CHAPTER 21 – VIRUSES, VIROIDS, AND PRIONS: INFECTIOUS BIOLOGICAL PARTICLES
- When you receive a vaccine (such as an influenza shot), your body produces antibodies against
that virus, protecting you against subsequent infection by any of those specific strains
Antibodies: highly specific protein molecules produced by the immune system that recognize and
bind to specific proteins of pathogen, such as the proteins in a virus’s coat
Virus: Lack many of the properties of life shared by all organisms and so are not considered to be
- Viruses cannot reproduce on their own and lack a metabolic system to provide energy for their
- Instead, they depend on the host cells that they infect for these functions.
- They are infectious biological particles rather than organisms
- The structure of a virus is reduced to the minimum necessary to transmit its genome from one
host cell to another
- A virus is simply one or more nucleic acid molecules surrounded by a protein coat or capsid.
Some capsids may be enclosed within a membrane or envelope derived from their host cell’s
- The nucleic acid genome of a virus may be either DNA or RNA and can be either single stranded
or double stranded
- Viral genomes range from just a few genes to over a 100; all viruses have genes that encode at
least their coat proteins, as well as proteins involved in regulation of transcription. Genomes of
enveloped viruses also include genes required for synthesis of envelope proteins. Some viral
genomes also include virus specific enzymes for nucleic acid.
Most viruses take one of 2 basic structural forms:
Helical viruses: the protein subunits assemble in a rodlike spiral around the genome. A number of
viruses that infect plant cells are helical
Polyhedral Viruses: The coat proteins from triangular units that fit together like the parts of a soccer
ball. The polyhedral viruses include forms that infect animals, plants and bacteria.
In some polyhedral viruses, proteins spikes that provide host cell recognition extend from the
corners, where the facets fit together. Both helical and polyhedral viruses can be enveloped in a
membrane derived from the host’s membrane
In eneveloped viruses, proteins synthesized from the viral genome in the host cell are transported to
and embedded in the membrane before the virus particle buds through the host cell. These proteins
allow the virus to recognize and bind to host cells. More than 4000 species of viruses have been classified into more than 80 families. The family
names end in –viridae and may refer either to the geographic region where the virus was first
discovered or to the structure of the virus. Some viruses are named the disease they cause.
Usually a virus infects only a single species or a few closely related species. A virus may even infect
only one organ system or a single tissue or a cell type in its host. However, some viruses are able to
infect unrelated species, either naturally or after mutating.
Of the roughly 80 viral families described to date, 21 includes viruses that cause human diseases.
Viruses also cause diseases of wild and domestic animals. Plants viruses cause annual losses of
tonnes of crops.
Some virus are annoying like virus that causes cold sores, chicken pox, while others are lethal that
cause deadly human diseases such as AIDS, encephalitis and Ebola hemorrhagic fever.
NOT all virsuses are BAD though! Some benefit their hosts by protecting them from pathogenic
viruses. The “protective” virus interferes with replication or other functions of the pathogenic
A huge number of viruses get destroyed by viruses known as bacteriophages. Viruses also provide a
natural means to control some insect pests, such as spruce budworm.
Viruses are vital components of ecosystems and may be the dominant “entity” in some ecosystems,
such as the oceans.
21.2 Viruses Infect Bacterial, Animal, and Plant Cells by Similar Pathways
Viral particles move by random molecular motions until they contact the surface of a host cell. For
infection to occur, the virus or the viral genome must then enter the cell. Inside the cell, the viral
genes are expressed, leading to replication of the viral genome and assembly of progeny viruses.
The new viral particles or virions, as the extracellular form of a virus is known, are then released
from the host cell, a process that often ruptures the host cell, killing it.
21.2a Bacteriophages Are Viruses That Infect Bacteria
Virulent bacteriophages: They kill their host cells during each cycle of infection
Temperate bacteriophages: Enter an inactive phase inside the host cell and can be passed on to
several generations of daughter cells before b