Download Viral Structures

Major Parts of a Virus - Bacteriophage
Capsid
(whole outer coat)
Collar
II. Viruses
A. What is a virus?
Virus – particle made up
of nucleic acid, protein,
& in some cases lipids
that can replicate only by
infecting living cells.
“Virus” is Latin for
poison.
Viral characteristics
1. Viruses are NOT cells.
2. Have No cell structures.
3. Can’t move, feed, or grow
outside of another
organism’s cells.
4. Reproduce inside living
cells.
5. Are very small – bacteria
seem big.
*A typical virus is made up of
a core of DNA or RNA
surrounded by a protein coat
(capsid).
Major Parts
1. Capsid – protein coat that
makes up most of a virus &
gives it shape.
2. Nucleic Acid – genetic
information in the form of
DNA or RNA.
3. Envelope – an additional
protective coating usually
made of lipids, proteins, &
carbs.
*Found only in some viruses
that infect animal cells.
Host Cell Dependency
-The capsid of each virus is
specific to the type of host
that it invades.
-Are classified by the hosts
that they invade.
Host – a living thing that
another organism lives in.
-Hosts shelter & nourish
invaders, ex : bacteria, plants,
or animals.
-Viruses that infect bacteria
are called bacteriophages.
Viral Infection
Are viruses living or nonliving?
-Nonliving- can’t reproduce without a host cell.
-Don’t fit into any of the 6 kingdoms.
B. Viral Infection
*Viruses can begin replication immediately
after they enter host cells or they may remain
dormant (inactive) for long periods of time.
2 Types of Viral Replication
1. Lytic cycle – (virulent cycle) the viral
replication process that rapidly kills a host
cell.
2. Lysogenic cycle – (temperate cycle) the viral
replication process in which a virus doesn’t
immediately kill a host cell.
Steps of the Lytic (Virulent) Cycle
1. Virus attaches itself to the
host cell using its tail
fibers.
2. The genetic material (DNA
or RNA) is injected into the
host.
3. Viral DNA/RNA directs cell
to make new viruses.
4. Viral pieces are made &
assembled, finishing the
copies of the virus.
5. Cell lyses (bursts) open
and releases new viruses.
Steps of the Lysogenic (temperate) Cycle
1. Attachment to host cell.
2. Injection of viral nucleic acid.
3. Recombination – viral DNA
segment (prophage) waits in
the host’s chromosomes and
gets replicated with the host’s
chromosomes.
4. Activation by environmental
stimulus.
5. Lysis of cells. Cycle may take
years.
C. Retroviruses
Retrovirus – a virus containing
RNA as its genetic information,
ex : HIV – causes AIDS.
-Retroviruses work in reverse,
“retro” referring to backward.
1. They inject a DNA copy of
their RNA into their hosts.
2. Viral & host DNA combine.
3. Viral DNA is dormant until
activated.
D. Viruses & Living Cells
-Viruses must infect a living
cell in order to grow &
reproduce.
-They must also take
advantage of the host’s
respiration, nutrition, & other
life functions, making them
parasites.
Parasite – organism that
depends entirely upon
another living organism for
existence, harming that
organism in the process.