Chapter 13 Flashcards
Viruses
1
Q
What are distinctive features of viruses
A
- Contain a single type of nucleic acid either DNA or RNA
- Contain a protein coat that surrounds the nucleate acid
- Multiply inside living cells by using the synthesizing machinery of the cell
- Causes the synthesis of specialized structures that can transfer the viral nucleic acid to other cells.
2
Q
The protein coat of a virus that surrounds the nucleic acid sometimes is enclosed by
A
an envelope made up of lipids, proteins and carbohydrates
3
Q
How were viruses originaly distinguished from other infectious agents
A
They are especially small in size, filterable, and they are obligatory intracellular cellular parasites
4
Q
Do viruses generate their own metabolism
A
Viruses have few or no enzymes of their own for metabolism, they lack enzymes for protein synthesis and ATP generation
5
Q
How do viruses multiply
A
To multiply viruses take over the metabolic machinery of the host cell
6
Q
Host range
A
The host range of a virus is the spectrum or variety of host cells the virus can infect.
7
Q
What are examples of the host range of viruses
A
Some viruses can infect invertebrates, vertebrates, plants, protists, fungi, and bacteria. Most viruses infect specific type of cells of only host species. Rarely, viruses cross the host-range barrier.
8
Q
Bacteriophages
A
Aka phages. Viruses that infect bacteria.
9
Q
What is required for the virus to infect a host cell
A
For the virus to infect a host cell, the outer surface of the virus must chemically interact with specific receptor sites on the surface of the host cell.
10
Q
Where are receptor sites on host cells
A
It can be part of the cell wall of the host, it can be part of the fimbriae or flagella, for animal viruses the receptor sites are on the plasma membrane of the host cells
11
Q
Virus size range
A
20 to 1000 nm in length
12
Q
Virion
A
A complete, fully developed, infectious viral particle composed of nucleic acid, and surrounded by a protein coat, and is a vehicle of transmission from one host cell to another.
13
Q
How are viruses classified
A
Based on their nucleic acid and by differences in the structures of their coats
14
Q
Describe the nucleic acid of a virus
A
It has either DNA or RNA, never both. The nucleic acid can be single stranded or double stranded. It can be linear, or circular, or in several separate segments.
15
Q
Capsid
A
A protein coat that protects the nucleic acid of a virus. It accounts for most of the mass of a virus.
16
Q
What determines the structure of the capsid
A
The nucleic acid
17
Q
Capsomeres
A
Protein subunits that compose the capsid. The protein subunits are of a single type or can be made of several types of protein.
18
Q
How do capsomeres help identify a virus
A
Their arrangement is characteristic of a particular type of virus
19
Q
Envelope
A
Some viruses have this, which covers the capsid and is usually made up of a combination of lipids, proteins, and carbohydrates
20
Q
Animal viruses released from the host cell by an extrusion process have an envelope that is made up of
A
The extrusion process coats the virus with a layer of the host cell’s plasma membrane, that layer becomes the viral envelope. In some cases, the envelope contains proteins determined by the viral nucleic acid and materials derived from the normal host cell components .
21
Q
Sometimes the envelopes are covered by
A
Spikes made up of carbohydrate-protein complexes (glycoprotein) that project from the surface of the envelope.
22
Q
How are spikes beneficial to viruses
A
They allow viruses to attach to host cells
23
Q
What advantages do spikes have for identification of a virus
A
They can cause viruses to bind to RBCs and form bridges resulting in clumping. This is the basis for several useful laboratory tests.
24
Q
Hemagglutination
A
The clumping of red blood cells.
25
What would you call viruses whose capsids are not covered by an envelope
Nonenvelope viruses (naked virus)
26
How is a nonenveloped virus protected
The capsid protects the nucleic acid from nuclease enzymes in biological fluids and promotes the virus's attachment to susceptible host cells.
27
How is it that some viruses can escape antibodies and prevent their inactivation
This is due to regions of the genes that code for these virus's surface proteins that are susceptible to mutations. Their progeny have altered surface proteins and antibodies are not able to react with them.
