1. 19-2 Viruses
- virus means “poison”
What is a Virus?
• non-living – no metabolism and
can’t reproduce on their own
• DNA or RNA core surrounded
by a protein coat
• use a living cell’s internal
structures to reproduce
themselves
• Capsid – protein coat - often
with proteins on it that help it
invade a host cell – often
highly specific
• once inside the host the viral
genes get the host cell to make
viral proteins and DNA/RNA
2.
3. - bacteriophage – a virus that
attacks bacteria
Viral Infection
1. Lytic Infection
- virus enters a cell; has copies
of itself made; and causes cell to
burst and release more viruses
2. Lysogenic Infection
- virus enters a cell and has its
DNA integrated into the DNA of
the host cell
- as cell replicates its DNA it also
replicates the viral DNA
- prophage – viral DNA
embedded in host DNA
4. Lytic
Pathway
Lysis of host cell is induced;
infectious particles escape.
Tail fibers and other parts are
added to coats.
Virus particles bind to wall of suitable host. Viral
genetic material enters cell cytoplasm.
Viral protein molecules are assembled into
coats; DNA is packaged inside.
Viral DNA directs host
machinery to produce viral
proteins and viral DNA.
Stepped Art
Fig. 21.20 Page 358
5. Lysogenic
Pathway
Viral DNA usually becomes
integrated into the bacterial
chromosome.
Prior to prokaryotic fission, the
chromosome and integrated viral
DNA are replicated.
Viral DNA is excised from
chromosome and cell enters
lytic pathway.
After binary fission, each
daughter cell will have
recombinant DNA.
Stepped Art
Fig 21.20 (2)
Page 358
6. - may stay inactive for a long time
- eventually something causes the DNA of
the prophage to be activated and it enters
the Lytic cycle and destroys the host cell
Retroviruses
• Viruses which have RNA as genetic material
• When they infect a cell the cell first makes
DNA from RNA template (reverse
transcription)
• DNA inserts into cell’s DNA and can stay
dormant
• Can cause some cancers, HIV causes AIDS,
colds
(Copy Figure 19-11)
7. 19-3 Diseases Caused by
Bacteria and Viruses
While most bacteria and viruses are not
harmful a few are pathogens (disease
causing)
- Disease results when a virus or
bacteria interfere with the host’s
ability to get enough nutrients or
energy
- Louis Pasteur developed the Germ
Theory of Disease when he proved that
bacteria can cause disease
Bacteria cause disease by directly
breaking down cells for food or by
releasing toxins that travel throughout
the body and interfere with normal
activity in the host.
8. - Vaccines are used to stimulate the immune
system by exposing it to a dead or
weakened pathogen.
- Antibiotics are used against bacterial
infections that the immune system doesn’t
beat.
- Bacteria can be controlled by sterilization
with heat; disinfectants; and correct food
processing and handling.
10. Viral Disease
- Viruses produce disease by disrupting the
body’s normal equilibrium
- Most viruses are very specific in what cells
they attack – polio only attacks motor neurons
- Viral infections cannot be treated with
antibiotics.
