VIRUSES Campbell and Reece Chapter 19 Virus an

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 1 st to describe concept of virus 1935: Stanley crystallized the virus

Tobacco Mosaic Virus

Viral Genomes DNA: dbl stranded DNA: 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 1 st 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 m. RNA 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. 2. natural selection will favor mutants having receptors that no longer allow phage to attach 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 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: DNA: dbl stranded 1. � ds. DNA: single stranded 2. � ss. DNA RNA: dbl stranded 3. � ds. RNA: single stranded 4. � ss. RNA: template for m. RNA synthesis 5. � ss. RNA template for m. RNA synthesis RNA: template for DNA synthesis 6. � ss. RNA template for DNA synthesis

Classes of Animal Viruses

Animal Viruses vs. Phages Animal Viruses many have both envelope & RNA some with DNA also have envelope Phages few have envelope or RNA

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: can serve directly as m. RNA immediately after infection can translate viral proteins � genomes Class V: � genome serves as template for m. RNA synthesis � C’ strands of RNA made which serve as templates for both m. RNA & new RNA strands

RNA Viruses Class VI: � retroviruses enzyme: reverse transcriptase transcribes RNA template DNA (opposite normal direction of information flow) � HIV a retrovirus: enveloped with 2 identical molecules ss. RNA & 2 reverse transcriptase � have

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 ds. DNA strand c’ to the virus’s RNA

Replication of HIV 4. reverse transcriptase catalyzes synthesis of 2 nd DNA strand c’ to the 1 st 5. ds. DNA incorporated as a provirus into host cell’s genome 6. proviral genes transcribed into ss. RNA which are genomes for next generation & as m. RNAs 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 1 st cells appeared

Evolution of Viruses most accepted hypothesis: evolved from naked bits of nucleic acids that moved from 1 cell another plasmids or tranposons 1 st virus � transposon: a transposable element that moves w/in a genome by means of a DNA intermediate � possibly

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) ds. DNA 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: H 1 N 1 (2009) was a new combination of pig, human, & avian viral genes