Ch 19 Viruses

What tools are in the medical arsenal against human viral diseases?

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.

What are restriction enzymes?

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

What was Wendell Stanley's contribution to our knowledge of viruses?

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

phospholipids and membrane proteins

derived from: host cell

proteins and glycoproteins

derived from: viral origin

What are the four forms of viral genomes?

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

Distinguish between horizontal transmission and vertical transmission in plants.

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

What are capsomeres?

large number of protein subunits that build capsids

What are two elements that nearly all animal viruses have?

nearly all animal viruses have an RNA genome and an envelope

Fig 19.6

pg 386

What portion of a phage enters the host cell and how?

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

Compare and contrast a prophage and a provirus. Which one are you likely to carry?

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

lytic mode of bacteriophage reproduction

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

plasmids

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

transposons

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

How does a DNA virus reproduce its genome?

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

The 2009 flu pandemic is H1N1. What is a pandemic? What does the name of the flu mean?

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).

What is a viroid? What important lesson do they teach?

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.

Fig. 19.3

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

Name four diseases caused by prions.

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

Name one viroid disease.

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

What is a capsid?

the protein shell enclosing the viral genome

Distinguish between virulent and temperate phages.

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

What are three ways bacteria may win the battle against the phages?

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.

What are two alarming characteristics of prions?

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.

What might trigger the switchover from lysogenic to lytic mode?

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

Why do we recover completely from a cold but not from polio?

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

What different shapes may capsids have?

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

What is meant by host range?

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

What is the role of an envelope in animal viruses?

help the virus infect their host

What are three processes that contribute to this sudden emergence of emerging viruses?

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

What are three ways that viruses make us ill?

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.

What was some early evidence of the existence of viruses? Why were they difficult to study?

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.

How do viruses spread throughout plant bodies?

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.

What are prions, how are they transmitted, and what do they do?

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.

Distinguish between a virus with a broad host range and one with an extremely limited host range, and give an example of each.

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

Two Nobel Prizes have been awarded for the study of prions. One went to Carlton Gajdusek, who worked with the Fore people of Papua, New Guinea in the 1960s to determine the cause of a kuru epidemic. Who got the second Nobel Prize in this area, and when?

Stanley Prusiner, 1997

Why don't restriction enzymes destroy the DNA of the bacterial cells that produce them?

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

What components of the host cell does a virus use to reproduce itself?

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.

Compare the host range for the rabies virus to that of the human cold virus.

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.

Viruses are obligate intracellular parasites. What does this mean?

Viruses can replicate only within a host cell

What is a prophage?

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

What are bacteriophages?

a virus that infects bacteria; also called a phage

What is a retrovirus?

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

lysogenic mode of bacteriophage reproduction

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

How do restriction enzymes prevent viral infection of bacteria?

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

How do most RNA viruses replicate their genome?

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

What property of a virus determines its attachment to a host cell membrane?

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