Virus are microscopic organisms. Till date there is no complete understanding of its structure. Chemically, each virus consists of two main parts. Nucleic acid and capsid. However, some virus have an envelope. The structure of virus is discussed.
1. Nucleic Acid: At the center of every virus is an acidic core made up of nucleic acid. It is called nucleoid. It is made up of DNA or RNA. DNA and RNA never coexist in virus. DNA is double-stranded and RNA is single-stranded. Usually animal and bacteriophage virus contain DNA and plant viruses contain RNA. It carries the genetic characteristics of the virus and plays a role in the synthesis of new virus.
2. Capsid: The coat outside the nucleic acid of the virus is called capsid. Numerous protein molecules join together to form the capsid. Each protein molecule is called a capsomer. There are two types of capsomia. Pentamer and Hexamer. A pentamer is composed of a single cluster of 5 and a hexamer of 6 monomers. Protein molecules arrange to form helical helix and spherical polyhedron structures. ΦX174 virus is composed of 12, herpesvirus 162 and TMV 2130–2200 capsomeres. Apart from proteins, the capsid also contains lipids and proteins. The capsid may have thorns or spikes on the outside. The capsid acts as an antigen. It protects the nucleic acid and helps in attacking the host.
3. Envelope: Some virus have a coat outside the capsid. It is called an envelope. It is 10-15 nanometers thick. It is made up of proteins, lipids and carbohydrates. Each structural molecule of the envelope is called a pelpomere. Virus that have an envelope are called lipovirus. Virus that do not have an envelope are called naked virus. The outer layer of the envelope may have spines called spikes.
4. Enzymes: Virus are non-trophic and contain no metabolic enzymes. However, bacteriophage has lysozyme, influenza virus has neurominidase and HIV has reverse transcriptase enzyme. Besides, virus contain enzymes like RNA polymerase, RNA transcriptase etc. Dr. Siddiq Publications