VIRUSES
Discovery of viruses
1883-A. Meyer, a
German scientist, demonstrated that TMV (tobacco mosaic disease) was contagious
by spraying sap from infected plants onto healthy plants, which then developed
the disease. He was unable to identify
a microbe that might cause the disease when he examined the sap under the
microscope.
1890- D. Ivanosky, a
Russian scientist, proposed that TMV disease was caused by a bacterium that was
either too small to be trapped by a filter or that produced a filterable toxin. Filtered sap to remove bacteria. Filtered
sap still transmitted disease.
1897-Martinus Beijerinck,
a Dutch microbiologist, proposed that the disease was caused by a reproducing
particle much smaller and simpler than bacteria. He infected a series of plants with sap over several
generations. He concluded that the
pathogen must have been reproducing because its ability to infect was undiluted
by transfers from plant to plant. He
also noted that unlike bacteria, the pathogen 1) reproduced only within the
host it infected. 2) Could not be
cultured on media 3) could not be killed by alcohol.
1935-Wendell M. Stanley,
an American biologist, crystallized the infectious particle now known as
tobacco mosaic virus.
Composition of Viruses
In the 1950’s, the electron microscope allowed scientists to view the structure of viruses.
Viruses
may range from 20-to100 nm.
1)
Viral
Genomes
Depending upon the virus, viral genomes:
a)
May
be double-stranded DNA, single-stranded DNA, double stranded RNA or Single
stranded RNA.
b)
Are
organized as single nucleic acid molecules that are either linear or circular.
c)
May
have as few as four genes or as many as several thousand.
2)
Capsids
and Envelopes
a)
Capsid-
Protein coat that encloses the viral genome.
1)
Its
structure may be rod-shaped, polyhedral or complex.
2)
Composed
of many capsomeres-protein subunits made from only one or a few types of
proteins.
b)
Envelope-Membrane
that cloaks some viral capsids.
1)
Helps
viruses infect their host.
2)
Derived
from host cell membrane, which is usually virus-modified and contains proteins
and glycoproteins of viral origin.
Viruses
are obligate intracellular parasites, which can express their genes and
reproduce only within a living cell.
Each virus has a specific host range.
Host
Range- Limited number or range of host’s cells that a parasite can infect.
1)
Viruses
recognize host cells by a complimentary fit between viral proteins and specific
cell surface sites.
2)
Some
viruses have broad host ranges, which may include several species ( ex. Rabies).
3)
Some
viruses have host ranges so narrow that they can:
a)
infect
only one species.
b)
Infect
only a single tissue type of one species. AIDS-T lymphocytes and macrophages.
Viral
Life Cycle involves:
1)
Infecting
the host cell with the viral genome.
2)
Coopting
host cell’s resources to replicate the viral genome and manufacture capsid
protein.
3)
Assembling
newly produced viral nucleic acid and capsomeres into the next generation.
Methods
of Infection of Host cells:
1)
Tailpiece
to inject Nucleic acid into the host cell.
2)
Attach to membrane proteins to form portal.
Patterns
of Viral Genome Replication:
1)
DNA—DNA. If viral DNA is double-stranded, DNA
replication resembles that of cellular DNA, and the virus uses DNA polymerase
produced by the host.
2)
RNA—RNA. Since host cells lack the enzyme to copy
RNA, most RNA viruses contain a gene that codes for RNA replicase, an enzyme
that uses viral RNA as a template to produce complimentary RNA.
3)
RNA—DNA. Some RNA viruses encode reverse
transcriptase, an enzyme that transcribes DNA from an RNA template.
Virus
cells use the host cell’s enzymes, ribosome’s, tRNA’s, amino acids, ATP, and
other resources to make copies of the viral genome and produce viral capsid
proteins.
Reproductive
Cycles of Bacteriophages:
I.
Lytic
Cycle-A viral replication cycle that results in the death or lysis of the host
cell.
1)
Phage
attaches to cell surface.
2)
Phage
contracts sheath and injects nucleic acid.
3)
Hydrolytic
enzymes destroy host cell’s DNA.
4)
Phage
genome directs the host cell to produce phage components-nucleic acid and
capsid proteins.
5)
Cell
lyses releases phage particles.
II.
Lysogenic
cycle-A viral replication cycle that involves the incorporation of the viral
genome into the host cell genome.
1)
Phage
binds to surface of cell.
2)
Phage
injects it nucleic acid into the host cell.
3)
Viral
nucleic acid forms a circle and either begins a lytic cycle or a lysogenic
cycle.
4)
During
a lysogenic cycle the viral nucleic acid inserts by genetic recombination into
a specific site on the hosts chromosome and becomes a prophage.
