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Chapter 4

Biological Sciences 55-237 : Viruses and Other Infectious Particles


Department
Biological Sciences
Course Code
BIOL 2070
Professor
tanyanoel
Chapter
4

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Microbiology Topic 4 Viruses and Other Infectious Particles
Introduction to Viruses
Viruses
Viruses (as well as viroids, prions) are microbes that are not considered truly living
Virus requires host to replicate (including host ribosomes, etc.)
Viruses contain DNA or RNA (not both)
Not all viruses are necessarily harmful (though all are considered to be parasitic
with host)
Viruses are able to infect all sorts of different types of cellular organisms
Although viruses are not considered to be living, we recognize that they exhibit
some of the characteristics of life when they are in a host cell
Many viruses do have a significant impact on their host, including causing disease
Given this, along with their small size and infectious nature, the study of viruses
(virology) is part of microbiology
Viruses differ from living cells in a number of ways
They require a host cell in order to replicate
Viruses do not divide in the way that cells do
o They do not carry out binary fission or mitosis
Viruses do have a genome and a small number of genes
Viruses possess either DNA or RNA but you will not find both of these in a mature
virus
Being reliant on a host cell, viruses do not have the genes or enzymes required for
energy production
o Have to rely on host cell for ribosomes, enzymes, and metabolites for protein
and nucleic acid production
There are many types of viruses
o Most of the ones we study cause disease
Not all viruses cause harm to the host but because of their nature, they all exhibit a
parasitic relationship
We have viruses for probably every type of cell on the planet
o Plants, animals, bacteria
Bacteriophages infect bacteria
We have viruses that affect fungi, archaea, protozoa
If there is a cell, there are probably many viruses that have the ability to infect it
We are looking at really small particles when considering viruses
Going from a relatively large yeast cell (a eukaryote) to different sized bacteria and a
range of different types of viruses
Many viruses are on the lower end of the spectrum
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However, there are some that are much larger and approach the size of bacteria,
including the very complex pox viruses
Viruses differ in size from bacteria and eukaryotic cells
Video
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While viruses are not cells and not living, they still posses a genome that encodes
information that they need to replicate
Viruses to rely on host cells to provide the energy and materials required for viral
replication
There are different types of viral genomes
o Some are DNA and others are RNA
You will only find one type of nucleic acid in a particular variant
In addition to variation in type of nucleic acid, we also see some differences within
the type
There are ssDNA viruses as well as dsDNA viruses; we also see ssRNA genomes as
well as dsRNA genomes
We are also going to see variations among the ssRNA viruses
Viral genomes are typically much smaller than the genomes of the host cell
Genome of a bacteriophage is much smaller than the E. coli genome
Viruses are considered to be obligate, intracellular pathogens
They have some things in common with other types of intracellular pathogens
As viruses, they have a lot of differences as well
Rickettsias and Chlamydias are intracellular bacteria
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