Viruses
What is a virus?
• A virus is classified as a non-cellular
particle made up of protein-covered
genetic material that can invade living cells
• Viruses are not considered to be alive
because they do not respire or metabolize
energy
• Viruses cannot reproduce unless they are
in a living cell
Sizes of viruses
How a virus reproduces
• Viruses grow and develop in the cells of
specific hosts and depend on these cells
for respiration, nutrition, and all other
functions of life to enable them to
reproduce.
• When viruses enter the host’s cells, they
“take over” by altering the host cells’
genetic make up.
Make-up of a virus
• Viruses have two components:
genetic material and
a protein covering called a capsid.
• A virus’s genetic material can either be
DNA or RNA, depending on the type of
virus.
The Lytic Cycle
The Lytic Cycle
The Lytic Cycle
The Lytic Cycle
The Lytic Cycle
Bacteriaphage
Lysogenic Cycle
Lysogenic cycle
Lysogenic cycle
Lysogenic cycle
Lysogenic cycle
Lysogenic cycle
RNA Viruses
• RNA viruses contain RNA, which can act
as mRNA once in the cell.
• The mRNA can manufacture viral proteins
and so bypass the host DNA.
• All RNA viruses are lytic since there is no
way for the viral RNA to become part of
the host’s DNA structure.
Retroviruses
• Retroviruses also contain RNA as their genetic
material.
• When a virus infects a cell, it produces a copy of
viral DNA from the viral RNA code.
• The new DNA becomes part of the hereditary
apparatus of the infected human cell.
• The host cell does not burst, but it changes
permanently in shape, metabolism, and growth.
• Because the host cell does not burst, the cycle
of infection is lysogenic.
Viruses and Disease
• Most viruses cause some kind of disease
in their hosts.
• Some common animal diseases caused
by viruses are chickenpox/shingles,
hepatitis (A, B, and C), measles, warts,
distemper, mumps, yellow fever, infectious
mononucleosis, equine encephalitis, and
rabies.
Plant Viruses
• Most plant viruses tend to be RNA viruses.
• Plant viruses can stunt plant growth and cause
low crop yields.
• Viruses can be spread from plant to plant by
insects carrying the virus, or by farmers and
gardeners using tools that are infected with the
virus.
• Some examples of plant viral diseases are
strawberry ringspot, bean leaf roll, tobacco
mosaic, potato mottle, apple chlorotic,
Homework
1. Explain why it is difficult to classify
viruses.
2. Draw diagrams to compare the lytic and
lysogenic cycles of a bacteriophage.
3. Compare the life cycle of a eubacterium
with the reproductive cycle of a virus.
4. Make a chart to compare how RNA
viruses are similar to and different from
DNA viruses.