Prophage- a phage genome that is incorporated into a
specific site on the bacterial chromosome.
a)
Most
phage genes are inactive.
b)
Prophage
genes are copied along with cellular DNA when the host cell reproduces. As the cell divides, both prophage and
cellular DNA are passed on to daughter cells.
c)
A
prophage may be carried in the host’s chromosomes for many years (latent
period).
5)
Occasionally
a prophage may leave the chromosome.
a)
This
may be spontaneous or caused by environmental factors such as radiation.
b)
The
excision process may begin the phages lytic reproductive cycle.
c)
Virions
produced during the lytic cycle may begin either a lytic or lysogenic cycle in
their new host cells.
6) Some
prophage genes in a lysogenic cell may be
expressed
and change the cell’s phenotype in a
process
called lysogenic conversion. This
occurs in
bacteria that cause diphtheria, botulism, and scarlet
fever. Pathogenicity results from toxins coded for
by
prophage genes.
REPRODUCTIVE
CYCLE OF ANIMAL VIRUSES:
I. Viruses with envelopes-some viruses are
surrounded by a membranous envelope, which is unique to several groups of animal viruses. This envelope is:
1)
Outside
the capsid and helps the virus enter the host cell.
2)
A
lipid bilayer with glycoprotein spikes protruding from the outer surface.
A.
Replication
cycle characterized by:
1)
Attachment-Glycoprotein
spikes protruding from the viral envelope attach to receptor sites on the hosts
plasma membrane.
2)
Entry-As
the envelope fuses with the plasma membrane, the entire virus (capsid and
genome) is transported into the cytoplasm by receptor-mediated endocytosis.
3)
Uncoating-
Cellular enzymes uncoat the genome by removing the protein capsid from viral
RNA.
4)
Viral
RNA and Protein Synthesis;
a)
Some
viral RNA polymerase is packaged in the virion.
b)
Viral
RNA polymerase(transcriptase) replicates the viral genome and transcribes viral
mRNA.
c)
Viral
mRNA is translated into viral proteins including:
Capsid proteins synthesized in free
ribosome’s.
Viral
envelope glycoproteins synthesized
By ribosomes bound the
the ER. Sent to
Golgi
Apparatus for processing.Golgi
Vesicle
transport glycoproteins to the
Plasma membrane, where they
cluster
viral
exit sites.
5) Assembly and release-New capsids surround
viral genomes. Assembled virions envelop with
the host plasma membrane as
they bud off
from the cells surface.
Some
envelopes such as those of the herpesvirus,
a DNA virus, contain
envelopes derived from the
hosts cell’s nuclear membrane. These reproduce
within the host
cell’s nucleus. They may
integrate their DNA into
the cell’s genome
as a provirus.
III.RNA Viruses:
Retroviruses-use reverse transcriptase to transcribe
DNA from the viral RNA genome. Most
complicated reproductive cycle.
1)
Attachment
and entry of the virion.
2)
Uncoating
of single stranded RNA genome.
3)
Reverse
transcription-Viral RNA is the template to produce DNA-the template for complementary
DNA strand.
4)
Integration-Newly
produced double-stranded viral DNA enters the nucleus. Viral DNA inserts into chromosomal DNA and
becomes a provirus (latent period).
5)
Viral
RNA and protein synthesis-Proviral DNA is transcribed into mRNA and is translated
into proteins. Transcribed RNA may
provide genomes for the next viral generation.
6)
Capsid
assembly and release of new virions.
EVOLUTION
OF VIRUSES:
Viruses are obligate intracellular parasites and cannot reproduce independently but they have a genome with the same genetic code a living organisms and they can mutate and evolve.
Viruses
probably evolved after the first cells, from fragments of cellular nucleic acid
that were mobile genetic elements (escaped genes).
Evidence:
1.
Genetic
material of different viral families is more similar to host genomes than to
that of other viral families.
2.
Some
viral genes are identical to cellular genes.
3.
Viruses
of eukaryotes are more similar in genomic structure to their cellular hosts
than to bacterial viruses.
4.
Viral
genomes are similar to certain cellular genetic elements such as plasmids and
transposons, all are mobile genetic elements.
VIRAL
DISEASES:
Simplex II-genital sores
Varicella zoster- chicken pox, shingles
Epstein Barr
virus-mononucleosis
Poxvirus-Small
pox, vaccinia, cowpox
Picornovirus-Poliovirus,rhinovirus
(common cold)
Togovirus-Rubella
virus,yellow fever virus
Rhabdovirus-rabies
Paramyoxovirus-measles,
mumps
Othomyxovirus-influenza
ANTIVIRAL
DRUGS