ch13

Question 1

virus

Answer 1

A virus is a minuscule, acellular, infectious agent usually
having one or several pieces of nucleic acid—either DNA or
RNA

Question 2

what do viruses lack

Answer 2

• cytoplasmic membrane
• cysotol and functional organelles
• not capable of metabolic activity on their own

Question 3

Viruses have an extracellular and an intracellular state.

Answer 3

• outside: virion = capsic + nucleic acid (maybe envelope)

- inside: capsid removed, just nucleic acid

Question 4

capsid

Answer 4

Basically, a virion consists of a protein
coat, called a capsid, surrounding a nucleic acid core.
- The capsid of a virus is
composed of proteinaceous subunits called capsomeres (or capsomers).
Some capsomeres are composed of only a single type of
protein, whereas others are composed of several different kinds
of proteins.

Question 5

envelope

Answer 5

Some virions have
a phospholipid membrane called an envelope surrounding
the nucleocapsid. The outermost layer of a virion (capsid or
envelope) provides the virus both protection and recognition
sites that bind to complementary chemicals on the surfaces
of their specific host cells.

Question 6

generalists

Answer 6

By contrast, some viruses are generalists;
they infect many kinds of cells in many different hosts.
An example of a generalist virus is West Nile virus

Question 7

viruses were first identified

Answer 7

from tobacco plants

Question 8

fungal viruses

Answer 8

• exist only within cells; that is, they seemingly
  have no extracellular state.
• Presumably, fungal viruses
  cannot penetrate a thick fungal cell wall. However, because
  fusion of cells is typically a part of a fungal life cycle, viral infections
  can easily be propagated by

Question 9

nucleocapsid

Answer 9

Together the nucleic acid and its capsid are
also called a nucleocapsid, which in many cases can crystallize
like crystalline chemicals

Question 10

viral shapes

Answer 10

• helical
• polyhedral (most common: icosahedron - 20 sides)
• complex

Question 11

matrix proteins

Answer 11

viral proteins called matrix proteins fill the region between
capsid and envelope.

Question 12

much-studied virus

Answer 12

dsDNA phage of e coli called type 4 (T4). T4 virions are complex, having
the polyhedral heads and helical tails seen in many bacteriophages.

Question 13

lytic replication cycle

Answer 13

• the cell undergoes lysis near the end of the cycle

- AESAR: attachment, entry, synthesis, assembly, release

Question 14

attachment

Answer 14

• Because phages, like all virions, are nonmotile, contact with a
  bacterium occurs by purely random collision
• The structures responsible for the attachment
  of T4 to its host bacterium are its tail fibers
• Attachment
  is dependent on the chemical attraction and precise fit between
  attachment proteins on the phage’s tail fibers and complementary
  receptor proteins on the surface of the host’s cell wall

Question 15

entry

Answer 15

• Upon contact with E. coli, T4
  releases lysozyme, a protein enzyme carried within
  the capsid that weakens the peptidoglycan of the cell wall.
• phage’s tail sheath then conctracts, forcing a tube within the tail through the cell wall and membrane
• phage injects genome thru tube and into bacteria
• empty capsid left outside

Question 16

viral enzymes

Answer 16

are either carried within the capsid or

coded by viral genes and made by the bacterium

Question 17

synthesis

Answer 17

• viral enzymes degrade bacterial DNA into its constituent nucleotides; As a result, the
  bacterium stops synthesizing its own molecules and begins
  synthesizing new viruses under control of the viral genome.
• Translation by the host cell’s ribosomes results
  in viral proteins, including head capsomeres, components of
  the tail, viral DNA polymerase (which replicates viral DNA),
  and lysozyme (which weakens the bacterial cell wall from
  within, enabling the virions to leave the cell once they have
  been assembled).

Question 18

assmelby

Answer 18

• viral assembly is a spontaneous process, requiring little

or no enzymatic activity

Question 19

transduction

Answer 19

Sometimes a capsid assembles around leftover pieces of
host DNA instead of viral DNA. A virion formed in this manner
is still able to attach to a new host by means of its tail fibers,
but instead of inserting phage DNA, it transfers DNA from the
first host into a new host. This process is known as transduction

Question 20

release

Answer 20

Newly assembled virions are released from the cell as lysozyme
completes its work on the cell wall and the bacterium disintegrates.

