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Viruses
General Characteristics of viruses
1. Depending on one’s viewpoint, viruses may be regarded
as exceptionally complex aggregations of nonliving
chemicals or as exceptionally simple living microbes.
How does it differ from a cell?
2. Viruses contain a single type of nucleic acid (DNA or
RNA) and a protein coat,
- sometimes enclosed by an envelope composed of
lipids, proteins and carbohydrates. They are not
cells or composed of cells.
General Characteristics of viruses
3. Viruses are obligatory intracellular
parasites.
4. A virion is a complete, fully developed
viral particle composed of nucleic acid
surrounded by a coat.
Host Range
1. Host range refer to the spectrum of host cells
in which a virus can multiply.
2. Most viruses infect only specific types of cells in
one host species.
3. Host range is determined by the specific
attachment site on the host cell’s surface and
the availability of host cellular factors.
Viral Size
1.
smaller than
bacteria.
2. Viruses rage from
20 to 14,000 nm
in length.
Classification of Viruses
1. Classification of viruses is based on type of
nucleic acid, morphological class, and presence
or absence of an envelope.
2. Virus family names end in -viridae; genus
names end in -virus; specific epithets have not
been assigned.
3. A viral species is a group of viruses sharing the
same genetic information and
ecological niche.
Nucleic Acid
1. Viruses contain either DNA or RNA, never
both, and the nucleic acid may be singleor double- stranded, linear or circular, or
divided into several separate molecules.
DNA or RNA
SS or DS
Linear or circular or divided
Capsid
1. The protein coat
surrounding the nucleic
acid of a virus is called
the capsid.
2. The capsid is composed
of subunits,
capsomeres, which can
be a single type of
protein or several types.
General Morphology
1. Helical viruses (for
example, tobacco
mosaic virus)
resembling long rods,
and their capsids are
hollow cylinders
surrounding the
nucleic acid.
Polyhedral viruses
2. Polyhedral viruses
(for example,
adenovirus) are
many-sided. Usually
the capsid is an
icosahedron. 20
triangular faces 12
corners example polio
virus
Complex viruses
3. Complex viruses
have complex
structures. For
example, many
bacteriophages have
a polyhedral capsid
with a helical tail
attached.
Capsid and Envelopes
3. The capsid of some
viruses is enclosed by an
envelope consisting of
lipids, proteins,
and carbohydrates.
4. Some envelopes are
covered with
carbohydrate-protein
complexes called spikes.
Attachment, absorption.
Example: influenza
Cultivation of Viruses
1. Viruses must be grown in living cells (cell
culture).
Animals and plant host are expensive and not easy
to maintain.
2. The easiest viruses to grow are bateriophages
(Phage = eater of bacteria).
Easy to manipulate bacterial cells and their viruses in
the laboratory
Growing Viruses
1. The plaque method mixes bacteriophages with
host bacteria and nutrient agar.
2. After several viral multiplication cycles, the
bacteria in the area surrounding the original
virus are destroyed; the area of lysis is called
plaque.
3. Each plaque originates with a single viral
particle; the concentration of viruses is given as
plaque-forming units (PFU)
Growth of Animal Viruses in the
Laboratory
1. Cultivation of some animal viruses requires
whole animals
Humans are the test subjects but processes are slow to see
the results
Simian AIDS (1986) and feline AIDS provide models for study
of human AIDS. Genetically engineered mice. SCID –Human
mouse
2. Some animal viruses can be cultivated in
embryonated eggs
Inoculation of an embryonated egg
Growth of Animal Viruses in the
Laboratory
3.
Cell cultures are cells growing in culture media in the laboratory
•
Primary cell lines (few generations) cell lines grow for a short time in vitro.
•
•
Embryonic cell lines (100 generations).
Continuous cell lines can be maintained in vitro indefinitely.
•
Transformed or cancerous cells
a.
HELA 1951
Viral multiplication
Bacteriophages
Multiplication of Bacteriophages
1. During a lytic cycle, a phage causes
the lysis and death of a host cell.
2. Lysogeny. DNA incorporated as a
prophage into the DNA of the host cell
Lytic Cycle
• The multiplication cycle of these phages
can be divided into five distinct stages:
– Attachment
– Penetration
– Biosynthesis
– Maturation
– Release
Lytic Cycle: Attachment
•
During the
attachment phase
of the lytic cycle,
–
–
–
Chance collision
Sites on the phage’s
tail fibers attach to
complementary
receptor sites on the
bacterial cell.
Lytic Cycle: Penetration
• Phage lysozymes opens a portion of the bacterial cell
wall,
• tail sheath contracts to force the tail core through the cell
wall,
• DNA enters the bacterial cell and the capsid remains
outside.
Lytic Cycle: Biosynthesis,
• Phage DNA is replicated
• Phage DNA produces mRNA coding for proteins
necessary for phage multiplication
• capsids and proteins are produced
Lytic Cycle: Maturation
• Phage DNA and
capsids are
assembling into
complete viruses
Lytic Cycle: Release (lysis)
• phage lysozyme
breaks down the
bacterial cell
wall, and the
multiplied
phages are
released
A Bacteriophage one-step growth curve
Vocabulary
• Burst time: The time from phage attachment to
release (AVG 20 to 40 min).
• Burst size: The number of newly synthesized
phages from a single infected cell (50-200).
• Eclipse period The time period when whole
virons can not be found. It is the time from the
end of penetration to the beginning of release.
Lysogeny
• Some viruses (lysogenic phages) do not
always cause lysis and death of the host
cell when they multiply.
• These viruses may incorporate their DNA
into the host cell’s DNA to begin a lyogenic
cycle.
• In lysogeny, the phage remains latent or
inactive
Characteristics of lysogeny
• Lysogenic cells are immune to reinfection
by the same phage.
– Repressor proteins stop transcription of all
other phage genes.
• Host cell may exhibit new properties
(phage conversion)
– Bacteria may acquire new genes from
previously infected cells
• Special transduction
Specialized transduction
Specialized transduction cont
Generalized Transduction
• Information is transported from one bacteria to
another via a phage.
• Bits of host DNA are packaged along with the
phage DNA in the capsid head.
• Unlike specialized transduction the transported
gene does not have to lie adjacent to the
prophage on the host chromosome.
– The host gene is randomly picked up in the cytoplasm
after the chromosome has been degraded.
Animal Viruses
• The multiplication cycle of these phages
can be divided into six distinct stages:
– Attachment
– Penetration
– Uncoating
– Biosynthesis
– Maturation
– Release
Multiplication of a DNA Papovavirus
Viral DNA enters
Cell’s nucleus
Enzymes synthesized
for DNA replication
RNA Picornaviruses
sense strand (+strand) virus
Single, + stranded, Picorna virus
Retrovirus
Things to know for the Exam
Animal virus life cycle