Viruses Flashcards
Chapter 21 in the Book
1
Q
Are viruses diverse entities?
A
Yes, b/c they vary in structure, methods of replication, and the hosts they infect.
2
Q
What are Virions?
A
Individual virus particles outside a host cell.
3
Q
How can virion surface structures be observed?
A
Scanning &Transmission Electron Microscopy
4
Q
How can virion internal structures be observed?
A
Images from a transmission Electron microscope.
5
Q
Do viruses fossilize?
A
No
6
Q
How do researchers speculate virus histories?
A
By using biochemical & genetic information.
7
Q
Do viruses have a common ancestor?
A
No
8
Q
What is the “devolution” or the Regressive hypothesis?
A
Suggests that viruses evolved from free-living cells, or from intracellular prokaryotic parasites.
9
Q
What is the “escapist” or the Progressive hypothesis?
A
Suggest that viruses originated from RNA or DNA molecules, or self-replicating entities similar to transposons or other mobile genetic elements, that escaped from a host cell with the ability to enter another.
10
Q
What is the Virus First hypothesis?
A
Suggest that viruses may have been the first self-replicating entities before the first cells.
11
Q
Are viruses noncellular?
A
Yes, b/c they lack most of the components of a cell, such as organelles, ribosomes, & a plasma membrane.
12
Q
What does noncellular mean?
A
A biological entity that does not have a cellular structure.
13
Q
What does a virion consists of?
A
- Nucleic Acid Core
- Capsid
- Envelope (SOMETIMES)
14
Q
What is a Capsid?
A
Protein coating of the viral core
15
Q
What is an Envelope?
A
Lipid bilayer that encircles some viruses.
16
Q
What is the Envelope made of?
A
Protein & phospholipid membranes derived from the host cell.
17
Q
Does the complexity of the host correlate with the complexity of the virion?
A
Not necessarily.
18
Q
What are bacteriophages?
A
Viruses that infect bacteria
19
Q
Do all virions have a nucleic acid genome covered by a protective capsid?
A
Yes
20
Q
What are capsomeres?
A
Protein subunits that make up the capsid
21
Q
What are the 4 groups that capsids are classified in?
A
- Helical
- Icosahedral
- Enveloped
- Head-and-tail
22
Q
Describe the structure of Helical capsids.
A
Long & cylindrical
23
Q
Describe the structure of Icosahedral capsids.
A
Shapes that are roughly spherical.
24
Q
Describe the structure of Enveloped capsids.
A
Have membranes derived from the host cell that surrounds the capsid
25
Describe the structure of Head-and-tail capsids.
They infect bacteria & have a head that is similar to isosachedral & a tail shaped like a helical.
26
What are Viral Receptors?
Glycoprotein used to attach a virus to a host via molecules on the cell.
27
What do viruses do with viral receptors?
Use them for penetration of a cell membrane; where after a virus can complete its replication inside the cell.
28
What do envelope viruses consist of?
1. Nucleic acid
2. Capsid Protiens
3. Phospholipid bilayer that contains virus-encoded proteins.
4. Glycoproteins embedded in the viral envelope that are used to attach to a host cell.
29
What are Matrix Proteins?
Envelope protein that stabilizes the envelope & often plays a role in the assembly of progeny virions
30
What do viruses use as their genetic material?
DNA or RNA
31
What is the Virus Core?
Contain the virus genome (the total genetic content)
32
What do viral genomes contain?
Genes that encode proteins, which the virus cannot get from the host cell. This genetic material may be single or double stranded. May be linear or circle.
33
What does the viral DNA do in DNA viruses?
The viral DNA directs the host cell's replication proteins to synthesize new copies of the viral genome & to transcribe & translate that genome into viral proteins.
34
What does the viral RNA do in RNA viruses?
To replicate their genomes in the host cell, the RNA viruses must encode their own enzymes that can replicate RNA into RNA.
35
Do mutations occur more frequently in DNA viruses or RNA viruses?