28
What are general morphology types of a virus
1. Helical
2. Polyhedral
3. Enveloped
4. Complex
29
Helical viruses
Long rods that may be rigid or flexible, nucleic acid is found within a hollow cylindrical capsid that has a helical structure
30
Example of helical virus
Rabies and ebola haemorrhagic fever
31
Polyhedral virus
Many sided shape, an icosahedron, a regular polyhedron with 20 triangular faces and 12 corners. The capsomeres of each face form an equilateral triangle
32
Example of polyhedral virus
Adenovirus and poliovirus
33
Enveloped virus
Roughly spherical. Helical or polyhedral.
34
Examples of enveloped viruses
Envelope polyhedral--> herpes simplex virus
| Enveloped helical--> influenza virus
35
Complex viruses
Some viruses, particularly bacterial viruses have complicated structures.
36
Example of a complex virus structure
Bacteriophage. Some have capsids to which additional structures are attached. Capsid head is polyhedral and the tail sheath is helical. Head contains the nucleic acid.
37
Example of a complex virus
Poxviruses
38
The International Committee on Taxonomy of Viruses groups viruses into families based on
Genomics and structure with the help of DNA sequencing
39
What suffix is used for genus names
-virus
40
Family names end in
-viridae
41
Order names end in
-ales
42
In formal usage, in what order are family and genus, viral species, and subspecies names used, example
FAMILY--> Herpeaviridae,
GENUS--> Simplexvirus,
Viral species--> human herpesvirus
Subspecies--> 2
43
Viral species
A group of viruses sharing the same genetic information and ecological niche (host range). Common names are used for species.
44
Subspecies
Different strains are designated by a number.
45
Spikes are
Also called peplomeres. They are made up of glycoproteins; proteins linked to sugars known as hemagglutinin (HA) and neuramidase (NA).
46
Where has much of our understanding about viruses come from
Bacteriophages, viruses that use bacteria as a host.
47
Where can bacteriophage be grown in the lab
Suspensions of bacteria in liquid media or in bacterial cultures on solid media.
48
What is made possible by growing bacteriophages on solid media
The plaque method for detecting and counting viruses
49
How would you perform the plaque method
A sample of bacteriophage is mixed with host bacteria and melted agar. This is then poured into a petri plate containing a hardened layer of agar growth medium. The mixture solidifies into a thin top layer that contains a layer of bacteria approximately one cell thick.
50
How do plaques form
Each virus infects a bacterium, multiplies, releases several new viruses that infect other bacteria in the immediate vicinity. After several viral multiplication cycles all the bacteria surrounding the original virus or destroyed. This produces a number of clearing or plaques visible against a lawn of bacterial growth on the surface of the agar.
51
Each plaque corresponds to
A single virus in the initial suspension
52
PFU
Plaque forming units, the concentration of viral suspension measured by the number of plaques
53
The most important taxonomic criteria are
1. Host organism
2. Particle morphology
3. Genome type
54
Nucleic acid classification
1. RNA viruses; single stranded or double stranded
| 2. DNA viruses; single stranded or double stranded
55
Subcategories of RNA viruses
1. Positive strand
2. Negative strand
3. Retrovirus
56
Animal inoculation with a virus is used as a
Diagnostic procedure for identifying and isolating a virus in the clinical specimen. The animal is observed for signs of disease or is killed for examination of infected tissues
57
Can all human viruses be grown in animals
No because they do not all cause the disease or symptoms of the disease in animals. They cannot be used to study the effects of viral growth and disease treatments.
58
What is a convenient and inexpensive form of growing animal viruses
Inoculation of an embryonated egg
59
How is an embryoated egg inoculated
A hole is drilled in the shell of the egg and a viral suspension or suspected virus containing tissue is injected into the fluid of the egg
60
How do you know viral growth has occurred in an embryo
1. Death of an embryo
2. Embryo cell damage
3. Formation of typical pocks of lesions on the egg membranes
61
What is the embryo method used mostly for today
Viral vaccine preparation
62
What are the different membranes in an egg that the virus can be injected into
1. Chorioallantoic membrane
2. Amniotic membrane
3. Allantoic membrane
4. Yolks sac
63
What type of viral culture has replaced embryonated eggs
Cell cultures
64
What do cell cultures consist of
Cells from animal tissues grown in a culture medium
65
What are cells from cell cultures suspended in
They are suspended in a solution that provides the osmotic pressure, nutrients, and growth factors needed for the cells to grow
66
How are cell culture lines prepared
A slice of animal tissue is treated with enzymes that separates the individual cells
67
What are the 3 basic types of cell cultures widely used in clinical and research virology
1. Primary cell cultures
2. Diploid fibroblast strains
3. Continuous cell lines
68
How do normal cells in a cell culture grow
They tend to adhere to the glass or plastic container and reproduced to form a monolayer. They have a type of on/off switch for growth.
