Question

Give a brief account of viruses with respect to their structure and nature of genetic material. Also name four common viral diseases.

Answer 1

Virus (L. poisonous fluid) is a group of ultramicroscopic, non-cellular, highly infectious agents that multiply only intracellularly- inside the living host cells without involving growth or division. Outside the host cells, they are inert particles. They are nucleoproteins having one or more nucleic acid molecules, either DNA or RNA, encased in a protective coat of protein or lipoprotein.

A virus consists of two parts: nucleoid (genome) and capsid. An envelope and few enzymes are present in some cases,

1. Nucleoid: The nucleic acid present in the virus is called nucleoid and it represents the viral chromosome. It is made up of a single molecule of nucleic acid. It may be linear or circular and nucleic acid can be DNA or RNA. It is the infective part of the virus which utilises the metabolic machinery of the host cell for the synthesis and assembly of viral components.
2. Capsid: It is a protein covering genetic material. Capsids have protein subunits called capsomeres. Capsid protects nucleoids from damage from physical and chemical agents.
3. Envelope: It is the outer loose covering present in certain viruses, like animal viruses (e.g., HIV) but rarely present in plant and bacterial viruses. It is made of proteins of viral origin and, lipids and carbohydrates of the host. Outgrowths called spikes may be present. Envelope proteins have subunits called peplomers. A virus without an envelope is a naked virus.
4. Enzymes: Rarely, lysozymes are found in bacteriophages. Reverse transcriptase enzyme (catalyses RNA-to-DNA synthesis) is found in some RNA viruses, like HIV. Some common viral diseases are – influenza, polio, measles, chickenpox, hepatitis, AIDS, bird flu, SARS (Severe Acute Respiratory Syndrome), etc

Answer 2

1. Viruses are sub-microscopic infectious agents that can infect all living organisms. A virus consists of genetic material surrounded by a protein coat. The genetic material may be present in the form of DNA or RNA.
2. Most of the viruses, infecting plants, have single stranded RNA as genetic material. On the other hand, the viruses infecting animals have single or double stranded RNA or double stranded DNA.
3. Bacteriophages or viruses infecting bacteria mostly have double stranded DNA. Their protein coat called capsid is made up of capsomere subunits. These capsomeres are arranged in helical or polyhedral geometric forms.
4. A.I.D.S, small pox, mumps, and influenza are some common examples of viral diseases.