Chapter 19 (Study Modules)

Which of the following is an example of vertical transmission of a virus in plants?

An infected plant produces seeds that contain the virus, giving rise to infected progeny.

A phage that inserts itself into the host DNA is called __________.

lysogenic

Vaccines for viral diseases are __________ and help prevent infection by __________.

harmless derivatives of pathogenic viruses; stimulating the immune system to mount a defense against the actual pathogen

Some viruses have membranous envelopes. Where do viral envelopes typically originate from?

Membranes from the host cell

How do prions propagate and replicate themselves?

Prions convert normal proteins into the misfolded prion version.

Which of the following statements correctly describes one difference between virulent phages and temperate phages?

Virulent phages replicate through the lytic cycle only, and temperate phages replicate using both the lytic and the lysogenic cycles.

Why are viruses often considered to be nonliving?

Viruses do not carry out metabolic processes, a primary characteristic of living organisms.

Viruses can vary with respect to all of the following characteristics except __________.

the presence or absence of metabolic machinery

Viruses that infect bacteria are called __________.

bacteriophages

How does a retrovirus like HIV enter a host cell?

Glycoproteins on the viral envelope bind to receptors on the host cell. The viral envelope fuses with the host cell's plasma membrane, facilitating uptake of the virus by the host cell.

Why can flare-ups of herpes virus infection recur throughout a person's life?

Herpes virus can leave its DNA behind as minichromosomes in nerve cell nuclei. Stress can trigger another round of virus production, producing characteristic blisters and sores.

When a virus infects an E. coli cell, what part of the virus enters the bacterial cytoplasm?

Only the nucleic acid

Reverse transcription, carried out by retroviruses, is the process by which __________.

RNA information is copied into DNA

When comparing DNA and RNA viruses, which mutate more quickly, and why?

RNA viruses, because no proofreading is done on RNA molecules

How are retroviruses different from other types of viruses?

Retroviruses use the enzyme reverse transcriptase to transcribe a copy of DNA from their own RNA.

Which of the following events occurs during the lytic life cycle of phages?

The host cell usually dies, releasing many new copies of the virus.

Prions are __________ that are thought to cause disease by __________.

abnormally shaped proteins; inducing similar but normally shaped proteins in the brain to adopt the abnormal form

Restriction enzymes help defend bacteria against viral infections by __________.

cutting viral DNA once it has entered the cell

A widespread outbreak of a viral disease is called a(n) __________, and a global outbreak is called a(n) __________.

epidemic; pandemic

The avian flu virus H5N1 is considered a greater long-term threat than the swine flu virus H1N1 because __________.

it has a significantly higher mortality rate

The phage reproductive cycle that kills the bacterial host cell is a __________ cycle, and a phage that always reproduces this way is a __________ phage.

lytic; virulent

Animals that harbor and can transmit a particular virus but are generally unaffected by it are said to act as a __________ for that virus.

natural reservoir

Which of the following is not a class I virus?

parvovirus

All of the following types of viruses have envelopes except __________.

picornovirus

Candidates for the original source of viral genomes include __________.

plasmids and transposons

Viral DNA incorporated into a bacterial chromosome is known as a(n) __________.

prophage

Class IV viruses are characterized by which of the following types of genomes?

single-stranded RNA that serves as mRNA

HIV, the virus that causes AIDS, only infects certain cells within the immune system. This is because __________.

the virus binds to specific receptors that are only present on certain immune cells

Circular RNA molecules that function like a virus in plants are termed __________.

viroids