VIRUSES

Campbell and Reece Chapter 19

Virus

an infectious particle incapable of replicating outside of a cell, consisting of RNA or DNA genome surrounded by a protein coat (capsid) & for some viruses a membranous envelope

SEM of Phage Attack on E. coli

Discovery of Viruses

1883: German scientist, Mayer, discovered that he could transmit tobacco mosaic disease (stunts growth of tobacco plant) by rubbing sap of affected plant on healthy plant

Discovery of Viruses

Mayer then hypothesized there was an unusually small bacteria that could not be seen with microscope

10 yrs later Ivanowsky put sap of infected plant thru filter designed to remove all bactreria ….filtrate still caused healthy plants to get disease

Discovery of Viruses

next hypothesis: bacteria produce a toxin that causes disease so it would have been in filtered sap ….Beijerinck proved the filtered sap contained infectious agents that reproduced but only inside host it infected

Beijerinck credited with being 1st to describe concept of virus

1935: Stanley crystallized the virus

Tobacco Mosaic Virus

Viral Genomes

DNA: dbl stranded DNA: single

stranded RNA: single

stranded RNA: dbl stranded

Viral Genomes

smallest have 4 genes largest several 100 named as DNA virus or RNA virus

Capsid

protein shell enclosing genome shapes:

rod polyhedral icosohedral

Viral Envelopes

membranous accessory structure derived from host cell membranes + proteins & glycoproteins of viral

origin

Bacteriophages

virus that infects bacteria

1st 7 identified infected E. coli named Type 1 –

Type 7 Types 2, 4, & 6

very similar:

Structure of Viruses

smallest virus 20 nm in diameter (smaller than a ribosome)

largest virus several 100 nm barely visible with light microscope

Host Cells

each particular virus can only infect cells of limited # of host species called the host range of the virus

specificity result of viral recognition system

most: protein on viral surface fits into receptor of host cell

Simplified Viral Replicative Cycle

1. virus enters cell & is uncoated releasing viral DNA & capsid proteins

2. host enzymes replicate the viral genome

3. different host enzymes transcribe the viral genome into viral mRNA translated by host ribosomes to make viral proteins

4. viral genomes & capsid proteins self-assemble into new virus particles which exit cell

Replication of RNA Viruses use virally encoded RNAP that can

use RNA as a template

Lytic Cycle

Lytic Cycle

culminates in death of host cell phages that replicate only by lytic

cycle called virulent phage

Bacterial Defenses

1. natural selection will favor mutants having receptors that no longer allow phage to attach

2. viral DNA may be recognized as foreign cut up by bacterial enzymes called restriction enzymes (restrict ability of phage to infect bacterium)

Phage Natural Selection

allows phage mutants that can bind to altered receptors or are resistant to particular restriction enzyme

so this parasite-host relationship in constant evolutionary flux

The Lysogenic Cycle

Lysogenic Cycles

does not destroy the host phages capable of using both modes

of replication called temperate phages

Λ phage (lambda) used widely in biological research

Prophage

viral DNA that has been integrated into bacterial loop of DNA

when that bacterium replicates the viral DNA is passed on to all daughter cells & so on & so on

when λ genome induced to leave the loop of bacterial DNA lytic cycle & cell (bacterium) dies

Phage Genes in Bacteria

diptheria, botulism, & scarlet fever would not be so harmful to humans w/out certain prophage genes that cause the host bacteria to make toxins

difference between E.coli that lives in out GI tract (no problem) & the one that’s found in food poisoning: presence of prophages

Animal Viruses nature of viral genome basis for classification of viruses

that infect animals:

1. DNA: dbl stranded dsDNA

2. DNA: single stranded ssDNA

3. RNA: dbl stranded dsRNA

4. RNA: single stranded ssRNA

5. RNA: template for mRNA synthesis ssRNA template for mRNA synthesis

6. RNA: template for DNA synthesis ssRNA template for DNA synthesis

Classes of Animal Viruses

Animal Viruses vs. Phages

many have both envelope & RNA

some with DNA also have envelope

few have envelope or RNA

Animal Viruses Phages

Viral Envelopes

outer membrane around capsid used to enter host cell viral glycoproteins protrude that

will bind to specific receptors on surface of host cell

Replicative Cycle of an Enveloped RNA Virus

RNA as Viral Genetic Material

includes most plant viruses & some RNA viruses

broadest variety infect animals

RNA Viruses

Class IV: genomes can serve directly as mRNA

immediately after infection can translate viral proteins

Class V: genome serves as template for mRNA

synthesis C’ strands of RNA made which serve as

templates for both mRNA & new RNA strands

RNA Viruses

Class VI: retroviruses have enzyme: reverse transcriptase

transcribes RNA template DNA (opposite normal direction of information flow)

HIV a retrovirus: enveloped with 2 identical molecules ssRNA & 2 reverse transcriptase

Replication Cycle of HIV

Replication of HIV

1. envelope glycoproteins allow virus to bind to specific receptors on certain WBCs

2. virus fuses with cell’s plasma membrane & capsid proteins removed viral RNA & proteins

3. reverse transcriptase catalyzes synthesis of a dsDNA strand c’ to the virus’s RNA

Replication of HIV

4. reverse transcriptase catalyzes synthesis of 2nd DNA strand c’ to the 1st

5. dsDNA incorporated as a provirus into host cell’s genome

6. proviral genes transcribed into ssRNA which are genomes for next generation & as mRNAs for translation into viral protein

Replication of HIV

7. viral proteins include capsid proteins & reverse transcriptase (made in cytosol) & envelope glycoproteins (made in ER)

8. vesicles transport the glycoproteins to host cell plasma membrane

9. capsids assemble around viral genome + reverse transcriptase molecules

10. new viruses bud off from host cell

Evolution of Viruses

there are viruses that infect every known form of life

use same universal genetic code as all living things

probable that viruses evolved after the 1st cells appeared

Evolution of Viruses

most accepted hypothesis: evolved from naked bits of nucleic acids that moved from 1 cell another possibly plasmids or tranposons 1st

virus transposon: a transposable element

that moves w/in a genome by means of a DNA intermediate

Plasmids

small, circular DNA molecules found in bacteria & in unicellular yeast (eukaryotic)

replicate independently of genome occasionally transferred between

bacteria

Viruses, transposons, & plasmids are all mobile genetic elements

virus may have more in common genetically with its host cell than with other viruses that infect same species

Mimivirus

largest virus to date (size of small bacterium)

dsDNA mimi: mimicking microbe ~1,000 genes: some code for proteins

used in translation, DNA repair, protein folding, & polysaccharide synthesis

Viruses, Viriods, & Prions

viruses cause disease in all life forms

viriods cause disease in plants prions cause disease in animals

Viral Disease in Animals

How viruses cause disease: damage or kill cells by release of

hydrolytic enzymes from lysosomes cause host cell to produce toxins have molecular components (in

envelope) that are toxic to host many of symptoms ass’c with viral

infection result of immune system reacting to infection (fever, chills, aches)

Vaccines

harmless variant or derivative of a pathogen that stimulates a host’s immune system to mount defenses against the pathogen

“New” Viral Diseases in Humans

usually caused by existing viruses that expand their host territory

example: H1N1 (2009) was a new combination of pig, human, & avian viral genes

Viral Infection in Plants

enter host cells thru damaged cell walls (horizontal transmission) or are inherited from a parent (vertical transmission)

Viriods

naked RNA molecules that infect plants & disrupt their growth

can cause misshaped potatoes

Prions

slow-acting, virtually indestructible infectious proteins that causes brain diseases in mammals

Model for How Prions Propagate