Virology BSCI 437: Lecture 1
Impact: It is important to study viruses because:
· Many important infectious diseases that afflict humankind are caused by viruses. These can be
fatal, uncomfortable and very contagious, cause congenital defects, or carcinogenic.
· Viruses can also affect the food supply, infecting crop plants and food animals.
· The relatively simple nature of viruses makes them useful as model systems for many of the basic
problems in biology.
History:
· Evidence of viral diseases in humans date back to 1500 BC: Polio, Smallpox, Rabies. (Figure of
Egyptian Polio, fig. 1.1., Tulip Mosaic Virus, Smallpox, 1.3)
· The existence of viruses became evident at the end of the 19th century. The newly acquired
expertise in handling of bacteria led to the germ theory of disease.
· Viruses are Small!
· Ultrafiltration methods made it clear that the causative agents of some diseases were even
smaller than bacteria. (Filters figure, 1.6)
· Infection by ultrafiltrate: The first example of this was demonstrated by Iwanowski with
Tobacco Mosaic Virus (1892), followed by Loeffler and Frosch with Foot and Mouth Disease
(1898).
· In 1898, Beijerinck introduced the term “virus” (Latin for “poison”) to the literature. The term
“Virus” became the operational definition of particulate infectious agents that are smaller than
bacteria and which are unable to multiply outside of living cells.
· In 1911, Rous discovered a virus that produced malignant tumors in chickens (Rous Sarcoma
Virus). This “Oncovirus’ turns out to be the first Retrovirus discovered.
Discovery of new viruses was very rapid.
· During the next 25 years, virology diverged into three areas: Plants, bacteriophages and animal
viruses.
· The ability to isolate large amounts of viruses from plants permitted extensive chemical and
physical analyses, eg:
o The first demonstration that viruses consisted of proteins and nucleic acids.
o The crystallization of TMV by Stanley (1935) was a paradigm shift in that it
demonstrated that agents able to reproduce in living cells could also behave like
macromolecules.
· Bacteriphage research initially focused on the hypothesis that these viruses could be used for
antibacterial therapies, i.e. that they could be injected into people to destroy bacteria inside of the
body. Although this proved to be untenable, this work set the technological foundation for
molecular biology as we know it. Examples include the discovery that nucleic acids are the
molecules of genetic inheritance (Hershey & Chase 1952, also credit Oswald, MacLeod and
McCarthy, 1944), the first model systems for DNA replication (M13), control of gene expression
and recombination (λ), discovery of mRNA, the elucidation of the factors that control initiation
and termination of both transcription and translation of genetic information, and the discovery of
restriction endonucleases.
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· Animal virus research concentrated on the pathogeneis of viral infections and epidemiology. The
need to study animal viruses spurred the development of techniques for growing animal cells in
vitro. Animal virus systems also played a large role in the development of immunology.
· The “New” Molecular Biology is founded on Virology.
o Understand cellular functions such as DNA (SV40) replication and repair,
o RNA splicing (adenoviruses),
o translation (picornaviruses, poli),
o protein-protein interactions,
o gene expression (retroviruses),
o Cancer and malignancy (Tumor viruses, papilloma and oncogene carrying
retroviruses). Oncogenes (genes within cells that are associated with cancer when
they mutate or are over or under-expressed depending on the particular oncogene)
were originally discovered in retroviruses. The virus captured these genes early
on in their evolution from cells they infected. Expression of these genes by the
virus in the infected cell can lead to the cell becoming cancers although, because
of the complex nature of cancer, this is a rare event. Still, a single cancerous cell
is all that is required to start a tumor.
· Therapeutics: Vectors to introduce foreign genes into bacteria (insulin) or animals (gene
therapy and vaccine development. Commonly used vectors are based on poxviruses,
retroviruses, adenoviruses (among others). A particular vector may be able to home in on
particular cell types (ex. Adenovirus-respiratory tract cells, retrovirus-immune system cells)
while others may be more general.
The Origin of Viruses. Three possible origins:
· Products of regressive evolution from free living cells. Best candidate are the Poxviruses.
· Derived from cellular genetic material that has acquired the capacity to exist and function
independently.
· Leftovers from the pre-biotic RNA world.
Definition of a Virus. A genetic element containing either RNA or DNA that is able to alternate
between intra- and extracellular states, the latter being the infectious state. Viruses are obligate
intracellular parasites. They are absolutely dependent on the host cells’ synthetic and energyyielding apparatus. Viruses consist of a nucleic acid genome that is protected by a protein
component (typically surrounded by a protein shell called a nucleocapsid). Frequently, there is a
second outer shell composed of lipids and proteins.