Question 21

burst time

Answer 21

For any phage undergoing
lytic replication, the period of time required to complete
the entire process, from attachment to release, is called the
burst time

Question 22

burst size

Answer 22

the number of new virions released from each

lysed bacterial cell is called the burst size

Question 23

lysogeny

Answer 23

• Some bacteriophages have a modified replication
  cycle in which infected host cells grow and reproduce normally
  for many generations before they lyse.
• temperate / lysogenic phages

Question 24

much-studied temperate phage

Answer 24

lambda phage

- another e coli parasite

Question 25

lysogenic conversion

Answer 25

Lysogenic phages can change the phenotype of a bacterium,
for example from a harmless form into a pathogen—a process
called lysogenic conversion.

Question 26

prophage

Answer 26

temperate phage enters cell and remains inactive. Such an inactive bacteriophage is
called a prophage. A prophage remains inactive by
coding for a protein that suppresses prophage genes. (One prophage gene codes for a protein that prevents transcription
of most of the other prophage genes.) A side effect
of this repressor protein is that it renders the bacterium resistant
to additional infection by other viruses of the same type.
- the prophage is inserted into the DNA of the bacterium, becoming
a physical part of the bacterial chromosome

Question 27

induction

Answer 27

when the lambda genome is induced to exit the bacterial chromosome (by recombination?)
and initiate a lytic cycle. Inductive
agents are typically the same physical and chemical agents
that damage DNA molecules, including ultraviolet light, X rays,
and carcinogenic chemicals.
- After induction, the lytic steps of synthesis, assembly,
and release resume from the point at which they stopped. The
cell becomes filled with virions and breaks open.

Question 28

lysogenic vs lytic

Answer 28

lysogenic: The viral DNA enters the cell, just as occurs with phage
T4, but the host cell’s DNA is not destroyed, and the phage’s genome
does not immediately assume control of the cell.

Question 29

bacteriophages vs animal viruses: structure

Answer 29

Unlike the bacteriophages we have examined, animal
viruses lack both tails and tail fibers. Instead, animal viruses
typically have glycoprotein spikes or other attachment molecules
on their capsids or envelopes.

Question 30

animal virus entry mechanisms

Answer 30

at least three different
mechanisms: direct penetration, membrane fusion, and
endocytosis.

Question 31

direct penetration

Answer 31

Some naked viruses enter their hosts’ cells by direct
penetration—a process in which the viral capsid attaches and
sinks into the cytoplasmic membrane, creating a pore through
which the genome alone enters the cell.

Question 32

membrane fusion

Answer 32

• the entire capsid
  and its contents (including the genome) enter the host cell
• viral envelope and the
  host cell membrane fuse, releasing the capsid into the cell’s
  cytoplasm and leaving the envelope glycoproteins as part of the
  cell membrane

Question 33

endocytosis

Answer 33

• the entire capsid
  and its contents (including the genome) enter the host cell
• Attachment of the virus to receptor
  molecules on the cell’s surface stimulates the cell to endocytize
  the entire virus

Question 34

uncoating

Answer 34

For those viruses that penetrate a host cell with their capsids
intact, the capsids must be removed to release their genomes
before the viruses can continue to replicate. The removal of a
viral capsid within a host cell is called uncoating, a process that
remains poorly understood. (membrane fusion and endocytosis)

Question 35

DNA vs RNA animal viruses

Answer 35

DNA viruses typically enter the nucleus,

whereas most RNA viruses are replicated in the cytoplasm.

Question 36

animal dsDNA virus synthesis

Answer 36

After messenger RNA is transcribed from viral DNA in the nucleus
and capsomere proteins are made in the cytoplasm by host
ribosomes, capsomeres enter the nucleus, where new virions
spontaneously assemble.

Question 37

animal ssDNA virus synthesis

Answer 37

Cells do not use
ssDNA, so when a parvovirus enters the nucleus of a host cell,
host enzymes produce a new strand of DNA complementary
to the viral genome. This complementary strand binds to the
ssDNA of the virus to form a dsDNA molecule. everything else same

Question 38

parvovirus

Answer 38

• A human virus with a genome composed of
  single-stranded DNA (ssDNA) is a parvovirus.
• prominent one: causes contagious disease in dogs

Question 39

types of rna viruses

Answer 39

There are four types of RNA viruses:
positive-sense, single-stranded RNA (designated +ssRNA);
retroviruses (a kind of +ssRNA virus); negative-sense, singlestranded
RNA (−ssRNA); and double-stranded RNA (dsRNA).

Question 40

+RNA

Answer 40

Single-stranded viral RNA that can
act directly as mRNA is called positive-strand RNA (+RNA). Ribosomes translate polypeptides using the codons of such
RNA.

Question 41

-RNA

Answer 41

in many +ssRNA viruses, a complementary negative-strand RNA (−RNA)
is transcribed from the +ssRNA genome by viral RNA polymerase;
-RNA then serves as the template for the transcription of
multiple +ssRNA genomes. Such transcription of RNA from RNA
is unique to viruses; no cell transcribes RNA from RNA.

