Download Lytic Cycle

VIRUS
• “virus” is Latin for poison
• An infectious agent made of nucleic
acid and a protein covering.
• A person who studies viruses is
called a virologist.
Characteristics of Viruses
• ½ to 1/100 (100 times smaller) the size of a bacterium
• Non living (not made of cells)
• No respiration
• No growth
• No movement
• Named for organism they infect or disease they cause
• Only “alive” when inside a HOST cell
• Do NOT grow-only reproduce inside a host
Figure 18.1 Comparing the size of a virus, a bacterium, and a eukaryotic cell
The size of
a virus is
very small in
comparison
to an animal
cell or even
a bacterium
Viral Structure
• Viruses are made up of a nucleic acid (DNA or
RNA…but NEVER both!)
• The nucleic acid is enclosed in a protein coat
called a CAPSID.
• Some viruses are surrounded by an envelope or
spiky outer covering used for extra protection.
• Viruses have diverse sizes and shapes.
Figure 18.2 Viral structure
HELICAL
POLYHEDRAL
GLOBULAR
BINAL /
COMPLEX
Parts of a Virus
• Capsid (protein coat)
• Nucleic Acid (DNA)
• Tail
• Tail Fibers
Infection
• Viruses cannot reproduce on their own—they
REPLICATE by using a HOST cell
• Infects a host cell
• HOST: A living organism in which a virus is found
• Has a matching surface receptor for host cell (fits like
a puzzle)
• 3D shape determines which cells virus infects
• If it can’t attach – it doesn’t infect
Viral Infection
• Can not be “Killed” with the use of antibiotics
because viruses do not have cell walls.
(Antibiotics puncture cell walls)
• Must be killed by the body’s own immune
system (WBC and antibodies)
• Once the virus is inside the host cell, two
different processes may occur.
• http://www.youtube.com/watch?v=Rpj0emEGShQ
Viral Cycles
• Lytic Cycle:
• Virus enters the cell, makes copies of itself and causes
the cell to burstVirus lyses the host cell quickly and
then releases hundreds of virus particles that infect
other cells.
• Lysogenic Cycle: Virus may “hide” in the host cell
for a long period of time until causing symptoms of
infection.
• While hiding the virus
Figure 18.4 The lytic cycle of phage T4
LYTIC CYCLE
Cell is lysed
quickly
LYTIC CYCLE
1. Attach
2. Inject
3. Make parts
5. Release / lysis
4. Assemble
Bacteria cell
being lysed
Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage
LYSOGENIC CYCLE
Figure 18.7x1 HIV infection
The HIV virus is an example of a lysogenic
virus. Can be undetected for years because no
symptoms are present. Can only be detected
through blood testing.
Figure 18.7x2 Couple at AIDS quilt
The HIV epidemic is greatly
affecting all nations, including US!
•Africa is extremely affected.
Figure 18.x1 Smallpox
Virus that
Causes
Smallpox is
evident in this
child.
Figure 18.x2 Measles
A virus causing measles can now be prevented with the
use of vaccines.
•Vaccines are a weakened/dead virus that allows the
body to create a resistance (antibodies) against the
harmful strain.
Figure 18.x3 Polio
Polio
Figure 18.x4 Hepatitis
Hepatitis
strains vary
and plague all
nations.
Figure 18.8x Deer Mouse
The Haunta Virus
plagued
Southwestern region of
the US in late 1990’s.
Killing several people.
It was transferred
through rodents.
Figure 18.x5 Influenza epidemic
Figure 18.9 Viral infection of plants
Figure 18.9x Tobacco mosaic virus
Figure 18.x6 Herpes
A cold sore (herpes virus) is a lysogenic virus that
integrates some of its nucleic acid into your DNA
and reappears.
Figure 18.8 Emerging viruses
Although viruses are not a living
organism they have the ability to
MUTATE and do that rather
well…Leading the the emergence of
new virons.
Retrovirus
• A certain type of virus that contains RNA instead
of DNA. Once in the host, it transcribes its RNA
into DNA using an enzyme called reverse
transriptase. It then inserts the new DNA into the
host’s DNA. Virus uses the cell to make itself.
• Example: HIV: Human Immunodeficiency Virus
(Causes AIDS)
•Viroids & Prions
• Viroids: similar to a virus. Infects only plants.
Contains RNA and has no capsid.
• Prion: similar to a virus. Infects only animals.
Made of protein only. Does not have RNA or
DNA.
• Example…Mad Cow Disease