Chapter 19

Question 1

What are viruses?

Answer 1

Small, non-cellular infectious particle consisting of genes packaged in a protein coat and, sometimes, a membrane

Question 2

Why aren’t viruses considered to be alive?

Answer 2

Do not consist of cells, cannot reproduce or metabolize outside of a host cell

Question 3

What are the molecular components of a virus?

Answer 3

A nucleic acid surrounded by a protein coat

Question 4

Describe viral genomes types.

Answer 4

Viral genomes consist of either double- or single-stranded DNA or double- or single-stranded RNA; Their genome has 3 to 2000 genes and may be a single linear or circular molecule of nucleic acid

Question 5

What are capsids?

Answer 5

the protein shell that encloses the viral genome

Question 6

What is the function of the capsid?

Answer 6

to hold the viral nucleic acid

Question 7

What are capsids composed of?

Answer 7

protein subunits called capsomeres

Question 8

How do capsids vary between viruses?

Answer 8

they vary in shape and size; may be spherical, helical, icosahedral, etc.

Question 9

What is a viral envelope?

Answer 9

A membrane that surrounds the capsids of influenza viruses and many other viruses found in animals; they contain a combination of viral and host cell molecules

Question 10

What are viral envelopes derived from?

Answer 10

From membranes of host cells

Question 11

Give an example of a virus that has an envelope.

Answer 11

influenza

Question 12

What is a bacteriophage?

Answer 12

Viruses that infect bacteria

Question 13

Describe the appearance of a phage

Answer 13

Have an elongated capsid head that encloses their DNA and attached to the protein tailpiece

Question 14

What does it mean that viruses are obligate intracellular parasites?

Answer 14

They can only replicate within a host cell

Question 15

Describe the general life cycle of a virus.

Answer 15

The virus enters the host cell and the nucleic acid and capsids separate. Once a viral genome has entered a cell, the cell begins to manufacture viral proteins, using host enzymes, ribosomes, tRNAs, amino acids, ATP, etc. The nucleic acid is replicated to produce copies of the viral genome and transcribed/translated to produce more capsomeres. These products self-assemble into new virus particles which then exit the cell.

Question 16

Lytic cycle

Answer 16

results in the death of the host cell; Produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses

Question 17

Lysogenic cycle

Answer 17

The viral DNA molecule is incorporated into the host cell’s chromosome without destroying the cell as a prophage; Every time the host divides, it copies the phage DNA and passes a copy to daughter cells

Question 18

What is a virulent phage?

Answer 18

A phage that reproduces only by the lytic cycle

Question 19

What is a temperate phage?

Answer 19

Phages that use both the lytic and lysogenic cycles

Question 20

What is a prophage?

Answer 20

the viral DNA molecule is incorporated into the host cell’s chromosome without destroying the cell

Question 21

How are viral envelopes generated by animal viruses?

Answer 21

They are made from the host’s plasma membrane as viral capsids exit

Question 22

Describe the process of production of viral DNA, RNA, and proteins in a cell.

Answer 22

In DNA viruses, the process of viral DNA transcription/translation is similar to normal gene expression. The viral DNA is transcribed into viral RNA which is translated into viral capsomeres
and/or glycoproteins. Additionally, replication is similar to regular replication. In non-retroviral RNA viruses, the viral RNA can be immediately translated into viral proteins such as capsomeres and/or glycoproteins. The viral RNA is replicated from the viral RNA template rather than from DNA.

Question 23

How is a retrovirus different from a typical animal virus?

Answer 23

It becomes a permanent resident of the host cell because the viral DNA is integrated into the host genome. After this integration, the DNA is expressed in the same manner as normal gene expression

Question 24

What is a provirus?

Answer 24

The viral DNA that is integrated into the host genome

Question 25

What is reverse transcriptase?

Answer 25

The enzyme that retroviruses use to copy their RNA into DNA

Question 26

Describe the ways that viral infections can cause damage to infected organisms.

Answer 26

Viruses may damage or kill cells by causing the release of hydrolytic enzymes by damaging lysosomes. Others have molecular components such as envelope proteins that are toxic

Question 27

What is an epidemic?

Answer 27

general outbreak of an illness

Question 28

How does it differ from a pandemic?

Answer 28

an epidemic is usually localized whereas a pandemic is a global outbreak

Question 29

What is a vaccine?

Answer 29

A medical treatment that contains harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the harmful pathogen

Question 30

How can it serve as a defense against epidemics?

Answer 30

It can prevent people from getting the viral illness

Question 31

Should antibiotics be used to treat viral infections?

Answer 31

No! they don’t work on viruses, only bacteria

Question 32

Describe the three processes that contribute to the emergence of new viral diseases.

Answer 32

1) RNA viruses have an unusually high rate of mutation
2) The disease can be disseminated from a small, isolated human population and can eventually spread around the world
3) About three-quarters of new human diseases originate by spreading from animals to humans

Question 33

What are zoonotic viruses?

Answer 33

human diseases that originate by spreading from animals to humans

Question 34

Give an example of a zoonotic virus.

Answer 34

Flu epidemics are caused by type A influenza viruses; these infect a wide variety of animals including birds, pigs, horses, and humans

Question 35

What does the tailpiece of a phage do?

Answer 35

Attaches the bacteriophage to the host and injects its DNA inside

Question 36

Give an example of a retrovirus in humans.

Answer 36

HIV