Viruses

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VIRUS

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a disease-causing, NONLIVING particle composed of an inner core of nucleic acids (DNA or RNA)
surrounded by a capsid; capable of replicating inside living cells called host cells.

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Why are they considered nonliving?

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1. they are not cells
2. they don’t carry out respiration, grow or develop
3. they cannot replicate without the help of the host cell

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virses are usually named by what?

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what organ they effect

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bacteriophages

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Viruses are also known to infect bacteria;

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Structure of a virus:

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inner core of nucleic acid (RNA or DNA) surrounded by an outer “coat” of protein
called a capsid. Some viruses have an additional envelope surrounding the capsid.

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The arrangement of proteins in the capsid determine:

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1. the shape of the virus
2. types of cells that can be invaded (similar to a puzzle or a lock and key)
a. some viruses are species specific and easy to eradicate – ex. humans (smallpox)
b. viruses that are not species specific are difficult to eradicate (ex. flu)
c. some viruses are cell type specific (polio viruses only infect human intestinal and nerve cells)
3. how the virus infects the cell

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How does the virus get inside the cell? 2 ways…

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1
1. The virus attaches to the plasma membrane then injects its nucleic acid into the cell (like a
vaccination) and the capsid stays attached to the outside of the cell.
2. A virus surround by an envelop attaches to the plasma membrane and the plasma membrane then
surrounds the virus to produce a virus-filled vacuole, which eventually bursts to release its nucleic
acid into the cell (ex. HIV and influenza)

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Lytic cycle

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viral replication cycle in which a virus takes over a host cell’s genetic material and uses the
host cell’s structures and energy to replicate until the host cell bursts, killing it.

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example of lytic cycle

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measels and a cold

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Virulent

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disease-causing and highly infective; viruses that undergo the lytic cycle.

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Lysogenic cycle

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viral replication cycle in which the virus’s nucleic acid (usually DNA) is integrated into
the host cell’s chromosome to create a provirus.

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provirus

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The provirus is inactive and is replicated each time the
host cell reproduces (mitosis); the host cell can continue in the lysogenic cycle (months or years) and is NOT
killed until the provirus leaves the chromosome and the lytic cycle is activated.

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example of lysogenic cycle

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herpes chicken pox

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Temperate

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refers to a nonvirulent virus that rarely causes disease; undergoes lysogenic.

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The newly formed viruses can leave the cell one of two ways:

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1. lysis – the bursting of the cell
2. exocytosis – active transport across the cell membrane
24.2 VIRAL DISEASES

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Retrovirus

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A type of virus in which RNA is its only nucleic acid that uses reverse transcriptase to make
DNA from viral RNA (a viral replication); the retroviral DNA is then integrated into the host cell’s chromosome
as a provirus.

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Reverse transcriptase

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enzyme carried in the capsid of a retrovirus that helps produce viral DNA
from viral RNA.

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White blood cells

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blood cells that protect the body from foreign substances and microscopic
organisms.

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ORIGIN OF VIRUSES

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• Viruses most likely originated from their host cells.
• Hypothesis – viruses are nucleic acids that break free from their host cell while maintaining the ability
to replicate parasitically within the host cell.

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Ebola virus

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An extremely contagious virus causing an acute, highly fatal hemorrhagic fever and spread
through contact with bodily fluids or secretions of infected persons and by airborne particles; after the Ebola
River in northwest Congo (formerly Zaire), where the disease was first observed.
Interested in a true Ebola story

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Herpes

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diseases caused by herpesviruses (genus Simplexvirus) and marked in one case by groups of
watery blisters on the skin or mucous membranes (as of the mouth and lips) above the waist and in the other
by such blisters on the genitals

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Polio

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an acute infectious disease caused by the poliovirus and characterized by fever, motor paralysis, and
of skeletal muscles often with permanent disability and deformity and marked by inflammation of
nerve cells in the spinal cord -- called also infantile paralysis.

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Vaccine

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a solution of a virus, bacterium,
or toxin, given to an uninfected individual,
that stimulates the body’s immune system to
provide protection against the pathogen; the
most effective method of prevention of viral
disease.

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vaccine's consist of

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inactivated or attenuated form of a virus

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Booster shot

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an additional dose of a
vaccine that can extend a person’s
protection against a virus.

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Antiviral drugs

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drugs administered to
infected patients that interfere with viral
nucleic acid synthesis.