19 Viruses Flashcards
KEY CONCEPTS 19.1 A virus consists of a nucleic acid surrounded by a protein coat 19.2 Viruses replicate only in host cells 19.3 Viruses, viroids, and prions are formidable pathogens in animals and plants (153 cards)
1
Q
Why are viruses not considered living?
A
They are unable to reproduce or undergo metabolic activities outside of their host.
2
Q
What observation did Mayer make?
A
He could transfer disease from one plant to another by rubbing sap from an infected plant onto a health one.
3
Q
What is ’TMV’?
A
The ’tobacco mosaic virus’ which causes the leaves to become mottled.
4
Q
How is genetic information carried in a virus?
A
Their genomes consist of double-stranded DNA, single-stranded DNA or single stranded RNA depending on the type of virus.
The genome is typically a single linear or circular molecule of nucleic acid. Some viruses have multiple nucleic acids (like multiple chromosomes in eukaryotes)
5
Q
How many genes does a virus have?
A
Anywhere from 4 to a thousand
6
Q
How many genes does a bacterium have?
A
200 to a few thousand.
7
Q
What are some examples of viruses with distinct structures?
A
’Tobacco mosaic viruses’, ‘adenoviruses’, ‘influenza viruses’ and ‘bacteriophage T4’
8
Q
What are some basic structures that many viruses have?
A
‘Capsids’ and ‘viral envelopes’
9
Q
What are capsids?
A
A protein shell which encloses the viral genome.
Capsids are made from a large number of protein subunits called ‘capsomeres’ (each capsid has very few unique types of protein)
10
Q
What are the protein coats of viruses called?
A
Capsids
11
Q
What are viral envelopes?
A
Envelopes derived from the membranes of the host cell which wrap around the virus.
They contain host cell phospholipids and membrane proteins.
They also contain proteins, glycoproteins and occasionally viral enzymes that are produced by the virus.
12
Q
What are the basic shapes of viruses?
A
‘Helical viruses’, ‘icosahedral viruses’ and other more complex forms.
13
Q
What is the structure of a ‘helical virus’?
A
It has a capsid made of a spiral of protein to form a tube. The viral genome is in the centre of this tube.
14
Q
What is the structure of a ‘icosahedral virus’?
A
The protein capsid is arranged so that it forms a polyhedron with 20 triangular faces i.e. a icosahedron.
(Glycoproteins may protrude from the surface)
15
Q
What is an example of a helical virus?
A
Tobacco mosaic virus
16
Q
What is an an example of an icosahedral virus?
A
Adenovirus
17
Q
What is the structure of ’tobacco mosaic virus’?
A
It is a ‘helical virus’
18
Q
What is the structure of an adenovirus?
A
It is an icosahedral virus with exactly 252 protein molecules in its capsid.
At each vertex of the capsid glycoproteins protrude.
19
Q
What are viruses that infect bacteria called?
A
Bacteriophages
20
Q
What are ‘bacteriophages’?
A
Viruses that infect bacteria.
21
Q
What is the structure of an influenza virus?
A
Influenza viruses have an outer ‘viral envelop’e studded with glycoprotein spikes.
The genome consists of eight different RNA molecules, each wrapped in a helical capsid.
22
Q
What are some examples of bacteriophages?
A
The first 7 were named T1 (type 1), T2, T3, T4, T5, T6 and T7
23
Q
What is a specific type of bacteriophage and why are they significant?
A
‘T-even’ phages (T2, T4 and T6) have a similar structure.
24
Q
What does ’T-even’ refer to?
A
A T2, T4 or T6 bacteriophage (coincidentally they have similar structures)
25
What are bacteriophages also called?
Phages
26
What are ‘phages’?
Bacteriophages i.e. viruses which infect bacteria.
27
What is the structure of a T-even phage?
It has an elongated icosahedral head which contains the DNA.
Attached to the head is a protein tail piece with fibers by which the phages attach to a bacterium.
This tail consist of a ’Tail sheath’ which is kinda like a body. At the bottom ’tail fibres’ branch off like legs.
28
Generally speaking, how specific are viruses in what hosts they will infect?
Most are quite specific and will infect only a few,
29
What are the hosts that a particular virus infects called?
The ‘host range'
30
What does ’host range’ refer to?
The range of host species that a particular vital species is capable of infecting.
31
What is the basic vital replicative cycle?
Virus enters cell and is uncoated, releasing viral DNA and capsid proteins.
Host enzymes replicate the viral genome.
Meanwhile, host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins.
Viral genomes and capsid proteins self-assemble into new virus particles, which exit the cell.
32
What species can the measles virus infect?
Only humans
33
What is an example of a virus that infects only one species?
Measles (only humans)
34
How does viral DNA enter a cell?
T-even phages use their tails to inject DNA.
