The Genetics of Viruses

I. Background

II. Structure

III. Life Cycles

Viral infections, past & present

Viral infection have been one of the major infectious

challenges of the human species, for as far back as we

can tell.

Polio

Ebola Virus

Herpes Virus

Severe acute respiratory syndrome (SARS)

Figure 18.11 A, B

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

(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.

Let’s size them up…

Compare the size of: eukaryotic cell, bacterial cell and a virus

0.5 m

I. Background Everyone is at risk

for infection! Bacteriophages

infect bacteria only All eukaryotes

(animals, plants, fungi, protist) all vulnerable

Size 20nm-250nm

Ghost phage

Viral Diseases in Plants

More than 2,000 types of viral diseases of plants are known.

Spots on leaves and fruits, stunted growth, and damaged flowers or roots

Tobacco Mosaic Virus

I. Background Virus Discovery:

Tobacco mosaic disease (1930’s)

Stunts growth produces the speckled coloration

I. Background Viral Evolution proposal: Fragments of

cellular nucleic acid Reproduce within host cells only

(non-living) Obligate intracellular parasites Host range

II. Structure of viruses

II. Structure of Viruses

1. Nucleic acid Enclosed in a protein coat Genomes may be

ds/ss DNA ds/ss RNA

18 250 mm 70–90 nm (diameter)

20 nm 50 nm(a) Tobacco mosaic virus (b) Adenoviruses

RNADNACapsomere

Glycoprotein

Capsomereof capsid

II. Structure of Viruses

2. Capsids Protein Various shapes &

structures Capsids are

produced by host

Viral structure

II. Structure of Viruses

3. Envelopes Found in many

animal viruses Glycoprotein and

lipids Derived by host “Spikes” fuse with

membrane or receptor mediated entry

80–200 nm (diameter)

50 nm(c) Influenza viruses

RNA

Glycoprotein

Membranousenvelope

Capsid

Receptor mediated model

Fusion Model

Basic Infection: Just Genome and Capsid

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III. Life Cycles

Bacteria and Eukaryotic Models

Figure 18.4d

80 225 nm

50 nm(d) Bacteriophage T4

DNA

Head

Tail fiber

Tail sheath

III. Life Cycles:

1. Bacteriophages complete two reproductive mechanisms:

lytic cycle lysogenic cycle

III. Life Cycles A. Lytic Cycle:

Digests the host’s cell wall, releasing the progeny viruses

Kills host Virulent phage

Phage assembly

Head Tails Tail fibers

Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell.

1 Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed.

2

Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell.

3Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms.

4

Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles.

5

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Life Cycles B. Lysogenic cycle

Incorporate viral DNA into bacteria genome (propahge)

Phage genome is replicated (for free!) w/o destroying the host

Temperate phages capable of using both cycles

The lytic and lysogenic cycles of phage , a temperate phage

Many cell divisions produce a large population of bacteria infected with the prophage.

The bacterium reproducesnormally, copying the prophageand transmitting it to daughter cells.

Phage DNA integrates into the bacterial chromosome,becoming a prophage.

New phage DNA and proteins are synthesized and assembled into phages.

Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle.

Certain factorsdetermine whether

The phage attaches to ahost cell and injects its DNA.

Phage DNAcircularizes

The cell lyses, releasing phages.Lytic cycleis induced

Lysogenic cycleis entered

Lysogenic cycleLytic cycle

or Prophage

Bacterialchromosome

Phage

PhageDNA

Figure 18.7

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III: Life Cycles

2. RNA viruses typically infect animals Retroviruses (HIV), use reverse

transcriptase to make cDNA Integrated into genome, provirus

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• The reproductive cycle of HIV, a retrovirus

Figure 18.10

mRNA

RNA genomefor the nextviral generation

Viral RNA

RNA-DNAhybrid

DNA

ChromosomalDNA

NUCLEUSProvirus

HOST CELL

Reverse transcriptase

New HIV leaving a cell

HIV entering a cell

0.25 µm

HIV Membrane of white blood cell

The virus fuses with thecell’s plasma membrane.The capsid proteins areremoved, releasing the viral proteins and RNA.

1

Reverse transcriptasecatalyzes the synthesis of aDNA strand complementaryto the viral RNA.

2

Reverse transcriptasecatalyzes the synthesis ofa second DNA strandcomplementary to the first.

3

The double-stranded DNA is incorporatedas a provirus into the cell’s DNA.

4

Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins.

5

The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).

6

Vesicles transport theglycoproteins from the ER tothe cell’s plasma membrane.

7 Capsids areassembled aroundviral genomes and reverse transcriptase molecules.

8

New viruses budoff from the host cell.9

III: Life Cycles Life after infection

Viruses may damage or kill cells Tissue damage Toxins that lead to disease symptoms

may be produced Asymptomatic

Sources http://www.aapsj.org/ http://www.stanford.edu/group/virus/1999/jchow/

rep.html http://pathmicro.med.sc.edu/mhunt/RNA-HO.htm http://www.brooklyn.cuny.edu/bc/ahp/LAD/C5/

C5_Viruses.html http://www.biology.com Campbell Reece Mitchell. Biology, Prentice Hall

1999, 2001 Sherris. Medical Microbiology: An introduction to

infectious disease, Appleton and Lange,1990

http://teachers.eastern.k12.nj.us/nabi/biology/index.html

Good links