Download 18.1 Studying Viruses and Prokaryotes

18.1 Studying Viruses and Prokaryotes
Viruses: Sections 18.1- 2
18.1 Studying Viruses and Prokaryotes
• Viruses are considered nonliving because they do not
respirate
move
grow, or
reproduce
18.1 Studying Viruses and Prokaryotes
• Viruses can reproduce only by using the structures of a
living cell called the host cell
• Viruses are pathogens = can cause an infectious
disease
18.1 Studying Viruses and Prokaryotes
• Viruses are not given Latin names, but are named after
the disease they cause or the host they infect
Ex:
bacteriophage = a virus that infects bacteria
rhinovirus = a virus that infects the nose
mumps = a virus that causes mumps
rabies = a virus that causes rabies
18.1 Studying Viruses and Prokaryotes
• The viral structure and shape determines the type of
host it invades
- the structure consists of two main parts:
1. DNA or RNA core
2. an outer protein coat (capsid)
18.1 Studying Viruses and Prokaryotes
Bacteriophage (virus that infects bacteria)
capsid
DNA
collar
tail sheath
tail fibers
tail spikes
18.1 Studying Viruses and Prokaryotes
- there are three viral shapes:
1. enveloped
capsid
nucleic acid
lipid
envelope
surface
proteins
enveloped
(influenza)
18.1 Studying Viruses and Prokaryotes
2. helical
Surface proteins
capsid
nucleic acid
lipid envelope
helical
(rabies)
18.1 Studying Viruses and Prokaryotes
3. polyhedral
surface
proteins
polyhedral
(foot-and-mouth
disease)
capsid
nucleic acid
18.1 Studying Viruses and Prokaryotes
* Viroids and Prions
• A virus is made of DNA or RNA and a protein coat.
– can infect many
organisms
18.1 Studying Viruses and Prokaryotes
• A viroid is made only
of single-stranded
RNA
– causes disease in
plants
– passed through
seeds or pollen
18.1 Studying Viruses and Prokaryotes
• A prion is made only of proteins.
– causes misfolding of other proteins
– results in diseases of the brain
18.1 Studying Viruses and Prokaryotes
• Viruses enter cells in various ways
– those that attack bacteria (bacteriophages) pierce
host cells
colored SEM; magnifications:
large photo 25,000; inset 38,000x
18.1 Studying Viruses and Prokaryotes
– those that attack eukaryotes enter through endocytosis
by fusing with the host’s cell membrane (Ex: HIV)
18.1 Studying Viruses and Prokaryotes
Viruses cause two types of infections.
1. Lytic cycle
- results in the death of a cell and the reproduction
and release of new viruses
13.1
Ecologists
Lytic
Cycle Study Relationships
host bacterium
3. release
1. attach & entry
The bacterophage attaches
and injects it DNA into a host
bacterium.
The host bacterium breaks apart,
or lyses. Bacteriophages are able
to infect new host cells.
The viral DNA directs the host cell to
produce new viral nucleic acids and
protein parts. The parts assemble into
new bacteriophages.
2. replication & assembly
The viral DNA
forms a circle.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.
18.1 Studying Viruses and Prokaryotes
2. Lysogenic cycle
- results in the incorporation of viral nucleic acids
into the host cell’s DNA
18.1 Studying Viruses and Prokaryotes
- in a lysogenic infection, the lysogenic cycle will
continue until some environmental stimuli, such as
fever, stress, or disease cause the dormant virus to
become active and enter the lytic cycle.
Ex:
Herpes
chicken pox

shingles
HIV

AIDS
13.1
Ecologists
Study Relationships
Lysogenic
Cycle
1. provirus formation
Continues until stimuli causes
it to enter into the lytic cycle.
The viral DNA is called a provirus (or
prophage) when it combines with
the host cell’s DNA.
3. division
Lysogenic Cycle
Although the prophage is not active, it
replicates along with the host cell’s DNA.
The host cell continues to
divide as usual.
2. replication