Virus is a small nucleoprotein complex and infectious agent, that replicates only inside the living cells of other organisms such as animals and plants. They are much smaller than bacteria (20-300 nm). They are obligate intracellular parasites of bacteria, protozoa, fungi, algae, plants, and animals. In 1890, D. Ivanovski and M. Beijerinck discovered the Tobacco mosaic virus that caused disease in tobacco plants. Friedrich Loeffler and Paul Frosch have discovered animal viruses that cause foot and mouth disease in cattle. They cannot be grown in a sterile medium but grow in specific host cells. They have only a single type of nucleic acid that serves as genetic material but lacks cellular structure and enzymes for metabolite processes. They are highly species-specific.
Properties of Viruses:
- They do not have cellular organization.
- They contain only one type of nucleic acid either DNA or RNA.
- They are obligate intracellular parasites.
- They lack enzymes for protein synthesis.
- They multiply by complex processes but not by binary fission.
- They are unaffected by antibacterial antibiotics.
Morphology of Viruses:
Viruses are acellular
which means they do not have cells as it has no nucleus, cytoplasm, or
organelles. They consist of two main parts viz. nucleic acid and capsid. In
nucleic acid, DNA or RNA is found at the core of the virus. They need a living
host to survive. The smallest infectious virus is 20 nm to 300 nm. The smallest
RNA virus is a picornavirus, the smallest DNA virus is parvovirus (20 nm) and
the largest virus is poxvirus (300 nm). They may be single or double-stranded.
The nucleic acid is linear or circular and either segmented or non-segmented. A
capsid is a protein coat that is surrounded by genetic material. The protein
coat is made up of repeating sub-units known as capsomeres. Based on the
arrangement of the morphological sub-unit, viruses are grouped into three types
namely Icosahedral symmetry, Helical symmetry, and Complex symmetry.
(a) Icosahedral symmetry: Capsomeres are arranged in
20 triangles (Fig.1), Example: Adenovirus.
(b) Helical symmetry: They are arranged in a hollow
coil that appears rod-shaped (Fig.2), Examples: Influenza virus, Rabies virus,
etc.
(c) Complex symmetry: They have complexity in their
structure of viruses and they do not show icosahedral or helical symmetry.
Example: Pox viruses. Some viruses have an outer envelope that is made of
lipids and proteins covering the capsid. They are parasites. Most viruses are
spherically shaped but some are different types pox virus is brick-shaped, the bacteriophage
is tadpole-shaped, and the tobacco mosaic virus is rod-shaped. They do not have
ribosomes and do not undergo mitosis or binary fission.
Functions of Viral Capsid:
- It protects the genetic materials.
- It serves as an antigenic determinant.
- It induces antibody production.
- It provides the structural symmetry of the virus.
- It facilitates the assembly and packaging of viral genetic information.
- It serves as a vehicle of transmission from one host to another.
Fig.1: Icosahedral symmetry
Fig.2: Tobacco mosaic virus (rod-shaped)
Viral Envelope: It is a lipoprotein derived from the
host cell membrane and virus-specific protein. It is composed of glycoproteins
which look like spikes. It confers instability to the virus because of the loss
of infectivity due to the disruption of lipids. They are more sensitive to
heat, lipid solvents, and detergents.
Functions:
- They help to attach to host cell receptors.
- They are antigenic determinants.
- They stimulate antibody production.
Examples: Herpes virus, HIV virus, Hepatitis virus,
etc.
Virion: It is an entire virus particle, consisting of
an outer protein shell called a capsid and an inner core of nucleic acid. They
are present in naked viruses, identical to nucleo capsid whereas in enveloped
viruses develop envelop.
Peplomers: In mature virus particles, the
glycoprotein often appears as projecting spikes on the outer surface of the
envelope, which are known as peplomers. Example: Influenza virus carries two
types of peplomers viz. hemagglutinin and neuraminidase.
Functions:
- It helps for attachment of the virus to the host cell receptors to initiate the entrance of the virion into the cell.
- It has antigenic properties.
- It has enzymatic activity like neuraminidase which cleaves neuraminic acid from host cell glycoproteins.
- It attaches to the receptors on the RBC and causes these cells to agglutinate.