Other viruses enter by endocytosis and release the DNA held in their capsid
35
What is the purpose of the tail of a T-even phage?
It injects viral DNA into a cell.
36
How is the viral genome replicated?
Many DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA. In contrast, to replicate their genomes, RNA viruses use virally encoded RNA polymerases that can use RNA as a template. (Uninfected cells generally make no enzymes for carrying out this process.)
37
Why is the host needed for viral replication?
It provides nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids, ATP, and other components needed for making the viral proteins.
These components are not made by the virus as it can not undergo metabolic processes.
38
What allows the specificity of viruses and thus their limited ‘host range'?
Viruses usually identify host cells by a “lock- and-key” fit between viral surface proteins and specific receptor molecules on the outside of cells.
39
What can the reproductive cycles of phages be divided into?
The ‘lytic cycle’ and the ‘lysogenic cycle
40
What is the fundamental difference between the lytic cycle and the lysogenic cycle?
The lytic cycle eventually leads to the death of the cell whereas in the lysogenic cycle the cell continues to live as an infected cell.
41
What are the stages of the lytic cycle? (in the context of a T4 phage)
Attachment, Entry of Phage DNA & Degradation of Host DNA, Synthesis of Viral genome/proteins, Assembly and Release.
Attachment: The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell.
Entry of phage DNA and degradation of host DNA: The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolysed.
Synthesis of viral genomes and proteins: The phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell.
Assembly: Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms.
Release: The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and bursts.
42
What is a phage that can only use the lytic cycle?
A virulent phage
43
What is a ‘virulent phage’?
A phage that reproduces exclusively by the lytic cycle
44
How does a bacteria protect itself from phages?
It release ‘restriction enzymes’ which breakdown the viral DNA.
45
What are ‘restriction enzymes’?
Enzymes that are realised by bacteria to destroy the DNA of phages that are infecting them.
(also used in biotechnology)
46
How does a bacteria protect its own DNA from destruction by the restriction enzymes its uses to defend against phages?
Its own DNA is methylated in a way which prevents attack
47
What are phages which can use both the lytic and lysogenic cycles called?
Temperate phages.
48
What are ’temperate phages'
Phages which can use both the lytic and lysogenic cycles
49
What is an example of a temperate phage?
Lambda (phage λ)
50
What happens during the ‘lysogenic cycle’?
Phage DNA integrates into the bacterial chromosome, becoming a ‘prophage’.
The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells.
Many cell divisions produce a large population of bacteria infected with the prophage.
Occasionally, a prophage
exits the bacterial chromosome, initiating a lytic cycle.
51
What does ’prophage’ refer to?
Viral DNA that has integrated into the bacterial chromosome during the lysogenic cycle
52
What is viral DNA that has integrated into the bacterial chromosome during the lysogenic cycle called?
‘Prophage'
53
Why is the lysogenic cycle important?
Each time the infected bacterium replicates exponentially more viruses are created.
54
What triggers the change form lysogenic to lytic cycle?
Environmental factors.
55
What are animal viruses grouped in?
Classes (I, II, III, IV, V and VI)
56
What characterises Class I viruses?
They have 'Double-Stranded DNA (dsDNA)
57
What characterises Class II viruses?
They have ’Single-Stranded DNA (ssDNA)'
58
What characterises Class III viruses?
They have ‘Double-Stranded RNA (dsRNA)'
59
What characterises Class IV viruses?
They have ’Single-Stranded RNA (ssRNA)’ which serves as mRNA
60
What characterises Class V viruses?
They have ’ssRNA’ which acts as a template for mRNA synthesis.
61
What characterises Class VI viruses?
They have ‘ssRNA’ which serves as a template for DNA synthesis
62
What does ’dsDNA’ refer to?
Double-Stranded DNA
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What does ’ssDNA’ refer to?
Single-Stranded DNA
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What does ’dsRNA’ refer to?
Double-Stranded RNA
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What does ’ssRNA’ refer to?
Single-Stranded RNA
66
Do Class I viruses have envelopes?
Some do
67
Do Class II viruses have envelopes?
No
68
Do Class III viruses have envelopes?
No
69
Do Class IV viruses have envelopes?
Some do
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Do Class V viruses have envelopes?
Yes
71
Do Class VI viruses have envelopes?
Yes
72
What viruses are classed as Class I?
Adenovirus, Papovirus, Herpesvirus and Poxvirus
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What viruses are classed as Class II?
Parvovirus
74
What viruses are classed as Class III?
Reovirus
75
What viruses are classed as Class IV?
Picornavirus, Coronavirus, Flavivirus and Togavirus
76
What viruses are classed as Class V?
Filovirus, Orthomyxovirus, Paramyxovirus, Rhabdovirus
77
What viruses are classed as Class VI?
Retrovirus
78
Under what class are Adenoviruses?