69
Cytopathic effect (CPE)
The visible effect viruses have on cells
70
How is CPE counted
In the same way as plaques in a lawn of bacteria and are reported as PFU per ML
71
Primary cell cultures
Come directly from animal tissue slices, and if repeatedly subcultured, one cell type will become dominant (cell strain)
72
What is a disadvantage of primary cell cultures
They don't last long and tend to die off after only a few generations
73
Diploid fibroblast strains
Most widely used strain and support growth of a wide range of viruses that require a human host. They are developed from human embryos and can be maintained for about a 100 generations.
74
Continuous cell lines
This is used when viruses are routinely grown in a lab. These are transformed cancerous cells that can be maintained through an indefinite number of generations
75
Continuous cell lines are sometimes called
Immortal cell lines
76
What is one of the most famous continuous cell lines
HeLa cell line. It was isolated from the cancer of a woman, Henrietta Lacks who died in 1951.
77
How do transformed cells from a cell line grow
These are continuous cell cultures that do not grow in a monolayer. They do not have an on/off switch to stop growth.
78
What's an example of a cytopathic effect
The cell deterioration by a virus infecting a monolayer of normal cells as they attempt to multiply
79
What are the common cytopathic effects in a viral cell culture
Changes in cell shape and detachment from adjacent cells or culture container
80
An experienced Virologist can use CPE to
To make a preliminary ID of the infecting virus
81
Syncytia
Giant multinucleate cells caused by fusion of adjacent cells
82
What are methods used to identify a virus
1. Evaluating cytopathic effects
2. Seroloical tests
3. Evaluating nucleic acids
83
What is the most common selogical method for identifying viruses
Western blotting
84
What do Serological tests for viral identification look for
* Detect antibodies against viruses in a patient
| * Use antibodies to identify viruses in neutralization tests, viral hemagglutination and Western blot
85
What viral identification methods include nucleic acids
▪︎Restriction fragment length polymorphism (RFLP)
| ▪︎ Polymerase chain reaction (PCR)
86
What instrument is used to visualize and identify viruses
An electron microscope
87
What is PCR used for
To amplify viral RNA, to identify a virus
88
What is provided by a host cell once a virus has infected it
Enzymes needed for protein synthesis, ribosomes, tRNA, and energy production, are used for synthesizing viral proteins
89
What are the 2 mechanisms by which bacteriophages can multiply
1. Lytic cycle
| 2. Lysogenic cycle
90
The lytic cycle ends with
Lysis and death of the host cell
91
The lysogenic cycle is different from the lytic cycle in that the host cell
Remains alive
92
Stages of multiplication in lytic cycle
1. Attachment
2. Penetration
3. Biosynthesis
4. Maturation
5. Release
93
Virions of T-even bacteriophages
Large and complex, nonenveloped, characteristic head and tail structure,
94
Attachment
Tail fibers attach to cell wall proteins. An attachment site on the virus attaches to a complimentary receptor site on the bacterial cell. A chemical interaction takes place in which weak bonds are formed
95
Penetration
After attachment. The T-even bacteriophage injects its DNA (nucleic acid) into the bacterium.
96
Describe the penetration process
The bacteriophage's tail releases an enzyme, phage lysozyme, which breaks down part of the bacterial cell wall. During penetration, the tail sheath contracts to force tail core into the cell wall until the tip reaches the plasma membrane, and the DNA is inserted in the host cell.
97
Biosynthesis
The bio synthesis of viral nucleic acid and protein occurs.
