1. Viruses
What is a virus?
Viruses are composed of nucleic acids enclosed in a
protein coat and are smaller than the smallest
bacterium.
• Most biologist consider viruses to be nonliving
because they don’t exhibit all the criteria for life.
They don’t carry out respiration, grow, or develop.
• All viruses can do is replicate-make copies of
themselves- and they can’t even do that without
the help of living things.

2. Virus
• A cell in which a virus replicates is called the
host cell.
• Because they are nonliving, viruses were not
named in the same way as organisms. Viruses,
such as rabies viruses and polioviruses were
named after the diseases they cause.
• A virus that infects a bacterium is called a
bacteriophage.

3. Viral Structure
• A virus has an inner core of nucleic acid, either
RNA or DNA, and an outer protein coat called
a capsid.
• Some large virus may have and additional
layer, called and envelope, surrounding their
capsid. Envelope is composed of same
materials found in the plasma membrane.
• Viral nucleic acid either DNA or RNA and
contains instructions for making copies of the
virus.

4. Viral Structure
• The arrangement of proteins in the capsid of a
virus determines the virus’s shape.
• The protein arrangement also plays a role in
determining what cell can be infected and
how the virus infects the cell.

6. Attachment of host cell
• Before a virus can replicate, it must enter a
host cell.
• Virus do not infect all types of cells.
• A virus recognizes and attaches to a host cell
when one of its proteins interlocks with a
molecular shape that is the receptor site on
the host cell’s plasma membrane.

7. • In general, viruses are species specific, and
some also are cell-type.
• For example, polio viruses normally infect only
intestinal and nerve cells.

8. Viral replication cycles
• Virus have two ways of getting into host cells.
1. The virus may inject its nucleic acid into the host
cell like a syringe injects a vaccine into your arm.
2. An enveloped virus, the plasma membrane of
the host cell surrounds the virus and produces a
virus-filled vacuole inside the host cell’s
cytoplasm. Then, the virus burst out of the
vacuole and releases its nucleic acid into the
cell.

9. Viral replication cycle
• Once the DNA is inside the cell it becomes
part of the cells DNA.
• There are two kinds of cycles. Lytic cycle which
kills the cell and the lysogenic cycle which
does not kill the cell the cell.
• Lytic cycle-the virus takes over the cell and the
cell just makes viruses and the cell burst and
release the new viruses.

10. Viral replication cycle
• Lysogenic cycle- The virus DNA becomes part
of the host cells DNA. It does not take over the
cell’s genetic material immediately. The host
cell still carries out it’s metabolic activity.
Provirus is viral DNA that is integrated into the
host cell’s chromosome.

11. Disease symptoms of proviruses
• The lysogenic process explains the
reoccurrence of cold sores, which are caused
b the herpes simplex I virus remains in your
provirus.
• Herpes, hepatitis B, shingles.