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Lecture 3

55-237 Lecture Notes - Lecture 3: Brainstem, P53, Cervical Cancer

Biological Sciences
Course Code
BIOL 2070

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Chapter 13: Viruses, Viroids and Prions
General Characteristics of Viruses
1930: Began to be called viruses instead of contagious fluid (virus is Latin for
1935: Wendell Stanley isolated tobacco mosaic virus (first isolated virus), and
around the same time the invention of the electron microscope made it possible to
see viruses
Viruses are not considered to be alive outside of host cells, however are alive once
inside of host cells (when viral replication begins are considered alive)
Viruses are very small (filterable) and obligatory intracellular parasites (need host
cells to replicate)
Contain a genome as a single type of nucleic acid (either DNA or RNA, won’t
find both in one virus).
Contain a protein coat that surrounds the nucleic acid, usually in the form of
capsids – repeating units of capomers.
Multiply inside living cells by using the synthesizing machinery of the cell
Cause the synthesis of specialized structures that can transfer viral nucleic acid to
other cells
Have no enzymes for their own metabolism (cant synthesize protein or ATP)
To stop viruses need to shut down function of host cell, antiviral drugs cant do
that because it would be toxic to the host
Host Range
Host Range: the spectrum of host cells that the virus can infect
Viruses can infect invertebrates, vertebrates, plants, protists, fungi, and bacteria,
but generally only specific types of cells in the host species
Viruses can expand their host range
Bacteriophages/Phages: Viruses that infect bacteria
To infect a host cell the outer surface of the virus must interact with the receptor
site of the cell. Many interactions between receptors and the virus cause a strong
bond between the virus and host cell

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Receptor sites: Bacteriapahages – cell wall of host or fimbriae or flagella. Animal
viruses – plasma membrane of host cells.
Phage Therapy: using bacteriophages to treat bacterial infections
Oncolytic: tumour destroying viruses
Viral Size
Is determined through the use of electron microscopy
Range from 20 to 1000 nm in length
Viral Structure
Virion: a complete, fully developed, infectious particle composed of nucleic acid
and surrounded by a protein coat outside of a host cell, and is a vehicle
transmoission from one host cell to another.
Viruses are classified by their nucleic acid and differences in the structures of
their coats
Nucleic Acid
Can have either DNA or RNA
Can be ds or ss
Can be linear or circular or in several different segments
The amount of nucleic acid to protein varies from 1% to about 50%
The total nucleic acid varies from a few thousand nucleotides to as many as 250,
Capsid and Envelope
Capsid: A protein coat that protects the nucleic acid of the virus
The capsid accounts for most of the mass of the virus and the structure is
determined by a particular virus’s nucleic acid.
Capsomeres: protein subunits that make up the capsid, can be the same protein or
different depending on the nucleic acid of the virus

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Envelope: a combination of proteins, carbohydrates, and lipids that covers the
capsid, the proteins are generally determined by the viruses nucleic acid and uses
materials derived from the host cell
Spikes: carbohydrate-protein complexes that project from the surface of the
envelope, some viruses use spikes to attach to the host cell
Hemagglutination: clumping that occurs due to spikes forming bridges between
multiple cells
Nonenveloped Viruses: Viruses whose capsids are not covered by an envelope.
The capsid provides resistance to nucleases and promotes the viruses attachment
to host cells
The immune system produces antibodies (proteins that interact with the surface
receptors/proteins of viruses) when a viral infection occurs. Virus surface
receptors/proteins have a high mutation rate and therefore antibodies can’t interact
with them after a mutation and the infection continues.
General Morphology
Helical Viruses: resemble long rods that may be rigid or flexible. Capsid is hollow
and cylindrical. Ie. Rabies and Ebola hemorrhagic fever
Polyhedral Viruses: Many sided virus. Capsid is generally and icosahedron (20
traingluar faces and 12 corners). Capsomeres of each face form an equilateral
triangle. Ie. Poliovirus and adenovirus.
Enveloped Viruses: Roughly Spherical. Can be enveloped helical viruses (ie.
Influenza), or enveloped polyhedral virus (ie. Herpes simplex virus).
Complex Viruses: generally bacterial viruses, have a complex structure such as
capsids with additional structures attached (polyhedral head and helical tail
sheath). Ie. Poxviruses
Taxonomy of Viruses
Used to be based off of symptomatology, but was not scientifically acceptable.
The suffix –virus is used for genus name
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