98
How does the phage attach to host cell
It attaches by tail fibers to host cell
99
Genetic controls regulate when ...
Different regions of phage dna are transcribed into mRNA during the multiplication cycle
100
Eclipse period
Takes place during biosynthesis. The period during viral multiplication when complete and infective virions are not yet present. Only separate components, DNA and protein can be detected.
101
Maturation
Viral components: bacteriophage DNA and capsids, are assembled into complete virions. This occurs spontaneously. The phage heads and tails or separately assembled from protein subunits, the head is filled with phage DNA and attached to the tail.
102
Release
The final stage. The plasma membrane breaks open which is caused by lysozyme produced within the host cell, encoded by a phage gene. Virions are released from the host cell and bacteriophages infect other susceptible cells in the vicinity.
103
Lysogenic phages are also called
Temperate phages
104
Lysogeny
A type of life cycle where a bacterial phage infects a type of bacterial host cell and can remain latent or inactive
105
Participating bacterial hosts cells infected by a lysogenic phage are called
Lysogenic cells
106
What is a well studied lysogenic phage
Lambda
107
Describe the lysogenic cycle
1. Upon penetration into an E. coli cell, the originally linear phage dna forms a circle.
2. Instead of multiplying and being transcribed it enters the lysogenic cycle. The DNA circle recombines with and becomes part of the circular bacterial DNA. The prophage remains latent.
3. Every time the host cell's machinery replicates the bacterial chromosome also replicates the prophage dna.
4. A spontaneous event can lead to the excision of the phage dna and initiate the lytic cycle
108
What events can bring a lysogenic phage out of its latent stage
The action of UV light, certain chemicals
109
The inserted phage DNA is called
A prophage
110
How are prophage genes repressed
Most are repressed by 2 repressor proteins that are the products of phage genes. These repressors stop transcription of all the other phage genes by binding to operators. The phage genes that would otherwise direct the synthesis and release of new virions are turned off.
111
What are the 3 important results of lysogeny
1. Lysogenic cells are immune to reinfection by the same phage, but not to other types
2. Phage conversion
3. It makes specialized transduction possible
112
Phage conversion
The host cell may exhibit new properties
113
What is an example of phage conversion
Corynebacterium diphtheriae's disease producing properties are related to the synthesis of a toxin. It can only produce this toxin when it carries a lysogenic phage because the prophage carries the gene coding for the toxin.
114
Specialized transduction
When a prophage is excised from the host chromosome, adjacent genes from either side may remain attached to the phage dna. The lysogenic phage packages bacterial DNA along with it's own DNA in the same capsid after lysing of the cell. Once the phage infects a new host, the prophage along with the new bacterial genes become integrated into the new host's DNA.
115
What happens to viruses that infect animals and remain latent in cells for long periods without multiplying and causing disease
The virus can be inserted into a host chromosome or remain separate from the host DNA in a repressed state
116
Multiplication of animal viruses stages
1. Attachment
2. Entry
3. Uncoating
4. Biosynthesis or chronic infection and then biosynthesis
5. Maturation and Release
117
What are the differences in animals viruses when compared to bacteriophages
▪︎Mechanism have entering the host cell
▪︎The synthesis and assembly of the new viral components
▪︎They have different enzymes than those found in phages
▪︎ The mechanism of maturation and release
▪︎ The effects on the host cell
118
Animal virus attachment
Animal viruses have attachments sites that attached to complimentary receptor sites on the host cell's surface. Receptor sites of animal cells are proteins and glycoproteins of the plasma membrane. The attachment sites for distributed over the surface of the virus and vary from one group to another.
119
When is attachment of animal viruses complete
When many sites are bound
120
Entry of animal viruses
Entry occurs by receptor mediated endocytosis or fusion
121
Receptor mediated endocytosis
If a virion attaches to the plasma membrane of a potential host, the host cell will enfold the virion into a fold of plasma membrane forming a vesicle and bringing it into the host cell.
122
Fusion
Enveloped viruses can enter by this alternative method. The viral envelope fuses with the plasma membrane and releases the capsid into the cell's cytoplasm.
