Ch 19 Viruses

In 1883, Adolf Mayer discovered that he could transmit tobacco mosaic disease from plant to plant by rubbing sap extracted from diseased leaves into healthy plants. Viruses were difficult to study because of their size.

crystallized the infectious particle, now known as the tobacco mosaic virus (TMV), thus confirming earlier hypotheses about the nature of this mysterious agent.

double-stranded DNA, single-stranded DNA, double-stranded RNA, single-stranded RNA

the protein shell enclosing the viral genome

depending on the type of virus, the capsid may be rod-shaped, polyhedral, or more complex in shape

large number of protein subunits that build capsids

derived from: host cell

derived from: viral origin

help the virus infect their host

See page 383 in text for the labeled figure. a. What type of virus is this? Bacteriophage, T4 b. What does its name mean? Bacteria eater c. What is its host? E. coli d. Is the genome of this virus DNA or RNA? DNA

through a "lock-and-key" fit between viral surface proteins and specific receptor molecules on the outside of cells

Viruses can replicate only within a host cell

each particular virus can infect cells of only a limited number of host species

Some viruses have broad host ranges, such as West Nile virus, which can infect mosquitoes, birds, horses, and humans. Other viruses have a host range so narrow that they infect only a single species, and are sometimes limited to particular tissue. Possible examples include human cold viruses

The rabies virus has a broad host range, able to infect most species of mammals, while the human cold virus has a narrow host range, only infecting the tissue of the cell lining of the upper respiratory tract in humans.

The host cell provides the nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins.

Many DNA viruses use the DNA polymerase of the host cell to synthesize new genomes along the templates provided by the viral DNA

they use virally encoded RNA polymerase that can use RNA as a template

a virus that infects bacteria; also called a phage

virulent: a phage that replicates only by a lytic cycle which destroys the host cell temperate: can undergo a lysogenic cycle, which allows the phage genome to be replicated without destroying the host

phage DNA enters the host cell. After the bacteriophage binds to a specific receptor on the outer surface of the host cell, the sheath of the tail contracts, injecting the DNA into the cell and leaving an empty capsid capsule

an endonuclease (type of enzyme) that recognizes and cuts DNA molecules foreign to a bacterium. It then cuts at specific nucleotide sequences (restriction sites)

they identify and cut up viral DNA that is detected as foreign

The bacterial cell's own DNA is methylated in a way that prevents attack by its own restriction enzymes

1. Natural selection favors bacterial mutants with receptors that are no longer recognized by a particular kind of phage. 2. When phage DNA successfully enters a bacterium, the DNA is often identified as foreign and cut up by restriction enzymes. 3. Instead of lysing their host cells, many phages coexist with them in a state called lysogeny.

a phage genome that has been inserted into a specific site on a bacterial chromosome

An environmental signal, such as a certain chemical or high-energy radiation

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results in the release of new phages by lysis (and death) of the host cell

the viral genome becomes incorporated into the bacterial host chromosome as a prophage, is replicated along with the chromosome, and does not kill the host

nearly all animal viruses have an RNA genome and an envelope

an RNA virus that replicates by transcribing its RNA and DNA and then inserting the DNA into a cellular chromosome

prophage: a phage genome that has been inserted into a specific site on a bacterial chromosome provirus: a viral genome that is permanently inserted into a host genome. Animal cells are most likely to carry a provirus

small, circular DNA molecules found in bacteria and in the unicellular eukaryotes called yeasts

DNA segments that can move from one location to another within a cell's genome

1. Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes. 2. Some viruses cause infected cells to produce toxins that lead to disease symptoms. 3. Some viruses have molecular components that are toxic, such as envelope proteins.

because the infected respiratory epithelial cells regenerate, but polio infects mature nerve cells that are not able to regenerate

Vaccines: Vaccines are a harmless variant or derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen. Antiviral drugs: Most antiviral drugs resemble nucleosides and as a result interfere with viral nucleic acid synthesis.

1. The mutations change existing viruses into new genetic varieties (strains) that can cause disease, even in individuals who are immune to the ancestral virus. 2. The dissemination of a viral disease from a small, isolated human population made possible by global travel and other social factors 3. The spread of existing viruses from other animals

A pandemic is a global epidemic. The name H1N1 identifies which forms of the two viral surface proteins are present: hemagglutinin (H) and neuraminidase (N).

horizontal: the plant is infected from an external route vertical: the plant inherits a viral function from a parent

Once a virus enters a plant cell and begins replicating, viral genomes and associated proteins can spread throughout the plant by means of plasmodesmata, the cytoplasmic connections that penetrate the walls between adjacent plant cells.

A viroid is a circular RNA molecule that infects plants. An important lesson from viroids is that a single molecule can be an infectious agent that spreads a disease.

One example of a viroid disease is cadang-cadang, which is killing millions of coconut palms.

Prions are infectious proteins which appear to cause a number of degenerative brain diseases in various animal species. Prions are most likely transmitted in food. They cause misfolding of proteins, particularly in the brain. The infection results in slow damage, but ultimately leads to death.

Scrapie in sheep, mad cow disease, Creutzfeld-Jakob disease, and kuru

1. Prions act very slowly, with an incubation period of at least ten years before symptoms develop. 2. Prions are virtually indestructible; they are not destroyed or deactivated by heating to normal cooking temperature.

Stanley Prusiner, 1997