Chapter 6 Exam 3 Flashcards
(94 cards)
1
Q
What are the two ways that viruses can exist?
A
extracellularly or intracellularly
2
Q
Describe extracellular viruses.
A
exist in an inactive state because they possess very few enzymes and cannot reproduce outside of living cells.
3
Q
Describe intracellular viruses.
A
exists as replicating nucleic acids that induce the host to synthesize viral components from which progeny irions are assembled and eventually released.
4
Q
What are bacteriophages?
A
viruses that affect bacteria.
5
Q
What is a virion?
A
a complete virus particle
6
Q
What is the size range of virions?
A
10-400nm
7
Q
Describe nucleocapsids.
A
surround the simplest virions.
composed of a nucleic acid and a protein coat called the capsid.
8
Q
What are protomers?
A
capsid proteins.
9
Q
What are the three types of capsid symmetry?
A
helical
icosahedral
complex
10
Q
What are enveloped viruses?
A
viruses with virions having an envelope.
11
Q
What are nonenveloped viruses?
A
viruses lacking an envelope.
12
Q
Describe helical capsids.
A
shaped like hollow tubes with protein walls.
the arrangement produces a rigid tube.
encloses an RNA genome, which is wound in a spiral and lies within a groove formed by the protein subunits.
13
Q
What is the size of a helical capsid influenced by?
A
protomers and the nucleic acid enclosed within the capsid.
14
Q
Describe icosahedral capsids.
A
regular polyhedron with 20 equilateral triangular faces.
Most efficient way to enclose a space.
constructed from capsomers.
15
Q
What are capsomers?
A
ring or knob shaped units made of five or six protomers.
16
Q
Describe complex symmetry.
A
virus structures that do not conform to the helical or icosahedral shape.
17
Q
What is binal symmetry?
A
the symmetry of a virus that has both helical and icosahedral shape.
18
Q
What is an envelope?
A
an outer membranous layer that usually arises from the plasma or nuclear membranes of the host cell.
19
Q
Where are envelope lipids and carbohydrates acquired from?
A
the host.
20
Q
Describe spikes.
A
envelope proteins that project from the envelope surface.
involved in virion attachment to the host cell surface.
can be used to identify specific viruses.
21
Q
What is neuraminidase?
A
Functions in the release of mature visions from the host cell.
Contained in certain spikes.
22
Q
What are hemagglutinin proteins?
A
Spikes that bind visions to red blood cells and cause hemagglutination.
23
Q
What are the four types of nucleic acid that can make up viruses?
A
Double stranded DNA, single stranded DNA, single stranded RNA and double stranded RNA
24
Q
Which types of nucleic acids are used by animal viruses?
A
dsDNA, ssDNA, dsRNA, ssRNA
25
Which types of nucleic acids are used by plant viruses?
ssRNA
26
Which types of nucleic acids are used by bacterial viruses?
dsDNA
27
What are segmented genomes?
genomes that consist of more than one piece of RNA.
| Present in most RNA genomes.
28
What is the first step in the life cycle of a virus?
attachment (adsorption) to a host.
29
What is adsorption followed by?
entry of either the nucleocapsid or the viral nucleic acid into the host.
30
What happens if the nucleocapsid enters the host?
uncoating of the genome usually occurs before the life cycle continues.
31
What happens once the nucleocapsid enters the host?
the synthesis stage begins. viral genes are transcribed and translated.
32
What happens after the synthesis stage occurs?
the assembly stage. new nucleocapsids are constructed by self-assembly of coat proteins with the nucleic acids.
33
What is the release step?
mature virions escape the host.
34
How is attachment to the host accomplished?
specific interactions between molecules on the surface of the virion and molecules on the surface of the host cell called receptors.
35
What happens when an animal virus particle attaches to a receptor?
causes conformational changes in virion proteins that facilitate interaction with secondary receptors, entry into the cell, and uncoating.
36
What does receptor specificity account for?
the observation that animal viruses infect specific animals and in some cases, only particular tissues of that animal.