123
Uncoating
The enzymatic separation of the viral nucleic acid from its protein coat occurs once the virion is enclosed within the vesicle. Nonenveloped capsid may be released into the cytoplasm of the host cell
124
What enzymes are used for uncoated animal viruses
* Some use lysosomal enzymes other of the host cell
| * Others are specific enzymes encoded by viral DNA
125
Biosynthesis of animal DNA viruses
The DNA of most DNA viruses is released into the nucleus of the host cell. Transcription of viral DNA and translation produce viral DNA and, later capsid proteins. Capsid proteins are synthesized in the cytoplasm of the host cell.
126
When does maturation of a DNA virus happen
This happens after the capsid proteins migrate into the nucleus of the host cell. The viral DNA and capsid protein assemble to form complete viruses. They are then released from the host cell.
127
In most DNA by viruses early transcription is carried out with
The hosts transcriptase (RNA polymerase)
128
Why do viruses disappear during the eclipse period of an infection
Because they are taken apart inside the host cell
129
RNA virus biosynthesis
This occurs in the host cell's cytoplasm. Different mechanisms of mRNA formation occur among different groups of RNA viruses. After viral RNA and viral proteins are synthesized maturation occurs.
130
What occurs after the uncoating if a ssRNA, sense strand (+strand)
The RNA strand within the virion, acts as mRNA. , the single stranded viral RNA is translated into 2 principal proteins, which inhibit the host cells synthesis of RNA and protein and which form an enzyme called RNA dependent RNA polymerase.
131
What does the enzyme RNA dependent RNA polymerase catalyze
The synthesis of another strand of RNA which is complementary in base sequence to the original infecting strand
132
The new strand catalyzed by RNA dependent RNA polymerase is called
Antisense strand or - strand
133
What does the antisense strand do
It serves as a template to produce additional .+ strands. These strands serve as
1. mRNA for the translation of capsid proteins.
2. May become incorporated into capsid proteins to form a new virus
3. May serve as a template for continued RNA multiplication
134
What are the 3 pathways of RNA virus synthesis
1. ssRNA, +strand
2. ssRNA, -strand
3. dsRNA, +strand with -strand
135
Uncoating releases
Viral RNA (genome) and viral proteins
136
ssRNA virus with a - strand
These viruses must transcribe a + strand to serve as mRNA before the begin synthesizing proteins. The mRNA transcribes additional - strands for incorporation into capsid protein
137
dsRNA, +sense with - strand
mRNA is produced inside the capsid and released into the cytoplasm of the host. RNA polymerase initiates production of antisense strands. The mRNA and antisense strands form DNA that is incorporated as new viral genome
138
Togaviridae biosynthesis
+ strand of RNA; after - strand is made from + strand, 2 types of mRNA are transcribed from the - strand. One type of mRNA is a short strand that codes for envelope proteins, the longer strand serves as mRNA for capsid proteins and can become incorporated into a capsid
139
Multiplication of RNA viruses occurs in the
Cytoplasm of the host cell
140
RNA dependent RNA polymerase synthesizes
Double stranded RNA
141
Biosynthesis of retroviruses
These viruses carry reverse transcriptase, which uses viral RNA as a template to produce complementary double stranded DNA. The enzyme also degrades the original viral RNA. The viral DNA is then integrated into a host cell chromosome as a provirus.
142
What happens to the provirus
Sometimes the provirus remains in a latent state and replicates when the DNA of the host cell replicates. Other times it is expressed and produces new viruses which can infect adjacent cells.
143
What can induce expression of a provirus
Mutagens such as gamma radiation
144
What happens during transcription of a provirus
Transcription produces RNA for a for new retrovirus genome and RNA that encodes the retrovirus, capsid, enzymes, and envelope proteins
145
After transcription of the provirus, viral proteins are processed by
Viral protease
146
What does a mature retrovirus acquire when it leaves the host cell
An envelope and attachments spikes as it buds out
147
How does a retrovirus enter a host cell
By fusion between attachments spikes and the host cell receptor
148
What is the 1st step in viral maturation of an animal virus
The assembly of the protein capsid.