I (dsDNA)
79
Under what class are Papoviruses?
I (dsDNA)
80
Under what class are Herpesviruses?
I (dsDNA)
81
Under what class are Poxviruses?
I (dsDNA)
82
Under what class are Parvoviruses?
II (ssDNA)
83
Under what class are Reoviruses?
III (dsRNA)
84
Under what class are Picornaviruses?
IV (ssRNA that serves as mRNA)
85
Under what class are Coronaviruses?
IV (ssRNA that serves as mRNA)
86
Under what class are Flaviviruses?
IV (ssRNA that serves as mRNA)
87
Under what class are Togaviruses?
IV (ssRNA that serves as mRNA)
88
What are some examples of viruses that cause human disease and are in the family Adenovirus?
Respiratory viruses; tumor-causing viruses
89
What are some examples of viruses that cause human disease and are in the family Papovirus?
Papillomavirus (warts, cervical cancer); polyomavirus (tumors)
90
What are some examples of viruses that cause human disease and are in the family Herpesvirus?
Herpes simplex I and II (cold sores, genital sores); varicella zoster (shingles, chicken pox); Epstein-Barr virus (mononucleosis, Burkitt’s lymphoma)
91
What are some examples of viruses that cause human disease and are in the family Poxvirus?
Smallpox virus; cowpox virus
92
What are some examples of viruses that cause human disease and are in the family Parvovirus?
B19 parvovirus (mild rash)
93
What are some examples of viruses that cause human disease and are in the family Reovirus?
Rotavirus (diarrhea); Colorado tick fever virus
94
What are some examples of viruses that cause human disease and are in the family Picornavirus?
Rhinovirus (common cold); po- liovirus; hepatitis A virus; other
enteric (intestinal) viruses
95
What are some examples of viruses that cause human disease and are in the family Coronavirus?
Severe acute respiratory syn- drome (SARS)
96
What are some examples of viruses that cause human disease and are in the family Flavivirus?
Yellow fever virus; West Nile virus; hepatitis C virus
97
What are some examples of viruses that cause human disease and are in the family Togavirus?
Rubella virus; equine encephalitis viruses
98
What are some examples of viruses that cause human disease and are in the family Filovirus?
Ebola virus (hemorrhagic fever)
99
What are some examples of viruses that cause human disease and are in the family Orthomyxovirus?
Influenza virus
100
What are some examples of viruses that cause human disease and are in the family Paramyxovirus?
Measles virus; mumps virus
101
What are some examples of viruses that cause human disease and are in the family Rhabdovirus?
Rabies virus
102
What are some examples of viruses that cause human disease and are in the family Retrovirus?
Human immunodeficiency virus (HIV/AIDS); RNA tumor viruses
| leukemia
103
What virus causes shingles?
Varicella zoster.
104
What virus causes chicken pox?
Varicella zoster
105
What does the varicella zoster virus cause?
Chicken pox/shingles
106
What causes the common cold?
The Rhinovirus
107
What does Rhinovirus cause?
The common cold
108
What does the Ebola virus cause?
Hemorrhagic fever
109
What causes hemorrhagic fever?
The Ebola virus
110
What virus causes leukaemia?
RNA tumor viruses
111
What do RNA tumour viruses cause?
Leukaemia
112
What are some distinctive features of bacteria
Most have envelopes and many have RNA genomes.
113
How is the envelope of an animal-infecting virus produced?
Ribosomes bound to the endoplasmic reticulum (ER) of the host cell make the protein parts of the envelope glycoproteins; cellular enzymes in the ER and Golgi appara- tus then add the sugars. The resulting viral glycoproteins, embedded in host cell–derived membrane, are transported to the cell surface.
In a process much like exocytosis, new viral capsids are wrapped in membrane as they bud from the cell.
114
What is the envelop of a herpes virus?
Are temporarily cloaked in membrane derived from the nuclear envelope of the host; they then shed this membrane in the cytoplasm and acquire a new envelope made from membrane of the Golgi apparatus.
115
Are all viral envelopes produced form the plasma membrane?
No, as is seen in herpesvirus
116
What virus has a unique form of viral envelope?
Herpesvirus.
117
What are the stages of the reproductive cycle of an animal-infecteing enveloped RNA virus? (non-retro virus)
Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell, promoting viral entry into the cell.
The capsid and viral genome enter the cell. Digestion of the capsid by cellular enzymes releases the viral genome.
The viral genome functions as a template for synthesis of complementary RNA strands by a viral RNA polymerase.
New copies of viral genome RNA are made using complementary RNA strands as templates.
Complementary RNA strands also function as mRNA, which is translated into both capsid proteins (in the cytosol) and glycoproteins for the viral envelope (in the ER and Golgi apparatus).
Vesicles transport envelope glycoproteins to the plasma membrane.
