Chapter 19 Viruses

Front 1

AIDS (acquired immunodeficiency syndrome)

Back 1

The symptoms and signs present during the late stages of HIV infection, defined by a specified reduction in the number of T cells and the appearance of characteristic secondary infections.

Front 2

bacteriophage

Back 2

A virus that infects bacteria

Front 3

capsid

Back 3

The protein shell that encloses a viral genome. It may be rod-shaped, polyhedral, or more complex in shape.

Front 4

HIV (human immunodeficiency virus)

Back 4

The infectious agent that causes AIDS. HIV is a retrovirus.

Front 5

host range

Back 5

The limited range of host cells that each type of virus can infect.

Front 6

lysogenic cycle

Back 6

A type of phage reproductive cycle in which the viral genome becomes incorporated into the bacterial host chromosome as a prophage and does not kill the host.

Front 7

lytic cycle

Back 7

A type of phage reproductive cycle resulting in the release of new phages by lysis (and death) of the host cell.

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pandemic

Back 8

A global epidemic.

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phage

Back 9

A virus that infects bacteria

Front 10

prion

Back 10

An infectious agent that is a misfolded version of a normal cellular protein. Prions appear to increase in number by converting correctly folded versions of the protein to more prions.

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prophage

Back 11

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

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provirus

Back 12

A viral genome that is permanently inserted into a host genome.

Front 13

restriction enzyme

Back 13

An endonuclease (type of enzyme) that recognizes and cuts DNA molecules foreign to a bacterium (such as phage genomes). The enzyme cuts at specific nucleotide sequences (restriction sites).

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retrovirus

Back 14

An RNA virus that reproduces by transcribing its RNA into DNA and then inserting the DNA into a cellular chromosome

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reverse transcriptase

Back 15

An enzyme encoded by certain viruses (retroviruses) that uses RNA as a template for DNA synthesis.

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temperate phage

Back 16

A phage that is capable of reproducing by either a lytic or lysogenic cycle.

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vaccination

Back 17

See immunization.

Front 18

vaccine

Back 18

A harmless variant or derivative of a pathogen that stimulates a host’s immune system to mount defenses against the pathogen.

Front 19

viral envelope

Back 19

A membrane that cloaks the capsid that in turn encloses a viral genome.

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viroid

Back 20

A plant pathogen consisting of a molecule of naked, circular RNA a few hundred nucleotides long.

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virulent phage

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A phage that reproduces only by a lytic cycle.

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Viruses are smaller and simpler than some bacteria and other prokaryotes

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lacking the structures and metabolic machinery found in cells, most viruses are little more than genes packaged in protein coats.

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Latin root for the word virus means

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“poison” because they are capable of causing a wide variety of diseases and can be spread between organisms.

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Viruses cannot reproduce or carry out metabolic activities outside of a

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host cell.

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Simple phrase used by two researchers describes viruses aptly enough

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Viruses lead “a kind of borrowed life.”

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Molecular biology was born in the laboratories of biologists studying viruses that infect

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bacteria.

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Experiments with viruses provided important evidence that

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genes are made of nucleic acids, and they were critical in working out the molecular mechanisms of the fundamental processes of DNA replication, transcription, and translation.

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The study of viruses has led to the development of techniques that enable scientists to manipulate

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genes and transfer them from one organism to another.

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Viruses are used as

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agents of gene transfer in gene therapy.

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Tobacco Mosaic Disease

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stunts the growth of tobacco plants and gives theirs a mottled, or mosaic, coloration.

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1883, Adolf Mayer, discovered

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that he could transmit the disease from plant to plant by rubbing sap extracted from diseased leaves onto healthy plants, searched for the infectious microbe in the sap, suggested that the disease was cause by unusually small bacteria that were invisible under a microscope.

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Ivanowsky tested Mayer’s hypothesis, using a filter designed to remove

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bacteria, after filtration, the sap still produced mosaic disease, clung to the hypothesis that bacteria were small enough to pass through the filter or made a toxin that could do so.

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Ivanowsky’s second possibility was rule out by Beijerinck who carried out a series of experiments that showed that the infectious agent in the filtered sap could reproduce

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in fact, the pathogen reproduced only within the host it infected, showing that unlike bacteria used in the lab at that time the mysterious agent of mosaic disease could not be cultivated on nutrient media test tubes or petri dishes.

