The components of Viruses

 The components of Viruses

Capsid: 

Capsid is the protein shell enclosing the viral genome. The nucleic acid together with some associated basic proteins forms the core of the virion. The relatively small genomes of the viruses cannot make many proteins. Therefore the viruses cannot afford too many proteins for building the capsid. The capsid of tobacco mosaic virus, for example, contains only a single polypeptide. In the icosahedral viruses, the capsid is made from one to several polypeptides. The polypeptides fold to form the basic structural unit of the capsid, known as a protomer. The protomers may associate with each other to form a capsomer. In icosahedral viruses five protomers form a pentameric capsomer, or six protomers are associated to form a hexameric capsomer. In a single virion, both pentameric and hexameric capsomers are present to give the characteristic shape and size to virion. The number of total capsomers is characteristic for the particular virus

In helical viruses, the capsomers are linked to each other to form a long chain which folds into a helix with a hollow core through which the nucleic acid molecule passes. Each helical virus has a fixed diameter and its length is determined by the size of the nucleic acid. For example, TMV has a diameter 15 to 18 nm and a length of 300 nm. Its genome which is a single - stranded RNA has 6,000 nucleotides. The RNA molecule lies in a groove within the hollow core of the helical capsid consisting of a single kind of protomers. The capsid polypeptide of TMV contains 158 amino acid residues coded by only 474 nucleotides out of the total 6,000 nucleotides present in TMV RNA genome.

The envelope

Nucleocapsid is enclosed by an envelope. The envelope consists of lipids and proteins, and the latter is sometimes covalently linked to carbohydrates. The envelope lipids and carbohydrates generally originate from the cytoplasmic or nuclear membrane of the cell. The envelope proteins are usually specified by the viral genome. Because of the fluidity of the lipids, envelopes are flexible and form a loose covering. The enveloped viruses may, therefore, assume different shapes (pleomorphic).

Depending on the virus, the carbohydrate-protein complexes (glycoproteins) may form projections the outer surface of the envelope. These projections are called spikes or peplomers. The spikes have finite size and they are distributed evenly at definite intervals. The influenza virus has an enveloped virion with spikes. The spikes are 10 nm high and are at a distance of 8 nm from each other. The spikes of influenza virus have enzymatic activities of two kinds. Some spikes having haemagglutinin activity help in attachment to RBCs of host cells causing haemagglutination. Other spikes have neuraminidase activity which probably helps in gaining entry into the host cell. Neuraminidase can cleave sialic acid residues of carbohydrates found on the surface of many types of human cells.

Viral nucleic acids (genome)

The most important constituent of a virus is its nucleic acid. It encodes the genetic information for synthesis of viral proteins. The genome consists of either DNA or RNA. Its size varies widely ranging between only 100 codons in very small viruses and 100,000 codons in large viruses. For example polyoma virus has only 4-5 genes, while λ-phage has 46 genes, herpes virus has 150 genes and smallpox virus has 240 genes.

The nucleic acid, DNA or RNA may be single-stranded (ss) or double-stranded (ds) depending on the virus. On the basis of nucleic acids, viruses can be classified into four categories, viz. ss-RNA viruses, ds-RNA viruses, ss-DNA viruses and ds-DNA viruses. These categories are present in all the three major groups, like animal, plant and bacterial viruses. However, the occurrence of the four categories of genomes in these three groups is not uniform. For example, majority of plant viruses are ss-RNA viruses. Similarly, majority of bacteriophages have ds-DNA genome. Most of the mycophages have ds-RNA. Another important feature of viral nucleic acid  is that they maybe linear or circular.