37
Describe eukaryotic cell membrane receptors.
have microdomains called lipid rafts that are thought to be involved in both virion entrance and assembly.
38
How are plant viruses an exception?
they do not have cell receptors.
| damage of host cells is required for the virus to access and enter the host.
39
What is the best way for viruses to infect plants?
plant-eating insects carry virions from one plant to another.
the virions are deposited in plant tissues as the insect devours the plant.
40
What is uncoating?
when some viruses shed their capsid proteins.
41
What are the three modes of penetration and uncoating?
1. fusion of the viral envelope with the host cell membrane.
2. entry by endocytosis.
3. injection of viral nucleic acid into the cytoplasm of the host cell, leaving the capsid outside.
42
Wht does fusion of viral envelopes with the host cell plasma membrane involve?
viral envelope glycoproteins that interact with proteins in the plasma membrane of the host cell.
43
What kind of viruses enter host cells by endocytosis?
nonenveloped viruses.
44
Describe the process of endocytosis to enter host cells.
virions may be engulfed by receptor-mediated endocytosis.
| resulting endocytic vesicles are filled with virions and fuse with endosomes
45
Describe the synthesis stage in double stranded DNA viruses.
very similar to the typical flow of information in cells.
genetic information stored in DNA and replicated by enzymes called DNA polymerases, recoded as mRNA, and decoded during protein synthesis.
46
What is the advantage of the synthesis stage in double stranded DNA viruses?
these viruses only need to rely on the machinery of the host cell to create the new DNA. do not need extra organelles.
47
Describe the synthesis stage in RNA viruses.
must carry in their nucleocapsides the enzymes needed to complete the syntehsis stage, or they must be synthesized during the infection process.
48
Describe early proteins.
involved in taking over the host cell.
49
Describe late proteins.
involved in self-assembly and release.
50
Describe self-assembly.
the baseplate, tail fibers and head components are assembled separately. Once the baseplate is finished, the tail tube is built on it and the sheath is assembled around the tube.
The phage prohead is constructed with the aid of scaffolding proteins.
51
What is a packosome?
a complex of proteins that incorporates DNA into the prohead.
Consists of a protein called the portal protein and the terminase complex, which moves DNA into the prohead.
52
What supplies the energy for DNA to move into the prohead?
ATP--supplied by the metabolic activity of the host bacterium.
53
Describe the process of virion release.
nonenveloped viruses, lyse their host cells at the end of the intracellular phase.
enveloped viruses undergo budding.
requires the activity of viral proteins.
54
What are the viral proteins necessary for virion release?
lysozyme and holin, which creates holes in the plasma membrane of the host cell.
55
describe the process of budding.
when virions are released by budding, the host cell may survive and continue releasing virions for some time.
virus-encoded proteins are incorporated into the membrane.
nucleocapsid is simultaneously released and the envelope formed by membrane budding.
56
What is a virulent phage?
a bacteriophage that only has one option: to begin multiplying immediately upon entering its bacterial host, followed by release from the host by lysis.
57
What are temperate phages?
bacteriophages that have two options: can multiply and lyse the host cell, or they can remain within the host cell without destroying it.
58
What is lysogeny?
the relationship between a temperate phage and its host.
59
What is a prophage?
the form of the virus that remains within its host.
| the viral nucleic acid either integrated into the bacterial chromosome or free in the cytoplasm.
60
What are two characteristics of lysogenic bacteria?
1. cannot be reinfected by the same virus. (immunity to superinfection)
2. as they reproduce, the prophage is replicated and inherited by progeny cells.
61
What is induction?
caused by changes in growth conditions or ultraviolet irradiation of the host cell.
cause the prophage to initiate synthesis of phage proteins and to assemble new virions.
62
What is the result of induction?
the lysogenic cycle ends and the lytic cycle commences; the host cell lyses and progeny phage particles are released.
63
What is lysogenic conversion?
occurs when a temperate phage changes the phenotype of its host.
involves alteration in surface characteristics of the host
64
What are two advantages of lysogeny?
1. allows the viral nucleic acid to be maintained within a dormant host.