RNA Viruses
36
Do viruses have a common genomic sequence that they all share?
No
37
What is the most commonly used classification method today?
The Baltimore Classification Scheme, which groups viruses based on how messenger RNA (mRNA) is generated in each particular type of virus.
38
What are the elements by which viruses can be classified by?
1. The Viral Genome
2. The Type of Capsid
3. The Envelope Structure for enveloped viruses
39
Describe Viral Genome structures.
May vary in the type of genetic material (DNA or RNA) & its organization (single or double stranded, linear or circular, & segmented or non-segmented)
40
Describe the Types of Capsids classifications.
Classified as naked icosahedral, enveloped isosachedral, enveloped helical, naked helical, & complex.
41
What is Negative Polarity?
ssRNA viruses with genomes complementary to their mRNA.
42
What is Reverse Transcriptase?
Enzyme found in Baltimore groups VI & VII that converts sing-stranded RNA into double-stranded DNA.
43
What is Permissive?
Cell type/specific host cells that are able to support the productive replication of a virus. The molecular basis for this specificity is due to a particular surface molecule known as the Viral receptor on the host cell surface.
44
What must viruses use to replicate?
Its host-cell processes.
45
What are the Cytopathic Effects?
The dramatic biochemical & structural changes in the host cell, which may cause cell damage. Also can change cell functions or destroy the cell.
46
What are the 2 ways progeny virions can be released?
1. The infected cell dies through Lysis or Apoptosis & all the progeny virions release at once.
2. Budding, where the virions leave the cell individually without killing the infected cell.
47
What is lysis?
Bursting of a cell.
48
What is Apoptosis?
Programmed cell death or "cell suicide"
49
What is Budding?
Method of exit from the cell used in certain animal viruses, where virions leave the cell individually by capturing a piece of the host plasma membrane.
50
What are the steps in the virus replication cycle?
1. Attachment
2. Penetration
3. Uncoating
4. Replicating
5. Assembly
6. Release
51
Explain the Attachment step.
A virus attaches to a specific receptor site on the host cell membrane through attachmentproteins in the capsid or via glycoprotiens embedded in the viral envelope. The specificity of this interaction determines the host & the cells within the host that can be infected by a particular virus.
Think about there being several keys and locks, where each key will fit only one specific lock.
52
Explain the Penetration Step.
Viruses may enter a host cell either with or without the viral capsid. The nucleic of bacteriophages enters the host cell "naked," leaving the capsid outside the cell. Plant & animal viruses can enter through endocytosis~ the cell membrane surrounds & engulfs the entire virus.
53
Explain the Uncoating Step.
Once inside the cell, the viral capsid degrades & then the viral nucleic acid is released & becomes available for replication & transcription.
54
Explain the Replication & the Assembly step.
The replication mechanism depends on the viral Genome. DNA viruses usually use host-cell proteins & enzymes to replicate the viral DNA & to transcribe viral mRNA, which is then used to direct viral protein synthesis.
RNA viruses usually use the RNA core as a template for synthesis of viral genomic RNA & mRNA. The viral mRNA directs the host cell to synthesize viral enzymes & capsid proteins, & assemble new virions.
Retroviruses have an RNA genome that must be reverse transcribed into DNA , which then is incorporated into the host cell genome. To convert RNA to DNA, retroviruses must contain genes that encode the virus-specific enzyme reverse transcriptase that transcribes an RNA template to DNA.
55
Explain the Release Step.
A.K.A egress, releases new virions produced in the host organism, where they are able to infect adjacent cells & repeat the replication cycle.
Recall: Can occur in 2 ways, when the host cell dies or by budding.
56
Can Reverse transcription occur in uninfected host cells?
No, b/c the enzyme reverse transcriptase is ONLY derived from the expression of viral genes within the infected host cells.
57
Is it true that b/c there are many different types of viruses that nearly every living organism has its own set of viruses trying to infect its cells.
Yes, it is true.
58
What does Productive mean?
Viral infection that leads to the production of new virions.
59
What is the Lytic Cycle?
