Chemical composition of virus | capsid, viral symmetry, viral envelop, viral nucleic acid

 

Chemical composition of virus | capsid, viral symmetry, viral envelop, viral nucleic acid

Chemical composition of virus

Viruses is composed of nucleic acid core surrounded by a protein called capsid. Capsid consist of large number of capsomere which is composed of polypeptide molecules. The capsid with an enclosed nucleic acid is called nucleocapsid.

Functions of capsid

• ·         It provides protection to the viral genome from enzymatic inactivation by nucleases in biological material and physical destruction.
• ·         The binding site which enables the virus to attach to specific site on the host cell is provided by capsid.
• ·         Capsid facilitates the assembly and packaging of viral genetic information.
• ·         It acts as a vehicle of transmission from host to another.
• ·         Capsid is antigenic and specific for each virus.
• ·         It gives the structural symmetry to the virus particle.

Viral symmetry

Viral architecture can be categorized into three types on the basis of the arrangement of morphological subunits.

1. Icosahedral Symmetry

An icosahedral (meaning of icosa is 20 in Greek) is a polygon having 12 vertices or corners and 20 facets or sides. The shape of each facet is an equilateral triangle. Pentagonal capsomeres at the vertices (pentons) and hexagonal capsomeres forms the facets (hexon). Example is Adeno viruses.

2. Helical Symmetry

The capsomeres and nucleic acid are wound together in the form of helix or spiral. Examples are influenza virus, parainfluenza virus, and rabies virus etc.

3. Complex Symmetry 

Viruses that do not have either icosahedral or helical symmetry because of complexity of their structure are referred to have complex symmetry. For example, pox viruses.

Viral Envelop

Virions can be enveloped and nonenveloped (naked).

1. Enveloped Virus

The virus envelope containing lipid is obtained from the plasma membrane of the host cell during the release from the cell surface by budding. Envelope is glycoprotein in nature.

Enveloped viruses are susceptible to the action of lipid solvent such as ether, detergent, and chloroform. For example, herpes virus, Hepatitis B virus, HIV virus

2. Non enveloped virus

Viruses which do not contain any outer covering. They (naked viruses) are more likely to be resistant to lipid solvents like chloroform, ether and detergent.

Peplomers

The glycoproteins mostly appear as projecting spikes on the outer surface of envelope which are called peplomers in mature virus particle.

A virus can have more than one type of peplomers. For example, the influenza virus has two types of peplomers, the hemagglutinin and neuraminidase.

Viral nucleic acids

Viruses contain a single type of nucleic acid either DNA or RNA which encodes the genetic information essential for replication of the virus.

The genome may be single stranded or double stranded, segmented or non-segmented, circular or linear. Viruses can be classified into DNA viruses and RNA viruses according to nucleic acid present.