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

Viruses are non-cellular organisms with an inert crystalline structure outside the living cell; they are obligate parasites. A virus is a nucleoprotein — genetic material surrounded by a protein coat (capsid) made of subunits called capsomeres, arranged in helical or polyhedral forms. The genetic material is either RNA or DNA (never both): plant viruses usually have single-stranded RNA, animal viruses have single- or double-stranded RNA or double-stranded DNA, and bacteriophages are usually double-stranded DNA viruses. Four common viral diseases: **mumps, small pox, herpes and influenz …

Structure and nature of viruses

• Viruses are non-cellular organisms characterised by an inert crystalline structure outside the living cell. W.M. Stanley (1935) showed they could be crystallised.
• Once they infect a cell, they take over its machinery to replicate themselves, killing the host; hence they are obligate parasites.
• A virus is a nucleoprotein — its protein coat, called the capsid, is made of small subunits called capsomeres, which protect the nucleic acid.
• These capsomeres are arranged in helical or polyhedral geometric forms (e.g., the helical Tobacco Mosaic Virus and the polyhedral-headed bacteriophage).

Nature of genetic material

• In addition to proteins, a virus contains genetic material that is either RNA or DNA — no virus contains both RNA and DNA.
• In general: plant-infecting viruses have single-stranded RNA; animal-infecting viruses have single- or double-stranded RNA or double-stranded DNA; and bacteriophages (bacterial viruses) are usually double-stranded DNA viruses.
• The genetic material is infectious. …