Question 42

retroviruses

Answer 42

• a kind of +ssRNA virus
• don’t use their genome as mRNA
• use reverse transcriptase (carried in capsid) to synthesize DNA from +RNA
• this DNA serves as the template for more +RNA molecules, which act both as mRNA
  for protein synthesis and as genomes for new virions
• HIV

Question 43

-ssRNA

Answer 43

In order to synthesize a protein, a ribosome can use only mRNA
(i.e., +RNA) because −RNA is not recognized by ribosomes. The
virus overcomes this problem by carrying within its capsid an
enzyme, RNA-dependent RNA transcriptase, which is released into
the host cell’s cytoplasm during uncoating and then transcribes
+RNA molecules from the virus’s −RNA genome. Translation of
proteins can then occur as usual. The newly transcribed +RNA
also serves as a template for transcription of additional copies of −
RNA

Question 44

dsRNA

Answer 44

The positive strand of the molecule
serves as mRNA for the translation of proteins, one of which
is an RNA polymerase that transcribes dsRNA. Each strand of
RNA acts as a template for transcription of its opposite, which is
reminiscent of DNA replication in cells

Question 45

persistent infectioin graph

Answer 45

!!!!!!

Question 46

persistent infections

Answer 46

Infections with enveloped viruses in which
host cells shed viruses slowly and relatively steadily are called
persistent infections; a curve showing virus abundance over time
during a persistent infection lacks the burst of new virions seen
in lytic replication cycles

Question 47

budding

Answer 47

Enveloped animal viruses are often released via a process
called budding. As virions are assembled, they
are extruded through one of the cell’s membranes—the nuclear,
endoplasmic reticulum, or the cytoplasmic membrane. Each
virion acquires a portion of membrane, which becomes the
viral envelope.
- During synthesis, some viral glycoproteins are inserted into cellular membranes, and these proteins become
the glycoprotein spikes on the surface of the viral envelope.
- Because the host cell is not quickly lysed, as occurs in bacteriophage
replication, budding allows an infected cell to remain
alive for some time

Question 48

releasing naked animal viruses

Answer 48

Naked animal viruses may be released in one of two ways:
Either they may be extruded from the cell by exocytosis, in a
manner similar to budding but without the acquisition of an
envelope, or they may cause lysis and death of the cell, reminiscent
of bacteriophage release.

Question 49

one reason it is difficult to treat viral diseases

Answer 49

Because viral replication uses cellular structures and pathways
involved in the growth and maintenance of healthy cells,
any strategy for the treatment of viral diseases that involves
disrupting viral replication may disrupt normal cellular processes
as well.

Question 50

latency

Answer 50

Some animal viruses, including chicken pox and herpes viruses,
may remain dormant in cells in a process known as latency; the
viruses involved in latency are called latent viruses or proviruses.
Latency may be prolonged for years with no viral activity,
signs, or symptoms. Though latency is similar to lysogeny
as seen with bacteriophages, there are differences. Some latent
viruses do not become incorporated into the chromosomes of
their host cells, whereas lysogenic phages always do.

Question 51

provirus

Answer 51

On the other hand, some animal viruses (e.g., HIV) are more
like lysogenic phages in that they do become integrated into a
host chromosome as a provirus. However, when a provirus is incorporated
into its host DNA, the condition is permanent; induction
does not occur in eukaryotes

Question 52

neoplasia

Answer 52

if something upsets the genetic control, cells begin to divide uncontrollably.
This phenomenon of uncontrolled cell division in a multicellular animal is called neoplasia.

Question 53

tumor

Answer 53

a mass of

neoplastic cells is a tumor.

Question 54

benign

Answer 54

Some tumors are benign; that is, they remain in one place
and are not generally harmful, although occasionally such noninvasive
tumors are painful and rob adjacent normal cells of
space and nutrients.

Question 55

malignant

Answer 55

Other tumors are malignant, invading
neighboring tissues and even traveling throughout the body to
invade other organs and tissues to produce new tumors—a process
called metastasis. Malignant tumors are also
called cancers. Cancers rob normal cells of space and nutrients
and cause pain; in some kinds of cancer, malignant cells derange
the function of the affected tissues, until eventually the body can
no longer withstand the loss of normal function and dies.

Question 56

protooncogenes

Answer 56

genes that play a role in cell division. As long as protooncogenes
are repressed, no cancer results. However, activity of oncogenes
(their name when they are active) or inactivation of oncogene
repressors can cause cancer to develop.

