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

1. Entry into host cells: The virus comes into contact with a suitable host. Pierces the host body. Through the pore, the virus genome enters the host cell. The virus increases the number in the host’s body.
2. Obligate endoparasites: Virus are obligate endoparasites. It resides in living host cells. Virus cannot multiply without a living host. Cannot multiply even in culture medium.
3. Coercing living cells to obey instructions: Virus contain nucleic acid inside the capsid. Virus contain either DNA or RNA as nucleic acid. The nucleic acid of the virus combines with the nucleic acid of the host. Then it creates new virus under the instructions of the virus. Dr. Siddiq Publications
4. Host Specificity: Virus are host specific. Bacteriophages attack bacteria, cyanophages attack cyanobacteria, plant virus attack plants and animal virus attack animals. However, if the virus changes its host, it is called an emerging virus.
5. Attack Site Specificity: Virus are host specific and attack site specific. Hepatitis B and C virus attack liver cells, HIV white blood cells, poliovirus the spinal cord, papillomavirus the genitalia, herpes simplex virus the skin, Epstein Barr virus the nose and pharynx, and T-lymphotrophic virus the white blood cells.
6. Genetic modification: Genetic modification can occur in virus. Mutant virus are a serious threat to the ecosystem. Vaccines or drugs are not effective against the mutated generation. Virus such as flu virus and covid-19. For this reason, every year new antibiotics have to be taken.

1. Phaginae : Virus that attack bacteria are called Phaginae.
2. Phytophaginae : Virus that attack plants are called Phytophaginae.
3. Zoophaginae : Virus that attack animals are called Zoophaginae.

Virus that do not break the host’s body after multiplying are called lysogenic virus. For example, lambda phase.

Viruses that burst out of the host’s body after increasing in number are called lytic virus. Like- T2, T4, T6, T8 etc.

1. Lytic Virus: Viruses that burst out of the host’s body after increasing in number are called lytic virus. Like- T2, T4, T6, T8 etc.
2. Lysogenic Virus: Virus that do not break the host’s body after multiplying are called lysogenic virus. For example, lambda phase.

Virus that attack fungi are called microphages. For example – PsV-S (Penicillium).

Virus that attack cyanobacteria are called cyanophages. In 1963 Safferman and Morris discovered cyanophages. Eg Microphages, LPP1, LPP2 (L=Lyngbya, P=Plectonema, P=Phormidium), AS-1, N-1 etc.

Viruses that reside in the body of bacteria and multiply by destroying bacterial cells are called bacteriophages. Like- T2, T4, T6, T8 etc.

Virus that live in animal bodies are called animal virus. For example, vaccinia, HIV, novel corona virus etc.