Virus characteristics:
1. Virus is an infectious agent and obligate intracellular parasite.
2. Virus infectious cycle includes a phase in which the agent consists of a virion. The virion
consists of RNA or DNA coated with one or more proteins (capsid structure) which is sometimes
coated with a membrane containing lipid and glycoproteins.
3. A virus can initiate another infection when transferred to a suitable host.
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4. A virus carries genetic information in the form of RNA or DNA. This genomic nucleic acid
carries information which redirects the genetic and metabolic apparatus of the infected cell to
produce virions.
Terms:
Virion- Morphologically complete (mature) infectious virus particle.
Pathogen- Biological disease agent.
Bacteriophage- Viruses that infect bacteria. Phage is Greek for eating, since bacteriophage
produced hole on lawns of bacteria.
Virulence- the ability of an infectious agent to produce disease. Many viruses are virulent
sometimes and asymtomatic at other times. The Herpes virus Epstein Barr virus (EBV) for
example generally infects people but causes no disease. However, in some, especially immune
compromised or worn down individuals, the virus cause mononucleosis. It can also more rarely
cause B cell lymphomas and nasopharyngeal carcinoma. The retrovirus Human T cell leukemia
virus (HTLV) generally is asymtomatic during infection but somtimes causes life threatening T
cell leukemia.
Common tasks faced by viruses.
1. Cell attachment – binding to a cell surface receptor.
2. Entry via receptor—mediated endocytosis.
3. Release of genome into cytoplasm via membrane fusion.
4. Transcription of viral mRNAs and of new viral genomes (RNA viruses)
5. Viral protein synthesis and assembly of provirus.
6. Maturation of viral particle.
7. Release of virus from cell.
8. Evasion of host defense and transmission to new host.
Effects on Host Cells
Inhibition of Host Macromolecular Biosynthesis. Translation is the primary battleground
between the virus and host cell, and host protein synthesis is typically attacked first. Viruses
have evolved molecular strategies for inactivating translation of host cellular mRNAs, while
enhancing translation of their own mRNAs. Examples include picornavirus protease 2A
(cleaves eIF4GII, shutting down cap-dependent but not cap-independent translation).
Cessation of host cell protein synthesis quickly inhibits DNA replication and chromatin
maintenance. This is followed by inhibition of cellular mRNA synthesis.
In response to viral attacks, host cells have evolved ways of 1) recognizing self from non-self
encoded mRNAs (5' 7-methylGppp-caps and polyA tails), and 2) by killing translation
altogether in infected cells (eg. PKR and DAI in animal cells, RIPs in plant cells, SKI proteins
in all cells). In response, viruses have evolved molecular mechanisms to circumvent the host
cell defenses, eg. PKR inhibitors encoded by Influenza and Hepatitis C, Vaccinia encoded DAI
inhibitors.
Changes in Regulation of Host Gene Expression. In some cases, virus infection may also
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affect the regulation of host genome expression, eg. SV40 and Adenoviruses increase the
expression of many enzymes involved in nucleic acid biosynthesis in order to make NTPs
available for incorporation into new viral mRNAs and genomes.
Appearance of New Antigenic Determinants on the Cell Surface, and Immune Avoidance.
Insertion of viral envelope proteins into the outer cell membrane results in the presentation of
these to the host immune system as foreign antigens. These can be recognized by antibodies,
which are in turn recognized by various leukocytes (Macrophages, PNMs, NK cells), targeting
the infected cell for killing. Additionally, newly synthesized viral proteins can be picked up
intracellularly by the proteosome, where they are processed and presented on the cell surface in
the context of Class I MHC gene products. Presentation of viral proteins in this context targets
the infected cell for death by CD8+ (cytotoxic) T-cells.
Cell Fusion. Envelope glycoproteins are designed to allow entry of the nucleocapsid into cells by
facilitating fusion of the viral and cellular lipid membranes. Expression of these glycoproteins
on the surface of cells as a normal feature of viral infection can also allow fusion of the
infected cell membrane with adjacent, uninfected cells. Examples of this include Herpes-,
Paramyxo-, and Retroviruses. The resulting “Syncytia” are masses of cytoplasm bounded by a
single membrane which contains the nuclei of all of the parent cells (up to 1000s!). This has
been exploited in the laboratory to make hybrid cells, which can be used for multiple genetic
applications. Fig. of syncytium, Fig. 13. 21.
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