Type of virus replication in which virions are released through lysis, or bursting, of the cell.
60
What is Prophage?
Phage DNA that is incorporated into the host cell genome.
61
What is Latency?
Virus that remains in the body for a long period of time but only causes intermittent symptoms.
62
What is the Lysogenic cycle?
Type of virus replication in which the viral Genome is incorporated into the genome of the host cell.
63
What do most plant viruses have for their genome?
Single-stranded (+) RNA
64
How does a plant virus enter a new host?
Some type of mechanical damage must occur. The damage is often caused by weather, insects, animals, fire, or human activities.
65
What facilitates movement from cell to cell within a plant?
Modification of Plasmodesmata~ cytoplasmic threads that pass from one plant cell to the next).
66
Can plant offsprings inherit viral diseases from parent plants?
Yes, can be transmitted by a variety of vectors, through contact with an infected plant's sap, by living organisms such as insects & nematodes, & through pollen.
67
What is Horizontal Transmission?
Transmission of a disease b/w unrelated individuals.
68
What is Hyperplasia?
Abnormally high cell growth & division.
68
What is Vertical Transmission?
Transmission of disease from parent to offspring.
69
What is Hypoplasia?
Abnormally low cell growth & division.
70
What are galls?
Appearance of plant tumors.
71
What is Cell Necrosis?
Cell death
72
Do animal viruses have to penetrate a cell wall to gain access to the host cell?
No, b/c animal cells do not have a cell wall.
73
What are the 2 different ways non-envoloped or "naked" animal viruses enter a cell?
1. As a protein in the viral capsid binds to its receptor on the host cell, the virus may be taken inside the cell via a vehicle during the normal cell process of receptor-mediated endocytosis.
2. Capsid proteins undergo shape changes after binding to the receptor, creating channels in the host cell membrane. The viral Genome in then "injected" into the host cell through these channels in a manner analogous to that used by many bacteriophages.
74
What are the 2 different ways envoloped animal viruses enter a cell?
1. Receptor-mediated endocytosis
2. Fusion, which ONLY occurs for envoloped virions & use special fusion proteins in their envolopes to cause the envelope to fuse with the plasma membrane of the cell, thus releasing the genome & capsid of the virus into the cytoplasm.
75
What is an acute disease?
Disease where symptoms rise & fall within a short period of time.
76
What are chronic infections?
Describe when the virus persists in the body for a long period of time.
77
What is an asymptomatic infection?
Disease where there are no symptoms & the individual is unaware of being infected unless lab tests are performed.
78
What are Oncogenic Viruses?
Viruses that have the ability to cause cancer. They interfere with the normal regulation of the host cell cycle either by introducing genes that stimulate unregulated cell growth (oncogenes) or by interfering with the expression of genes that inhibit cell growth. These viruses can either be DNA or RNA viruses.
79
What is the primary method of controlling viral diseases?
Vaccination
80
What is a Vaccination?
81
What are Back Mutations?
82
What is Attenuating?
83
Are prions & viroids pathogens?
Yes
84
What are pathogens?
Agents with the ability to cause diseases.
85
What are prions?
Infectious particles that consist of proteins that replicate without DNA or RNA.
86
What is PrP (Prion Protein)?
An infectious structural variant of a normal cellular protein. It is this variant that constitutes the prion particle.
87
What 2 forms does PrP exist as?
1. PrP^c
2. PrP^sc
88
What is PrP^c?
Normal prion protein.
89
What is PrP^sc?
Infectious form of a prion protein. Once introduced into the body, the PrP^sc contained within the prion binds to PrP^c & converts it to PrP^sc. This leads to exponential increase of the PrP^sc protein, which aggregates. PrP^sc is folded abnormally, & the resulting conformation (shape) is directly responsible for the lesions seen in brains.
90
What are Viroids?
Plant pathogen that produces only a single-stranded, specific circular RNA. Do not have capsid or outer envelope. Can reproduce only within a host cell. They do NOT manufacture any proteins.