2. enables the survival of infected host cells within a population that has few uninfected cells.
65
What is a cytocidal infection?
an infection that causes death of the host cell.
66
What can eukaryotic infections cause?
microscopic or macroscopic degenerative changes or abnormalities in host cells and in tissues that are distinct from lysis. cytopathic effects.
67
What is a tumor?
a growth or lump of tissues resulting from neoplasia.
68
What is neoplasia?
unregulated abnormal new cell growth and reproduction.
69
Describe the form of tumor cells?
have aberrant shapes and altered plasma membranes that may contain distinctive molecules (tumor antigens).
70
What is anaplasia?
the reversion of a cell to a more primitive or less differentiated state.
71
Describe benign tumors.
formed when tumor cells remain in place to forma compact mass.
72
Describe malignant tumors.
formed when tumors actively spread throughout the body in a process known as metastasis.
73
What is carcinogenesis?
a complex, multistep process that involves the mutation of multiple genes.
74
What are oncogenes?
genes involved in carcinogenesis.
75
How are oncogenes formed in the body?
can be introduced into a cell by viruses; others arise from normal proto-oncogenes.
76
What are proto-oncogenes?
cellular genes required for normal growth but when mutated or overexpressed, they become oncogenes.
77
What are oncoviruses?
viruses known to cause cancer.
| encode proteins that bind to and inhibit the function of tumor suppressor proteins.
78
What do tumor supressor proteins do?
regulate cell cycling or monitor or repair DNA damage.
79
Describe Rb.
functions critical to normal cell cycling.
| when rendered inactive, they undergo uncontrolled reproduction and are hyperproliferative.
80
Describe p53.
normally initiates cell cycle arrest or apoptosis. When inactivated, it cannot do this and DNA damage continues.
81
What does the process of culturing temperate viruses require?
additional step of inducing the lytic phase of their life cycles.
82
How are animal viruses usually cultured?
by inoculating suitable host animals or embryonated eggs.
| also grown in tissue culture on monolayers of animal cells.
83
What are plaques?
formed when the virus causes its host cell to lyse on a culture.
enlarge as the virus replicates.
84
How can plant viruses be cultivated?
plant tissue cultures, cultures of separated cells, or cultures of protoplasts may be used.
viruses can also be grown in whole plants, usually after the leaves are mechanically inoculated.
85
What develops in infected plants?
a localized necrotic lesion develops due to the rapid death of cells in the infected area.
86
What is hemagglutination assay?
an indirect method of counting animal viruses.
if the ratio of virions to cells is large enough, virions will join the red blood cells together; that is, they agglutinate, forming a network that settle out of suspension.
87
What is a plaque assay?
several dilutions of a sample containing virus particles are plated with appropriate host cells.
when the multiplicity of infection is very low, each plaque in a layer of host cells is assumed to have arisen from the multiplication of a single virion. Therefore, a count of the plaques produced can be used to calculate the plaque forming units in the original sample.
88
What is the lethal dose?
the dilution that contains a concentration of virions large enough to destroy 50% of the host cells or organisms.
89
What is the infectious dose?
the dose that causes 50% of the host organisms to become infected.
90
What are viroids?
infections agents that consist only of RNA.
cause over 20 plant diseases.
covalently closed, circular ssRNAs.
rodlike shape.
do not encode gene products.
requires host-dependent RNA polymerase to function.
91
What is RNA silencing?
cell detects the presence of dsRNA and selectively degrades it.
viroids usurp this response by hybridizing to specific host mRNA molecules to which they have a complementary nucleotide sequence.
How viroids cause disease.
92
What are satellites?
consist only of nucleic acid.
may encode one or more gene products and need a helper virus to replicate and infect host cells.
three types: viruses, RNA, DNA.
need a helper virus in order to replicate.
93
What are prions?
cause a variety of neurodegenerative diseases in humans and other animals.
progressive degeneration of the brain and eventual death.
94
What are the two forms of prions?
infectious, abnormally folded form
normal cellular form.
the interaction between the two converts the cellular form into the abnormal form.
