Micro #6 Viruses

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micro #6 Viruses

The Search for the Elusive Virus

1. Viruses were too small to be seen with the first microscopes
2. The cause of viral infections was unknown for years
3. Louis Pasteur first proposed the term virus
4. 1890s
a. Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus
b. Loeffler and Frosch discovered an animal virus that causes foot -and-mouth disease in cattle
5. Many years of experimentation showed what we know today and by the 1950s virology had grown

Tobacco Mosaic Virus

1. Wendell Stanley crystallized TMV & found the virus to still be active.
2. Symptoms of Tobacco mosaic virus in tobacco. Note the narrowed and mottled or mosaic appearance of the leaves. (Courtesy H.D. Shew).

The Position of Viruses in the Biological Spectrum

1. Have evolved to be able to infect every type of cell
2. Cannot exist _Independently_ from the host cell, so aren't considered living things (copy genetic info & get out)
3. However, since they can direct life processes they are often considered more than lifeless molecules
4. Referred to as _infective particles_, either active or inactive
5. _Obligate__ intracellular parasites
6. Cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses

Size Range

1. Smallest infectious agents (above & beyond preons)
2. Most are so small, they can only be seen with an electron microscope
3. Bacterial viruses (_bacteriophages - aka phages_) are fairly large compared to their host cell.
4. Animal viruses - much variation in size
from 20 nm in diameter up to 450 nm in length

Viral Components: Capsids and Envelopes

1. Contain only those parts needed to invade and control a host cell
a. Capsid - outer components of subunits that give rise to their _crystaline__ appearance
i. Envelope - in 13 of the 20 families of _animal_ viruses (b/c outer most component is cell membrane. envelope usually a modified piece of hosts' cell membrane. difference = if cell wall then bursts)
ii. If no envelope, called naked virus

Viral Components: Nucleic Acids

b. Core - genetic material
1. DNA = circular or linear, single or double stranded
2. RNA = circular or linear, double stranded. if single stranded then (+) or (-)
3. have bits and pieces of genetic material (DNA & RNA), if they come in w/different DNA or RNA they would need their own enzymes.

- The capsid and the nucleic acid together are called the nucleocapsid
4. Fully formed virus that is able to establish an infection in a host cell- _virion__

The Viral Capsid: The Protective Outer Shell

1. Constructed from identical subunits called capsomeres__
2. Made up of _protein_____________ molecules
a. Can be single or multiple types
b. Morphology can vary
i. Helical (long tubes) and icosahedral (20 sided)
ii. Capsomers arranged to make different shaped viruses (dictated by genes)
iii. Arrangement dictated by genes of the nucleic acid


Examples: adenovirus & polio virus
1. structure is 3 dimensional, 20 sided & 12 evenly spaced corners
2. Although they all display this symmetry, there are wide variations

Morphology - Complex Viruses

1. Typically bacteriophages
2. Structures have different functions
3. Poxvirus (below) is an example

The Viral Envelope

1. Enveloped viruses take a bit of the host _cell membrane__ in the form of an envelope
2. In the envelope, some or all of the regular membrane proteins are replaced with viral proteins
3. Some proteins form a BINDING LAYER between the envelope and the capsid
4. Glycoproteins remain exposed as _spikes_ that are essential for attachment and pathogenicity
☼ hemagglutinin (H) (spikes)
☼ neuraminidase (N) (spikes)
☼ 16 H subtypes (or serotypes) and 9 N subtypes of influenza A virus and H1 N1 (swine flu)

Three Functions of the Viral Capsid/Envelope (14)

1. protects nucleic acid (benefit to virus)
2. help to introduce genetic materal into host cell (benefit to virus)
3. stimulate the immune system to produce antibodies to protect host cells in future (benefit to us)

Nucleic Acids: At the Core of a Virus

1. Viruses are often group based on their genetic material
2. Genome- the sum of all genetic material
3. Number of viral genes compared with a bacterial and human cell = human about 30,000, viruses have 4-5, bacteria have about 4,000. bacteriophages have more
4. They only have the genes necessary to invade host cells and redirect their activity
5. Some viruses are exceptions to the rules regardiing DNA and RNA
a. Parvoviruses contain single-stranded DNA
b. Reoviruses contain double-stranded RNA

DNA Viruses

1. ssDNA - example parvoviruses
2. dsDNA - most common
a. Linear and circular
b. Reason why this is most common type nucleic acid found in viruses = host cells are used to working w/ds DNA - so viruses are utilizing same genetic machinery

RNA Viruses

1. Mostly single-stranded
a. Positive-sense RNA: genomes that are ready for immediate translation into proteins (similar to mRNA)
b. Negative-sense RNA: genomes have to be converted into the proper form to be made into proteins. What is the proper form? the positive form, like mRNA
2. Some DS - reoviruses (respiratory and intestinal tract infections) Why is this unique? b/c we have seen dsRNA
3. Segmented- individual genes exist on separate pieces of RNA