Question 57

Viruses cause __% to __% of human cancers in several

ways:

Answer 57

20 - 25
Some viruses carry copies of oncogenes as part of their
genomes, other viruses promote oncogenes already present in
the host, and still other viruses interfere with normal tumor repression
when they insert (as proviruses) into repressor genes.

Question 58

Virologists have developed three types of media

for culturing viruses:

Answer 58

media consisting of mature organisms
(bacteria, plants, or animals), embryonated (fertilized) eggs, and
cell cultures.

Question 59

culturing viruses in bacteria

Answer 59

Phages can be grown in bacteria maintained either in liquid cultures
or on agar plates. In the latter case, bacteria and phages
are mixed with warm (liquid) nutrient agar and poured in a
thin layer across the surface of an agar plate. During incubation,
bacteria infected by phages lyse and release new phages
that infect nearby bacteria, while uninfected bacteria grow and
reproduce normally

Question 60

plaques

Answer 60

After incubation, the appearance of the
plate includes a uniform bacterial lawn interrupted by clear
zones called plaques, which are areas where phages have lysed
the bacteria.

Question 61

plaque assay

Answer 61

Such plates enable the estimation of
phage numbers via a technique called plaque assay, in which
virologists assume that each plaque corresponds to a single
phage in the original bacterium-virus mixture.

Question 62

Culturing Viruses in Chicken Eggs

Answer 62

Chicken eggs are a useful culture medium for viruses because
they are inexpensive, are among the largest of cells, are free of
contaminating microbes, and contain a nourishing yolk (which
makes them self-sufficient). Most suitable for culturing viruses
are chicken eggs that have been fertilized and thus contain a
developing embryo. Embroyonic tissues (called membrans) provide
ideal inoculation sites for growing viruses.

Question 63

2 types of cell cultures

Answer 63

diploid and continuous

Question 64

diploid cell cultures

Answer 64

are created from embryonic animal, plant, or human
cells that have been isolated and provided appropriate growth
conditions. The cells in diploid cell culture generally last no
more than about 100 generations (cell divisions) before they die.

Question 65

continuous cell cultures

Answer 65

are

longer lasting because they are derived from tumor cells.

Question 66

viroids

Answer 66

Viroids are extremely small, circular pieces of RNA that are infectious
and pathogenic in plants. Viroids are similar to
RNA viruses except that they lack capsids. Even though they are
circular, viroids may appear linear because of hydrogen bonding
within the molecule.

Question 67

viroidlike agents

Answer 67

Viroidlike agents—infectious, pathogenic RNA particles that

lack capsids but do not infect plants—affect some fungi.

Question 68

prions

Answer 68

a proteinaceous infectious
agent that was different from any other known infectious
agent in that it lacked instructional nucleic acid.

Question 69

PrP

Answer 69

All mammals make a cytoplasmic membrane protein
called PrP. PrP is anchored in lipid rafts and plays a role in
the normal activity of the brain, though the exact function of
PrP is unknown. The amino acid sequence in PrP is such that
the protein can fold into two stable tertiary structures: The normal, functional structure of cellular PrP has several prominent
α-helices, whereas a disease-causing form—prion PrP—is
characterized by β-pleated sheets.

Question 70

how

can prions carry the information required to replicate themselves?

Answer 70

Scientists have determined that prion PrP acts like a bad influence
in a crowd of teenagers, encouraging molecules of normal,
cellular PrP to misbehave by refolding into prion PrP molecules,
which then clump together.

Question 71

prion diseases

Answer 71

As clumps of prion PrP propagate
throughout the brain, neurons stop working properly and eventually
die, leaving holes and a spongy appearance.
Because of this characteristic, clinicians call prion diseases spongiform encephalopathies.

Question 72

Why don’t prions develop in all mammals, given that all

mammals have PrP?

Answer 72

Under normal circumstances, it appears
that other nearby proteins and polysaccharides in lipid rafts
force PrP into the correct (cellular) shape. Mutations in the PrP
gene can result in the initial formation of prion PrP, but human
cellular PrP visually misfolds only if it contains methionine as
the 129th amino acid.

Question 73

Prions are associated with several diseases:

Answer 73

including bovine
spongiform encephalitis (BSE, so-called mad cow disease),
scrapie in sheep, kuru (a human disease that has been eliminated),
chronic wasting disease (CWD) in deer and elk, and variant
Creutzfeldt-Jakob disease (vCJD) in humans.

Question 74

deactivating prions

Answer 74

Normal cooking or sterilization procedures do not deactivate
prions, though they are destroyed by incineration or by
autoclaving in concentrated sodium hydroxide.