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18.1 Studying Viruses and Prokaryotes
Characteristics of Living Organisms
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Made up of cells
Reproduce on their own
Have genetic information, DNA
Grow and Develop
Obtain and use materials and energy (metabolism)
Maintain and internal balance (homeostasis)
Respond to their environment
Evolve, change over time
18.1 Studying Viruses and Prokaryotes
Viruses, bacteria, viroids, and prions can all cause
infection.
• Any disease-causing agent is called a pathogen.
1 nanometer (nm) = one
billionth of a meter
100 nm
eukaryotics cells
10,000-100,000 nm
viruses
50-200 nm
prokaryotics cells
200-10,000 nm
viroids
5-150 nm
prion
2-10 nm
18.1 Studying Viruses and Prokaryotes
What is the difference between bacteria and viruses?
• A virus is made of DNA or RNA and a protein coat.
– non-living pathogen
– can infect many
organisms
• Bacteria are onecelled microorganisms
that can cause
infection.
– living pathogen
18.1 Studying Viruses and Prokaryotes
Viruses differ in shape and in ways of entering host
cells.
• Viruses have a simple structure.
– genetic material (DNA or RNA)
– Capsid (a protein shell)
– maybe a lipid envelope, a protective outer coat
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
18.1 Studying Viruses and Prokaryotes
• Viruses enter cells in various ways.
– bacteriophages pierce host cells
colored SEM; magnifications:
large photo 25,000; inset 38,000x
18.1 Studying Viruses and Prokaryotes
• Viruses enter cells in various ways.
– viruses of eukaryotes enter by endocytosis
18.1 Studying Viruses and Prokaryotes
The different viral shapes, play a role in how viruses work.
• It is how they identify their host cell.
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
18.1 Studying Viruses and Prokaryotes
Enveloped Virus
• Ex: influenza
18.1 Studying Viruses and Prokaryotes
Helical virus
• Example: Rabies
18.1 Studying Viruses and Prokaryotes
18.1 Studying Viruses and Prokaryotes
Polyhedral Virus
• Example: Foot and mouth (coxsackie virus)
18.1 Studying Viruses and Prokaryotes
18.1 Studying Viruses and Prokaryotes
Viruses are specific in what they infect
• Most viruses are species specific
– Ex. Small pox-humans only, tobacco mosaic virusplants only, bacteriophages-bacteria only
• can only affect a limited range of hosts
– host range is determined by the specificity of
attachment to the cells, which depends on properties
of both the virion's coat and specific receptors on the
cell surface
• Some can affect a number of species with in mammals
ex. Rabies
18.1 Studying Viruses and Prokaryotes
The genetic material in viruses differ from prokaryotes and
eukaryotes.
• They can have either DNA or RNA, but never both.
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
18.1 Studying Viruses and Prokaryotes
• A prion is made only of proteins.
– causes misfolding of other proteins
– results in diseases of the brain Ex: mad cow disease
18.1 Studying Viruses and Prokaryotes
• A viroid is made only
of single-stranded
RNA.
– causes disease in
plants
– passed through
seeds or pollen
– Examples: Potato
spindle tuber and
Coconut cadangcadang
18.1 Studying Viruses and Prokaryotes
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Viruses can only reproduce after they have infected
host cells.
They are simply packaged sets of genes that move from
one host cell to another.
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
18.1 Studying Viruses and Prokaryotes
• Bacteriophages infect bacteria.
capsid
DNA
tail sheath
tail fiber
18.1 Studying Viruses and Prokaryotes
Lytic infection
• 1. Bacteriophage injects DNA into cell
• 2. Viral DNA forms plasmid in host
• 3. May enter lysogenic cycle
• 4. Host makes new virus
• Final: Host explodes (lyses), spreading virus
• Well known lytic viruses:
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SARS
Common cold
Influenza
Rabies
AIDS mode of HIV
Tobacco mosaic virus
18.1 Studying Viruses and Prokaryotes
Lytic infection causes the cell to burst.
host bacterium
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
forms a circle.
The viral DNA directs the host
cell to produce new viral parts.
The parts assemble into new
bacteriophages.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.
18.1 Studying Viruses and Prokaryotes
Lysogenic Cycle
• 1. Bacteriophage injects DNA into cell
• 2. Viral DNA forms prophage (part of host DNA)
• 3. Viral DNA replicates with host
• 4. Cell reproduces producing many copies of virus
• Final: Cell has dormant virus or may enter lytic cycle
• Well known lysogenic viruses:
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HIV
Herpes
Hepatitis B
Varicella-Zoster (chicken pox)
18.1 Studying Viruses and Prokaryotes
• A lysogenic infection does no immediate harm.
The prophage may leave the
host’s DNA and enter the
lytic cycle.
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Many cell divisions produce a
colony of bacteria infected
with prophage.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
18.1 Studying Viruses and Prokaryotes
18.1 Studying Viruses and Prokaryotes
Retroviruses
– A retrovirus is an RNA virus
that is duplicated in a host
cell using the reverse
transcriptase enzyme to
produce DNA from its RNA
genome. The DNA is then
incorporated back into the
host's genome.
- This type of transcriptase has a
high rate of error causing a
higher frequency of mutations.
HIVinfected
white
blood cell
18.1 Studying Viruses and Prokaryotes
Vaccines are made from weakened pathogens.
• A vaccine stimulates body’s immune system to prepare
for attack from microbes
18.1 Studying Viruses and Prokaryotes
Vaccines
• 2 most commonly
used types of
vaccines. Both build
antibodies using
proteins from the
injected viruses.:
– Live attenuated:
virus is weakened
to simulate actual
infection (best).
– Inactivated: dead
viruses injected
(safest).