A capsid assembles around each viral genome molecule.
Each new virus buds from the cell, its enve- lope studded with viral glycoproteins embedded in membrane derived from the host cell.
118
What are the stages of the replicative cycle of HIV (a retrovirus)?
The envelope glycoproteins enable the virus to bind to specific receptors on certain white blood cells.
The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA.
Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA.
Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first.
The double- stranded DNA is incorporated as a provirus into the cell’s DNA.
Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral protein.
The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER).
Vesicles transport the glycoproteins to the cell’s plasma membrane.
Capsids are assembled around viral genomes and reverse transcriptase molecules. New viruses bud off from the host cell.
119
What does ’provirus’ refer to?
The genetic information of a virus which integrates into and replicated within the genome of the host cell.
(basically a prophage but not necessarily from a bacteriophage)
120
What is genetic information of a virus that has integrated into the host cell’s genome called?
Provirus
121
What are plasmids?
Small, circular DNA molecules found in bacteria and in yeasts
122
What are transposons?
DNA segments that can move from one location to another within a cell’s genome
123
What are small circular DNA molecules found in bacteria/yeasts called?
Plamsids
124
What are sections of DNA which can move within an organism’s genome called?
Transposons.
125
What is it called when genetic material can move?
They are called ‘mobile genetic factors'
126
What are some examples of mobile genetic factors?
Plasmids, Transposons and Viruses
127
What are plasmids an example of?
Mobile genetic factors
128
What are transposons and example of?
Mobile genetic factors
129
What are viruses an example of?
Mobile genetic factors
130
What can viruses, plasmids and transposons be grouped as?
Mobile genetic factors
131
What is a mimivirus?
A double-stranded DNA virus with an icosahedral capsid that is 400 nm in diameter.
(The beginning of its name is short for mimicking microbe because the virus is the size of a small bacterium.)
132
What are some examples of infectious particles that are similar to viruses?
Viroids and Prions
133
What is the difference between an epidemic and a pandemic?
An epidemic is more localised (i.e. outbreak in one city) than a pandemic, which occurs over a large area i.e. the entire world.
134
What is the typical structure of a plant virus?
Most are helical viruses
135
What are the basic routes that a virus takes to spread between plants?
‘Vertical transmission’ and ‘horizontal transmission'
136
What is ‘vertical transmission’?
Transmission of a virus from one parent to the offspring. This can be through sexual or asexual reproduction.
137
What is ‘horizontal transmission’?
The spread of a virus between plants by the wind or by herbivores (especially insects) which travel between plants and thus may act as carries for a virus.
138
What are ‘viroids’?
Circular RNA molecules, only a few hundred nucleotides long, that infect plants. Viroids do not encode proteins but can replicate in host plant cells using host cell enzymes.
These small RNA molecules seem to cause errors in the regulatory systems that control plant growth
139
What is a sign that a plant is infected by viroids?
It has abnormal development and stunted growth.
140
What ‘mobile genetic element’ does stunted growth suggest?
Viroids
141
What organisms do viroids infect?
Only Plants
142
What are prions?
Infectious proteins
143
How do prions arrise, replicate and lead to harm?
A prion is a misfolded form of a protein normally present in brain cells.
When the prion gets into a cell containing the normal form of the protein, the prion converts normal protein molecules to the misfolded prion versions. Several prions then aggregate into a complex that can convert other normal proteins to prions, which join the chain.
Prion aggregation interferes with normal cellular functions and causes disease symptoms.
144
What is the typical structure of a plant virus?
Most are helical viruses
145
What are the basic routes that a virus takes to spread between plants?
‘Vertical transmission’ and ‘horizontal transmission'
146
What is ‘vertical transmission’?
Transmission of a virus from one parent to the offspring. This can be through sexual or asexual reproduction.
147
What is ‘horizontal transmission’?
The spread of a virus between plants by the wind or by herbivores (especially insects) which travel between plants and thus may act as carries for a virus.
148
What are ‘viroids’?
Circular RNA molecules, only a few hundred nucleotides long, that infect plants. Viroids do not encode proteins but can replicate in host plant cells using host cell enzymes.
These small RNA molecules seem to cause errors in the regulatory systems that control plant growth
149
What is a sign that a plant is infected by viroids?
It has abnormal development and stunted growth.
150
What ‘mobile genetic element’ does stunted growth suggest?
Viroids
151
What organisms do viroids infect?
Only Plants
152
What are prions?
Infectious proteins
153
How do prions arrise, replicate and lead to harm?
A prion is a misfolded form of a protein normally present in brain cells.
When the prion gets into a cell containing the normal form of the protein, the prion converts normal protein molecules to the misfolded prion versions. Several prions then aggregate into a complex that can convert other normal proteins to prions, which join the chain.
Prion aggregation interferes with normal cellular functions and causes disease symptoms.