Other Substances in the Virus Particle

1. Other Substances in the Virus Particle
a. Can contain enzymes for specific operations within the host cell
b. Polymerases to synthesize DNA and RNA
☼ Purpose? (21)

c. Replicases to copy RNA
d. Reverse Transcriptase (22) synthesizing DNA from RNA

How Viruses are Classified and Named

A. Main criteria
1. Structure
2. Chemical composition
3. Similarities in genetic makeup
B. International Committee on the Taxonomy of Viruses, 2000
1. 3 orders
2. 63 families "-viridae"
3. 263 genera "-virus" (italicized)
4. Subspecies are designated by a number.
C. Some virologists use a species naming system, but it is not an official designation

Viral Taxonomy

1. Herpesviridae = family
2. Herpesvirus = genus
3. Human herpes virus HHV-1, HHV-2, HHV-3

Retroviridae (meaning [23]?)
Human immunodeficiency virus HIV-1, HIV-2

Parvoviridae -ss DNA

1. Fifth disease (AKA erythema infectiosum and as slapcheek) is one of several possible manifestations of infection by parvovirus B19
2. Parvo (Canine parvovirus type 2/ CPV2) is a contagious virus mainly affecting dogs


1. DS DNA, nonenveloped, 70-90nm
2. Papillomaviruses replicate exclusively in body surface tissues such as the skin, or the mucosal surfaces of the genitals, anus, mouth, or airways
3. Of the more than (24)______ types of HPV, more than 30 types can be passed from one person to another through sexual contact
4. Persistent HPV infections are now recognized as the major cause of (25) __________ cancer


1. Double-stranded DNA, enveloped, 200-300nm
2. Orthopoxvirus -The most famous member of the genus is Variola virus, which causes smallpox. It was wiped out using another orthopoxvirus, the (26) ____________ virus, as a vaccine.

Herpesviridae - DS DNA, enveloped

Type Synonym Symptom
HHV-1 Herpes simplex 1 Oral/genital herpes
HHV-2 Herpes simplex 2 Oral/genital herpes
HHV-3 Varicella zoster virus (VZV) Chickenpox and shingles
HHV-4 Epstein-Barr virus (EBV), Infectious mononucleosis
HHV-6,7 Roseolovirus Sixth disease -roseola
HHV-8 Kaposi's sarcoma -associated herpesvirus Kaposi's sarcoma

Picornaviridae ss RNA, + strand, n.e.

1. Why can these spread so fast? (27)
2. Enterovirus - 2nd most common in humans
a. Includes poliovirus
b. Coxsackie - hand, foot and mouth disease; herpangina, & hemorrhagic conjunctivitis
3. Rhinovirus- Most common in humans, common cold
4. Hepatovirus - Hepatitis A - causes acute liver disease lasting from a few weeks to several months.
5. Norovirus - also +ss RNA, n.e

SS RNA, positive strand, enveloped

1. Togaviridae
a. Rubivirus (rubella virus) - causes German measles.
2. Congenital rubella syndrome (CRS) can occur in a developing fetus of a pregnant woman who has contracted rubella during her first trimester.

SS RNA, positive strand, enveloped

1. Flaviviridae - generally spread through arthropod vectors
a. West Nile encephalitis, Dengue fever, Hepatitis C
2. Coronaviridae - upper respiratory infection and the common cold
a. Human infection by SARS (severe acute respiratory syndrome)
b. Coronavirus appears to be limited to the respiratory tract

Mononegavirales - 1RNA strand, negative

1. Paramyxoviridae - measles, mumps and canine distemper
2. Orthomyxoviridae - influenza virus A, B, & C;
3. Bunyaviridae - Hantavirus, associated with rodents and arthropods

Modes of Viral Multiplication (6 steps)

The host cell is absolutely necessary for viral multiplication
STEPS: (28)
1. adsorption
2. penetration
3. uncoating
4. synthesis
5. assembly
6. release


1. Virus encounters susceptible host cells
2. Adsorbs specifically to (29)________________sites on the cell membrane
Because of the exact fit required, viruses have a limited _host range__


1. Flexible cell membrane of the host is penetrated by the whole virus or its _nucleic acid_
2. Endocytosis_: entire virus engulfed by the cell and enclosed in a vacuole or vesicle
3. The viral _envelope_ can also directly fuse with the host cell membrane


1._Enzymes__ in the vacuole dissolve the envelope and capsid
2. The virus is now uncoated - simply genetic material


1. Free viral nucleic acid exerts control over the host's synthetic and metabolic machinery
2. DNA viruses- enter host cell's _nucleus_ where they are replicated and assembled
a. DNA enters the nucleus and is _transcribed_ into RNA
b. The RNA becomes a message for synthesizing viral proteins (_translation__)
c. New DNA is synthesized using host nucleotides
3. RNA viruses- replicated and assembled in the _cytoplasm__


1. Mature virus particles are constructed from the growing pool of parts
2. Primarily within the cytoplasm
3. What carries the message for how virus is assembled? (39)


1. Nonenveloped and complex viruses are released when the cell _lyses_/_ruptures__
2. Enveloped viruses are liberated by _budding__ or exocytosis
3. Anywhere from 3,000 to 100,000 virions may be released, depending on the virus
4. Entire length of cycle- anywhere from 8 to 36 hours

