA 5¢

mRNA

D

B

E

C

A

Inactive

repressor

Protein

Polypeptides that make up

enzymes for tryptophan synthesis

Tryptophan absent, repressor inactive, operon on

LE 18-21a

Repressible Operon

le 18 22b

lac operon

DNA

lacl

lacY

lacA

lacZ

RNA

polymerase

mRNA

mRNA 5¢

Transacetylase

Permease

-Galactosidase

Protein

Inactive

repressor

Allolactose

(inducer)

Lactose present, repressor inactive, operon on

LE 18-22b

Inducible Operon

positive gene regulation
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
le 18 23a

Promoter

DNA

lacl

lacZ

RNA

polymerase

can bind

and transcribe

Operator

CAP-binding site

Active

CAP

cAMP

Inactive

CAP

Lactose present, glucose scarce (cAMP level high): abundant lac

mRNA synthesized

LE 18-23a

Inactive lac

repressor

le 18 23b

Promoter

DNA

lacl

lacZ

CAP-binding site

Operator

RNA

polymerase

can’t bind

Inactive

CAP

Inactive lac

repressor

Lactose present, glucose present (cAMP level low): little lac

mRNA synthesized

LE 18-23b

slide67

Both plates of E.coli are transformed by pGLO plasmid as seen by growth on LB with ampicillin. The upper plate also contains arabinose, the inducer for the green fluorescent protein. This is visualized under UV light. The lower plate does not glow even though it has transformed cells because the media lacks arabinose.