Virus
Viruses are the smallest
known infective agents and the simplest form of life known. They do not possess
any cellular organization and they do not fall into the category of unicellular
microorganisms.
Properties
of Viruses
1. Viruses do not have a cellular
organization.
2. They contain only one type of nucleic
acid, either DNA or RNA but never both.
3. They are obligate intracellular
parasites.
4. They lack the enzymes necessary for
protein and nucleic acid synthesis and are dependent for replication on the
synthetic machinery of host cells.
Morphology
of viruses
Size: Viruses are much smaller than
bacteria. Their small size and ‘filterability’ (ability to pass through filters
that can hold back bacteria) led to their recognition as a separate class of
infectious agents. They were termed as filterable agents upon discovery.
Viruses vary widely in size from 20 nm to
300 nm. The largest among them is pox virus (300 nm) and is as large as the
smallest bacteria (mycoplasma). The
Smallest viruses are the parvovirus (about 20 nm).
They are too small to be seen under the
light microscope. Some of the larger viruses, such as poxviruses can be seen
under the light microscope when suitably stained.
Shape:
Shape of virus varies with different
groups of viruses. Most of the animal viruses are roughly spherical, some are
irregular and pleomorphic. Poxviruses are brick-shaped, rabies virus is
bullet-shaped, tobacco mosaic virus is rod-shaped. Bacteriophages have a
complex morphology.
They consist of nucleic acid core
surrounded by a protein coat called capsid. The capsid is composed of a large
number of capsomers which are polypeptide molecules arranged
symmetrically. The capsid with the
enclosed nucleic acid is known as nucleocapsid.
Functions of Capsid
i. It protects the viral genome from
physical destruction and enzymatic inactivation.
ii. It provides binding sites for the
virus to attach to specific receptor sites on the host cell.
iii. It facilitates the assembly and
packaging of virus after replication.
iv. It serves as a vehicle of transmission
from one host to another.
v. It is antigenic and specific for each
virus type and is of importance in the host’s defence to virus infection.
vii. It provides the structural symmetry
to the virus particle.
Viral architecture is grouped into three based
on the arrangement of morphologic subunits:
(1) Icosahedral symmetry (2) Helical
symmetry (3) Complex structures.
An icosahedral (icosa, meaning 20 in
Greek) is a polygon with 12 vertices or corners and 20 facets or sides. Each
facet is in the shape of an equilateral triangle. Two types of capsomers
constitute the icosahedral capsid. They are the pentagonal capsomers at the
vertices (pentons) and the hexagonal capsomers making up the facets
(hexons). Example - Adenovirus
2. Helical Symmetry
The nucleic acid and the capsomers are
wound together in the form of a helix or spiral. Examples: Tobacco Mosaic Virus and rabies.
3. Complex Symmetry
They do not have either icosahedral or
helical symmetry due to complexity of their capsid structure. Example is Poxviruses. Some bacteriophage such as T4 phage have icosahedral
head and helical tail and thus have complex symmetry.
Enveloped Virus : Virus may be enveloped
or nonenveloped (naked). In enveloped Virus, the envelope or outer covering of
virus is derived from the plasma membrane of the host cell during their
replication. The envelope is lipoprotein in nature. The lipid is of host cell
origin while the protein is virus encoded.
Enveloped viruses are susceptible to the
action of lipid solvents such as ether, chloroform and detergents, whereas most
viruses existing as naked capsids are more likely to be resistant to them.
In virus particle, the glycoproteins appear
as projecting spikes on the outer surface of the envelope. These are known as peplomers (from peplos,
meaning envelope). A virus may have more than one type of peplomers, e.g., the
influenza virus carries two kinds of peplomers, the hemagglutinin which is a
triangular spike and the neuraminidase which is a mushroom shaped structure.
Envelope confer chemical, antigenic and biological properties on viruses.
Functions of Peplomers
i. Many peplomers mediate attachment of
the virus to the host-cell receptors to initiate the entrance of the virion
into the cell.
ii. Some viral glycoproteins attach to
receptors on red blood cells, causing these cells to agglutinate
(hemagglutination).
iii. Some possess enzymatic activity like
neuraminidase which cleave neuraminic acid from host cell glycoproteins.
iv. Glycoproteins are major antigens
Viral Nucleic Acids
Viruses contain a single kind of nucleic
acid—either DNA or RNA—that encodes the genetic information necessary for
replication of the virus. The genome may be single-stranded or double-stranded,
circular or linear, and segmented or nonsegmented. The type, structure and size
of nucleic acid are used for classifying viruses.
Viral
replication
The genetic information necessary for
viral replication is in the viral nucleic acid but since virus lack
biosynthetic enzymes they depend on the host cell for replication. The viral replication
/ multiplication cycle can be divided into six sequential phases
1. Adsorption
or attachment
2. Penetration
3. Uncoating
4. Biosynthesis/Replication
5. Maturation
6. Release
1. Adsorption or Attachment
Virus come in contact with host cells by
random collision followed by specific adsorption or attachment. This is mediated by the binding of virion
surface structures, known as ligands, to receptors on cell surface. In case of
influenza virus, hemagglutinin (a surface glycoprotein) binds specifically to
sialic acid residue of glycoprotein receptor sites on the surface of
respiratory epithelium. In case of human immunodeficiency virus, surface glycoprotein
gp 120 acts as a ligand which binds to the CD4 glycoprotein on the surface of
mature T lymphocytes. Rabies virus binds to the acetylcholine receptor found on
neural cells.
2. Penetration
After adsorption, the virus particle is
taken up inside the cell.