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Beijerinck imagined a reproducing particle must smaller and simpler than a bacterium

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generally credited with being the first scientist to voice the concept of a virus.

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Beijerinck’s suspicions were confirmed in 1935 by

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Stanley who crystallized the infectious particle, now known as tobacco mosaic virus (TMV), now seen with the help of an electron microscope.

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Tiniest viruses re only

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20 nm in diameter – smaller than a ribosome, millions could easily fit on a pinhead.

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Largest know virus

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diameter of several hundred nanometers is barely visible in the light microscope.

Front 38

Examining the structure of viruses more closely reveals that they are

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infectious particles consisting of nucleic acid enclosed in a protein coat and, in some case, a membranous envelope.

Front 39

Viruses can vary with respect to all of the following characteristics except _____. ( Concept 19.1)

Back 39

Your Answer:

the presence or absence of metabolic machinery

Correct. Viruses are not classified by the presence or absence of metabolic machinery, because viruses do not contain metabolic machinery.

Front 40

A microbiologist analyzes chemicals obtained from an enveloped RNA virus that infects monkeys. He finds that the viral envelope contains a protein characteristic of monkey cells. Which of the following is the most likely explanation? ( Concept 19.1)

Back 40

Your Answer:

The virus fools its host by mimicking its proteins.

Correct Answer:

The viral envelope forms as the virus leaves the host cell.

No. Although this may be true, it is not an explanation of how the virus acquired the protein.

Front 41

Which of the following, if any, may be a component of a virus? ( Concept 19.1)

Back 41

Your Answer:

all of the above
Correct. Viral genomes may consist of double-stranded or single-stranded DNA, double-stranded or single-stranded RNA, and many contain an envelope made up of a phospholipid bilayer.

Front 42

Viruses that infect bacteria are called _____. ( Concept 19.1)

Back 42

Your Answer:

bacteriophages

Correct. Viruses that infect bacteria are called bacteriophages or, simply, phages.

Front 43

HIV, the virus that causes AIDS, only infects certain cells within the immune system. This is because _____. ( Concept 19.2)

Back 43

Your Answer:

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

Correct. The virus binds to receptors such as CD4 that are only present on certain cells such as helper T cells.

Front 44

Cancer cells often have protein receptor molecules on their surfaces that differ from those on normal body cells. Given this fact, how might viruses be used to treat cancer? ( Concept 19.2)

Back 44

Your Answer:

Viruses could be engineered to infect only cancer cells by altering viral surface proteins to recognize only the receptors on cancer cells.

Correct. The host specificity of viruses could be used to make cancer cells "sick" while normal body cells would not be infected. This approach would reduce the collateral damage seen in chemotherapy.

Front 45

Why are phages useful in treating bacterial infections in humans? ( Concept 19.2)

Back 45

Your Answer:

The first three answers are correct.

Correct. Phage therapy is useful because of its host specificity and ability to evolve to avoid the problems of resistance.

Front 46

Which of the following can a virus do without a host cell? ( Concept 19.2)

Back 46

Your Answer:

none of the above

Correct. Viruses are obligate intracellular parasites. They require a host cell to carry out all of the above cellular processes (and more).

Front 47

When a virus infects an E. coli cell, what part of the virus enters the bacterial cytoplasm? ( Concept 19.2)

Back 47

Your Answer:

only the nucleic acid

Correct. After attaching to receptors on the surface of the bacterial cell, the virus injects its DNA into the cell.

Front 48

The phage reproductive cycle that kills the bacterial host cell is a _____ cycle, and a phage that always reproduces this way is a _____ phage. ( Concept 19.2)

Back 48

Your Answer:

lytic; virulent

Correct. A lytic cycle ends with the lysis of the bacterial host cell.

Front 49

In the lytic life cycle of phages _____. ( Concept 19.2)

Back 49

Your Answer:

the cell typically dies, releasing many copies of the virus

Correct. After attaching to receptors on the surface of the bacterial cell, the phage injects its nucleic acid into the cell. The phage then hijacks the cellular machinery to manufacture many copies of itself.

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A phage that inserts itself into the host DNA is called _____. ( Concept 19.2)

Back 50

Your Answer:

a bacteriophage






Correct Answer:

lysogenic

No. "Bacteriophage" and "phage" are synonyms.