Damage to the Host Cell and Persistent Infections (part 1)

1. _Cytopathic (CPE)_ effects- virus-induced damage to the cell that alters its microscopic appearance
2. Inclusion bodies- (43) compacted masses of viruses or damaged cell organelles

Damage to the Host Cell and Persistent Infections (part 2)

1. Important for the diagnosis of viral infections
2. Some viral infections maintain a carrier relationship
a. The cell harbors the virus and is not immediately lysed__
b. Persistent infections- from a few weeks to the remainder of the host's life
2. Some viruses remain in a chronic _latent__state, periodically becoming activated
4. Some viruses enter their host cell and permanently alter its genetic material, leading to cancer
a. _Oncogenic__ viruses
b. Their effect is called transformation
c. Oncoviruses- mammalian viruses capable of initiating tumors

Viruses that Infect Bacteria

1. Called _bacteriophage aka phages______
2. Most contain dsDNA. Why?

3. Often make the bacteria they infect more pathogenic_ for humans

T-even Phages

1. Icosahedral capsid head containing DNA
2. Other parts? central tube, collar, base plate, tail pins and fiber
2. Similar stages as animal viruses
a. _adsorb__ to host bacteria
b. The nucleic acid _penetrates__ the host after being injected through a rigid tube inserted through the bacterial membrane and wall
c. Entry of the nucleic acid causes the _cessation_ of host cell DNA replication and protein synthesis
d. The host cell machinery is then used for viral replication and synthesis of viral proteins
e. As the host cell produces new parts, they spontaneously _assemble into bacteriophages___

What are two functions of lysozyme in bacteriophage infections? (55)


Lysogeny: The _Silent_ Virus Infection

1. Temperate phages- special DNA phages that undergo adsorption and penetration but are not replicated or released immediately
2. Instead the viral DNA enters an inactive _prophage_ stage
3. the cell's progeny will also have the temperate phage DNA
4. Lysogenic conversion: A bacterium acquires a new trait from its temperate phage

***Allowing pathogenicity

Medical Importance of Transduction

1.Clostridium botulinum causes botulism
a. Disease caused by botulism toxin
b. Gene for the toxin is carried by a bacteriophage
c. Bacteriophage tends to be lysogenic
2. Disease causing strains of Staphylococcus aureus, Corynebacterium diphtheriae and Vibrio cholera all have a single toxin protein encoded within the chromosomes of lysogenic bacteriophages


1. What are two types of viral infections in bacteria? (59)

2. Transduction is bacteriophages moving host genetic material -
a. Not on purpose
b. Transduction is an error in their reproduction
c. Virus packing host DNA is usually random
d. Can a virus with host DNA still attach & inject genes? (60)
e. Can occur in lytic or lysogenic infections

Techniques in Cultivating and Identifying Animal Viruses (part 1) - primary purpose

1. Primary purposes of viral cultivation
a. To isolate and identify viruses in clinical specimens
b. To prepare viruses for _vaccines____________
c. To do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells

Techniques in Cultivating and Identifying Animal Viruses (part 2) Live animal inoculation

1. Using _Live_ Animal Inoculation
a. Specially bred strains of white mice, rats, hamsters, guinea pigs, and rabbits
b. Occasionally invertebrates or nonhuman primates are used
c. Animal is exposed to the virus by injection

Techniques in Cultivating and Identifying Animal Viruses (part 3) Bird embryos

1. Using Bird Embryos
a. Enclosed in an egg- nearly perfect conditions for viral propagation
b. Chicken, duck, and turkey are most common
c. Egg is injected through the shell using sterile techniques

Using Cell (Tissue) Culture Techniques

1. Most viruses are propagated in some sort of cell culture
2. The cultures must be developed and maintained
3. Animal cell cultures are grown in sterile chambers with special media
4. Cultured cells grow in the form of a _monolayer___
5. Primary (64)
6. Continuous (65)

Medical Importance of Viruses

1. Most viruses must run there course, host must treat the (67)__________________
2. Most common cause of _acute_ infections that do not result in hospitalization
3. Most do not cause death but those that do can have very high mortality rates (rabies, AIDS, Ebola)
4. Others can lead to long-term debility (polio, neonatal rubella)

Other Noncellular Infectious Agents: Prions

1. Infectious (72) __________ with a long period of latency
a. Deposition of protein fibrils in the brain tissue-
2. Inherited and transmissible by ingestion, transplant, and surgical instruments
3. Spongiform encephalopathies: Sheep scrapie, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker syndrome, fatal familial insomnia, mad cow disease
4. PrPC: Normal cellular prion protein, on cell surface
5. PrPSc: Scrapie protein; accumulates in brain cells forming plaques

Treatment of Animal Viral Infections

1. Because they are not bacteria, _antibiotics___ are ineffective
2. Antiviral drugs block virus replication by targeting one of the steps in the viral life cycle (example - Tamiflu)
3. (70)_Interferon____ shows potential for treating and preventing viral infections
4. Vaccines stimulate (71)_immunity_

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