Some viruses accomplish this by receptor mediated
endocytosis or viropexis. Here uptake of
the ingested virus particles within endosomes occurs. Most nonenveloped viruses
enter the cell by viropexis.
Enveloped viruses fuse their membranes
with host cell membranes and deliver the nucleocapsid or genome directly into
the host cytoplasm.
3. Uncoating
This is the stripping of virus of its
capsid so that the nucleic acid is released into the cell. Uncoating is
effected by the action of host cell lysosomal enzymes. Genome of most RNA
viruses remain in the cytoplasm, while genome of DNA viruses, except
poxviruses, are delivered to the nucleus.
4. Biosynthesis/Replication
This phase includes synthesis of the viral
nucleic acid, capsid protein and enzymes necessary in the various stages of
viral synthesis, assembly and release. In addition to these, some ‘regulator
proteins’ are also synthesized which shut down the normal cellular metabolism
and direct the production of viral components.
Most DNA viruses synthesize their nucleic
acid in the host cell nucleus. The exception is the poxviruses, which
synthesize the components in the host cell cytoplasm.
Most RNA viruses synthesize their
components in the cytoplasm, except orthomyxoviruses, some paramyxoviruses and
retroviruses which synthesise few of the components in the nucleus.
Viral proteins are synthesised in the
cytoplasm.
Steps of Biosynthesis
i. Transcription of messenger RNA (mRNA)
from the viral nucleic acid.
ii. Translation of the mRNA into ‘early
proteins’. These are enzymes required for the synthesis of virus components and
to induce shutdown of host protein and host nucleic acid synthesis.
iii. Replication of viral nucleic acid.
iv. Synthesis of ‘late’ or structural
proteins, which are the components of daughter virion capsids.
5. Maturation
Assembly of the various viral components
into virions occur during maturation.
This may take place either in the nucleus (herpes and adenoviruses) or
cytoplasm (picorna and poxviruses).
In case of enveloped viruses, the
envelopes are derived from the host cell nuclear membrane (herpes virus) or
host cell plasma membrane (orthomyxoviruses and paramyxoviruses).
6. Release
Viruses are released from cells after
lysis of the cell or by exocytosis or by budding from the plasma membrane.
Viruses that exist as nucleocapsids are released
by the lysis of the host cell (polioviruses) or they may be extruded by a
process known as reverse phagocytosis.
Release of enveloped viruses occurs as
budding from the plasma membrane without killing the cell.
Eclipse
phase
During viral multiplication the virus
cannot be demonstrated inside the host cell from the stage of penetration till
the appearance of mature virions. This period is known as the ‘eclipse phase’.
The time taken for a single cycle of replication is about 15-30 minutes for
bacteriophages and about 15-30 hours for animal viruses.
Abnormal
replicative cycles
1. Incomplete Viruses
A proportion of daughter virions that are
produced may not be infective due to defective assembly. Such ‘incomplete viruses’ are seen in large numbers
when cells are infected with a high dose of influenza virus. The virus yield will have a high
hemagglutinin titer but low infectivity. This is known as the von Magnus phenomenon.
2. Abortive Infections
Abortive infections fail to produce
infectious progeny, either because the cell may be nonpermissive and unable to
support the expression of all viral genes or because the infecting virus may be
defective due to lack of some functional viral genes.
3. Latent Infection
During a latent infection, there will be the
persistence of viral genomes and expression of none or a few viral genes, but
the infected cell continue to survive.
4. Defective Viruses
Viruses which are genetically deficient
and incapable of producing infectious daughter virions without the helper
activity of another virus are known as’ defective viruses’. Progeny virions will
be formed only if the cells are simultaneously infected with a helper virus,
which is a normal virus.
Example
i. Hepatitis D virus and adeno-associated
satellite viruses which replicate only in the presence of their helper
viruses—hepatitis B and adenoviruses respectively.
ii. Rous sarcoma virus (RSV) cannot code
for the synthesis of the viral envelope in the absence of its helper virus
(avian leucosis virus). Infectious progeny
of RSV results only if the helper virus contribute to the synthesis of the
envelope.
Baltimore
classification of viruses based on replication mechanisms
Viruses have been categorized into six
classes by Baltimore (1970) based on their replication mechanisms.
|
Sl.
No |
Class |
Feature |
|
|
Single
stranded DNA viruses |
DNA
molecule moves into the host cell nucleus and is converted into the duplex
form. Transcription is achieved by host enzymes. Example - Parvovirus |
|
|
Double
stranded DNA viruses |
DNA
enters the host cell nucleus and uses the host cell enzymes for transcription Example
- Hepadnaviruses, Poxviruses |
|
|
Single
stranded RNA viruses Depending
on the method of mRNA transcription, these are classified into two categories |
Positive
strand (plus strand, positive sense): The viral RNA acts as the mRNA. Viral
RNA is infectious by itself and is translated directly into viral proteins in
the host cell cytoplasm. Example
-Picorna, Togaviruses. |
|
|
The
negative strand (minus sense) RNA viruses: The RNA is ‘antisense’, with
polarity opposite to mRNA. They possess their own RNA polymerases for mRNA
transcription Example
– Rhabdo virus, Orthomyxo virus, Paramyxovirus |
|
|
|
Double
stranded RNA viruses |
The
DS RNA is transcribed to mRNA by viral polymerases Example
- Reoviruses |
|
|
Retrovirus |
SS
RNA genome is converted into an RNA: DNA hybrid by the viral reverse
transcriptase enzyme. Double stranded DNA is then synthesized from the RNA: DNA
hybrid. The double stranded DNA form of the virus (provirus) is integrated
into the host cell chromosome. This integration may lead to transformation of
the cell and development of neoplasia. |
No comments:
Post a Comment