Chapter 19: Viruses

What was Wendell Stanley's contribution to our knowledge of viruses?
He crystallized the infectious particle of tobacco mosaic virus (TMV). Not even the simplest of cells can aggregate into regular crystals, so he knew it was something different.
What was some early evidence of the existence of viruses? Why were they difficult to study?
An infectious agent in the sap of tobacco plants. they couldn't be seen under a microscope.
What are the 4 forms of viral genomes?
1. Rod shape, helical viruses. Single type of capsid protein around RNA 2. icosohedral, glycoprotein spike at every vertex. DNA inside. 3. Spherical viruses: viral envelope with glycoproteins on it, with RNA in helical capsids inside. 4. Bacteriophages: phages that infect bacteria. DNA in icosohedral head, protein tail apparatus.
What is a capsid?
the protein shell enclosing the viral genome
what are capsomeres?
protein subunits that make up capsids
what different shapes may capsids have?
rod-shaped: helical viruses, or inside spherical viruses
icosohedral: icosohedral viruses or the heads of bacteriophages.
The viral envelope is derived from.....
the host cell.
What is the role of an envelope in animal viruses?
to protect the genome and infect easier: envelope has glycoproteins the same as the cell so it is accepted
What does bacteriophage mean?
phage that infects bacteria. All phages are bacteriophages.
What property of a virus determines its attachment to a host cell membrane?
if it has the glycoproteins to match the ones on the host cell membrane.
Viruses are obligate intracellular parasites, what does this mean
that they can only reproduce within a host cell, since they lack metabolic enzymes & equipment for making proteins
Host range of rabies virus vs. that of the human cold virus:
Rabies= broad, humans, raccoons, etc. Human Cold Virus= only infects the cells lining the upper respiratory tract of humans. Tissue-specific.
What components of the host cell does a virus use to reproduce itself?
host cell provides ATP, ribosomes, tRNA, and amino acids. Needs host enzymes to transcribe the viral genome into mRNA, which host ribosomes use to make more capsid proteins. Host enzymes also replicate the viral genome.
How does a virus use the host cell to replicate the viral genome? (DNA/RNA)
DNA virus uses host DNA pol to syn new viral DNA, an uses host DNA as template. RNA virus uses virally encoded polymerases that use host RNA as template.
List the 4 steps of viral entering to exiting host cell
1. glycoproteins on the viral envelope bind to the host cell, and the viral envelope merges with the plasma membrane. 2. viral DNA + proteins are released into the cell. 3. Host enzymes replicate the genome. 4. At the same time, host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins. 5. Viral genomes and capsid proteins self-assemble into new virus particles which exit the cell.
Distinguish between virulent and temperate phages:
virulent= phage that reproduces only by lytic cycle.
Temperate= phage that can reproduce both ways.
What portion of a phage enters the host cell and how does it do this?
After phage uses tail fibers to bind to host sites, the sheath of the tail contracts, and DNA only is injected into the cell.
What are restriction enzymes? What is their role in bacteria?
When foreign phage DNA enters a bacterium, it is recognized as foreign and these cellular enzymes cut it up, restricting the ability of the phage to infect the bacterium.
Why don't restriction enzymes destroy the DNA of the bacterial cells that produce them?
phage DNA contains a modified form of cytosine that isn't recognized by restriction enzymes.
What are three ways bacteria may win the battle against the phages/
1. restriction enzymes 2. natural selection favors bacterial mutants with receptors no longer recognized by a particular type of phage. 3. Instead of using up and killing host cells, some phages coexist with them in the lysogenic state.
What is a prophage?
the viral DNA integrated into the host cell's DNA. mostly silent within bacterium: one prophage gene silences others.
What might trigger the switchover from lysogenic to lytic mode?
an environmental trigger (chemically or radiation) induced the prophage to exit bacterial chromosome & initiate a lytic cycle.
3 Steps of the lysogenic cycle:
1. phage attaches to host cell and injects its viral DNA 2. phage DNA circularizes 3. phage DNA is integrated into the bacterial chromosome as a prophage 3. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells
Steps of the Lytic cycle:
1. phage attaches to host cell and injects its viral DNA 2. phage DNA circularizes 3. new phage DNA and proteins are synthesized and assembled into phages 4. cell lyses, releasing hundreds of phages
What are two elements that nearly all animal viruses have?