149
What are the capsids of many animal viruses enclosed by
Envelopes made up of protein, lipid, and carbohydrate
150
What encodes envelope proteins
Viral genes and are incorporated into the plasma membrane of the host cell
151
What encodes the lipids and carbohydrate of an envelope
Host cell genes and are present in the plasma membrane
152
How does the envelope develop
Around the capsid by a process called budding
153
What part of the host cell becomes part of the envelope
A portion of the plasma membrane adheres to the virus and becomes part of the envelope
154
How are animal viruses released from the host cell
▪︎Enveloped visues are released by budding
| ▪︎Nonenveloped viruses are released through ruptures in the host cell plasma membrane-->causing death of hist cell
155
Several types of cancer known to be caused by
Viruses
156
What are reasons that cancers caused by a virus go unrecognized
1. Most of the particles of some viruses infect cells but do not induce cancer
2. Cancer might not develop until long after viral infection
3. Cancers do not seem to be contagious as viral diseases are
157
Oncogenes
Parts of the genome affected by cancer causing alterations to cellular DNA
158
What has the potential to make a normal cell cancerous in a eukaryotic cell
Anything that can alter the genetic material
159
How are ocogenes activated to abnormal functioning
Mutagenic chemicals, high energy radiation, and viruses
160
Oncogenic viruses
Viruses capable of inducing tumors in animals
161
How our oncogenic viruses similar to lysogeny in bacteria
Their genetic material integrates into the host cell DNA and replicates along with the host cell's chromosome.
162
What happens when tumor cells undergo transformation
They acquire properties that are distinct from the properties of uninfected cells or from infected cells that do not form tumors
163
What is the virus specific antigen on the cell surface of a cell transformed tumor cell called
Tumor specific transplantation antigen (TSTA)
164
T antigen
An antigen in the nucleus of a transformed tumor cell
165
Activated oncogenes transform normal cells into
Cancerous cells
166
What are the hallmarks of transformed cells
1. They have increased growth
2. They have loss of contact inhibition
3. TSTA or T antigens
167
The genetic material of oncogenic viruses become integrated into
the host cell's DNA
168
What kind of viruses become ocogenic
DNA viruses and Retroviruses (RNA virus)
169
How can retroviruses induce tumors
This is related to their production of a reverse transcriptase . The provirus which is the double stranded DNA molecule synthesized from the viral RNA, becomes integrated into the host cell's DNA, new genetic material is thereby introduced into the host genome, and this is a key reason retroviruses can contribute to cancer.
170
What do retroviruses contain that turn on oncogenes or other cancer causing factors
Ocogenes or Promoters
171
Proteins produced by tumor viruses can cause
uncontrolled host cell division.
172
Proto-oncogene
A normal gene, that when under the control of a virus, can cause uncontrolled cell division and can act as an oncogene
173
How do oncogenes work
1. Product of oncogene can disrupt normal cell function leading to uncontrolled cell divisions
2. Controlled by viral regulators near the site of their integration into the host cell's chromosomes
174
Teratogenesis
The induction of defects during embryonic development
175
Teratogen
A drug or other agent that induces birth defects
176
What 3 human viruses account for a large number of teratogenic affects
1. Cytomegalovirus
2. HSV 1 and 2
3. Rubella
177
What type of infections are those caused by ocogenic viruses
Latent infections
178
Latent infection
The virus inhabits the host cells but causes no damage until it is activated by a stimulus. Viruses remain asymptomatic until they suddenly appear.
179
Persistent or chronic viral infection
Occurs gradually over a long period. These are detectable infectious viruses that gradually build up over long periods. Typically persistent viral infections are fatal.
180
Prion
Proteinaceous infectious particle, a mutated protein. Infection is inherited and transmissible by ingestion, transplant, and surgical instruments.
181
PrPc
Normal cellular prion protein on cell surface
182
PrPsc
Scrapie protein; accumulates in brain cells, forming plaques
183
How can a protein become infectious
If an abnormal prion protein enters a cell, it changes a normal prion protein to an abno6prion protein, which now can change another normal prion protein, resulting in an accumulation of the abnormal prion proteins
184
How do plant viruses enter plants
Through wounds or via insects
185
Viroids
Short pieces of naked RNA, only 300 to 400 nucleotides long, with no protein coat. They cause plant diseases.
