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Viruses
1. Identify the structures of viruses
2. Explain the differences between the lytic and
lysogenic cycles of viral reproduction
3. Recognize the medical importance of viruses.
What is a virus?
• You have probably had the flu sometime
during your life.
• Nonliving particles called viruses cause
influenza
• Viruses are composed of nucleic acid enclosed
in a protein coat
– Viruses are smaller than the smallest bacterium
cell
What is a virus?
• Most biologists consider viruses to be
nonliving because they do not possess all of
the criteria for life.
• Viruses do not carry out respiration, grow, or
develop. All that viruses can do is replicate –
make copies of themselves– and they can’t
even do that without the help of a living cell
• A cell in which a virus replicates is called a
host cell.
What is a virus?
• Some viruses are named after the disease
they cause (rabies viruses and polio viruses)
• Other viruses are named for the organ they
infect (Human Immunodeficiency Virus –HIV)
• A bacteriophage, or phage for short, is a virus
that infects a bacterium
Viral
Structure
Envelopes are composed primarily of the same materials
found in the cell membrane of all cells.
Viral
Structure
Some relatively large viruses such as human flu viruses have
an additional layer that surrounds the capsid called an
envelope.
Viral Structure
• Viral nucleic acid is either
DNA or RNA and contains
instructions for making the
virus.
• Some viruses have only 4
genes while others have
hundreds.
Viral Structure
• The protein arrangement
(a) also plays a role in
determining what cell can
be infected and how
viruses infects the cell.
Replication
• Before a virus can replicate, it must enter a host cell
• 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 cell
membrane
• Each virus has a specifically shaped attachment
protein. Therefore, each virus can usually only attach
to a few kinds of cells.
• In general, viruses are species specific, and some are
cell type specific.
Replication
• Attachment is specific
• The species specific characteristics of viruses
is significant for controlling the spread of viral
diseases.
Lytic Cycle
Lytic Cycle
1st step
What is the 1st step called?
Attachment
What happens during the first step? Virus attaches to host
cell
Lytic Cycle
2nd step
What is the 2nd step called?
Entry
What happens during the 2nd step? Virus injects nucleic
acid into host cell
Lytic Cycle
3rd step
What is the 3rd step called?
Replication
What happens during the 3rd step? Host cell replicates
viral nucleic acid
Lytic Cycle
4th step
What is the 4th step called?
Assembly
What happens during the 4th step? New viral particles are
being put together
Lytic Cycle
5th step
What is the 5th step called?
Lysis and Release
What happens during the 5th step? Host cell breaks open
& releases viruses
Examples of Viruses that follow the
Lytic Cycle
•
•
•
•
Common cold
Influenza (flu)
Rabies
What do they have in common?
The symptoms of the viral infection progress fairly
rapidly
Lysogenic Cycle
Lysogenic
Cycle
1st Picture
What is the 1st picture called?
Attachment and Entry
What happens in the 1st picture ?
Injects nucleic acid into
host cell
Lysogenic
Cycle
2nd to 6th
Picture
Provirus
What happens from the 2nd to the 6th picture?
Viral nucleic acid inserts into host cell chromosome;
called a provirus
Lysogenic
Cycle
7th picture
What is the 7th picture called? Cell replication
What happens in the 7th picture?
Viral nucleic acid is replicated with host cell
Lysogenic
Cycle
8th to 2nd
to 3rd
picture
What happens from the 8th to 2nd to 3rd picture?
Viral nucleic acid exits host chromosome and
enters lytic cycle
Lysogenic
Cycle
5th picture
What happens in the 5th picture?
Host cell breaks open & releases viruses
Provirus
• A provirus usually does not affect the functions of a host cell
and the host cell carries out its own metabolic activity.
• Every time the host cell reproduces, the provirus is
replicated also
Provirus
• Every cell that originated from the infected host cell has a
copy of the provirus
• The lysogenic phase can continue for many years before
becoming a lytic virus
Examples of Lysogenic Viruses
• Hepatitis B
• Chicken pox
• HIV
Retroviruses
• Many viruses, such as the HIV virus that
causes AIDS are RNA viruses (their genetic
material is only RNA)
• Once inside the host cell, the RNA is converted
into DNA
• The newly synthesized DNA is inserted into
the host cell’s chromosome and becomes a
provirus (and follows the lysogenic cycle)
Retrovirus
Review
Which characteristic of life do viruses have in
common with cells?
A. Viruses can grow
B. Viruses can respire.
C. Viruses can replicate.
D. Viruses can develop.
Review
Which is not a component of viruses ?
A. Capsid
B. Nucleic acid
C. Protein coat
D. Phage
Review
In the lytic cycle, after the host’s metabolic
machinery makes viral nucleic acid and viral
proteins, the next phase is-A. Lysis and release
B. Replication
C. Assembly
D. Attachment