1. can be composed of single stranded DNA/RNA, or double stranded DNA/RNA. 2. Many animal viruses have both an envelope and RNA genome
What is a retrovirus? How do they replicate their genome?
reverse transcriptase transcribes an RNA template into DNA. This is an RNA -> DNA info flow, the opposite of the normal direction.
Compare and contrast a prophage and a provirus. Which one are you likely to carry?
Prophages are integrated viral DNA in host cell genome. Infects bacteria. Proviruses are integrated viral DNA into host cell's genome, in animals. I would carry this one
9 steps of HIV infection:
1. virus binds to certain WBC's 2. the viral envelope fuses with the cell's plasma membrane. Capsid proteins are removed, releasing viral proteins and RNA. 3. Reverse Transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA 4. Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first 5. Double stranded DNA is incorporated as a provirus into cellular DNA 6. Proviral genes are transcribed into RNA molecules by host RNA pol. These serve as genomes for the next viral generation and as mRNAs for translation into viral protein. viral proteins include capsid proteins, reverse transcriptase, and envelope glycoproteins. 7. vesicles transport glycoproteins to the cells' plasma membrane 8. capsids are assembled around the viral genomes and reverse transcriptase molecules. 9. New viruses bud off from the host cell.
What are Transposons:
DNA segments that can move from one location to another within the cell's genome.
When an animal is infected with more than one strain of virus...
the different strains can undergo genetic recombination if the RNA molecules in their genomes mix and match during viral assembly. Coupled with mutation, these changes can lead to a more deadly strain with a larger host range.
What are emerging viruses?
existing viruses that mutate, spread more wildly in the current host species, or spread to a new host species.
What are three ways that viruses make us ill?
1. viruses can damage or kill cells by causing the release of hydrolytic enzymes from lysosomes 2. can cause infected cells to produce toxins that lead to disease symptoms 3. Some have molecular components that are toxic.
Why do we recover completely from a cold but not from polio?
the infection site of colds, the epilthelium of the respiratory tract, can efficiently repair itself and reproduce. Polio damages mature nerve cells, which can't repair themselves or reproduce so the damage is permanent.
What are vaccines and how do they work?
vaccines are harmless variants or derivatives of the pathogen. They stimulate the immune system to mount defenses against the harmful pathogen.
RNA viruses have an unusually high rate of mutation because...
errors in replicating their RNA genomes can't be corrected by proofreading. Some mutations change existing viruses into new strains that can cause disease even in individuals who had the previous strain.
What does H1N1 mean?
Influenza A version, made up of hemagglutinin (H) (helps virus attach to host cells) and neuraminidase (N) (enzyme that helps release new virus particles). H and N are both viral surface proteins, and there are many variations on them.
Horizontal Transmission in plants:
plant is infected by external source. virus must get past outer layer, so plant is more susceptible if damaged by ind, injury, or herbivores. Herbivores can also carry & transmit the virus to the plant.
Vertical Transmission in plants:
plant inherits viral infection from parent. Can occur in asexual reproduction or sexual reproduction with infected seeds.
How do viruses spread throughout plant bodies?
spreads through plasmodesmata, cytoplasmic connections that penetrate the walls between adjacent plant cells. Virally encoded proteins cause enlarging of plasmodesmata.
What is a viroid?
Infectious nucleic acids that cause disease in plants. They are circular RNA molecules only a few hundred nucleotides long. Smaller than viruses: they don't encode proteins but can replicate in host plant cells using host cell enzymes. They cause errors in the regulatory systems that control plant growth.
What are prions?
infectious proteins that cause degenerative brain diseases in animals. Prions are midfolded forms of a protein normally present in brain cells. when a prion encounters a normally folded version of the same protein, it induces it into the prions's abnormal shape. The resulting chain reaction: high levels of prion aggregation= cellular malfunction= eventual brain degeneration
What are two alarming characteristics of prions?
1. they act very slowly with an incubation period of at least 10 years before symptoms develop. This prevents sources of infection from being identified. 2. Virtually indestructible: not destroyed or deactivated by heat. No known cure.
Who made the model of how prions work
stanley pruisner
To cause a human pandemic, the H5N1 avian flu virus would have to mutate to...
have human-human transmission
RNA viruses require their own supply of certain enzymes because
host cells lack enzymes that can replicate the viral genome.