Q.Discuss the discovery and structural organisation of viruses.

[!TLDR]

The question asks for a full discussion of the discovery of viruses and how they are structurally organised.

[!ANSWER]

Viruses were discovered through Ivanowsky's (1892) and Beijerinck's (1898) work on tobacco mosaic disease, and shown to be a chemical entity when Stanley crystallised TMV (1935); structurally, every virus is a nucleic acid core (DNA or RNA, never both) enclosed by a protein capsid built from capsomeres, arranged with helical, icosahedral or complex symmetry, and sometimes surrounded by a lipid envelope.

The discovery of viruses began with a puzzle involving tobacco plants. In 1892, Dmitri Ivanowsky found that sap from tobacco plants affected by mosaic disease remained infectious even after being passed through a fine porcelain filter capable of trapping ordinary bacteria, showing that the disease agent was smaller than any known bacterium. In 1898, Martinus Beijerinck confirmed this and proposed that the agent was an entirely new kind of infectious entity, describing it as a 'contagium vivum fluidum' - effectively founding virology as a field. The chemical nature of this agent was finally revealed in 1935, when Wendell Stanley succeeded in crystallising it - now known as the Tobacco Mosaic Virus (TMV) - and showed it consisted largely of protein, demonstrating for the first time that a virus could be studied as a definite chemical substance rather than merely inferred from its infectious behaviour.

Structurally, all viruses share the same basic organisation revealed by this work and later research: a core of nucleic acid, carrying the virus's genetic information, enclosed by a protective protein coat called the capsid, which is built from repeating protein subunits called capsomeres. A single virus particle contains only one type of nucleic acid, either DNA or RNA, never both, and this may be single- or double-stranded. The capsomeres of the capsid are arranged in one of several recognised symmetrical patterns: helical symmetry, in which capsomeres spiral around the nucleic acid to give a rigid, rod-shaped particle (as in TMV, whose capsid winds helically around a single strand of RNA); icosahedral (polyhedral) symmetry, in which capsomeres form a roughly spherical, many-faced particle (as seen in many animal viruses); and complex symmetry, seen in bacteriophages such as the T-even phages, which combine an icosahedral head (enclosing double-stranded DNA) with a separately structured helical/cylindrical tail bearing a base plate and tail fibres. Some viruses additionally carry an outer lipid envelope, derived from a previous host cell's membrane, wrapped around the capsid.

[!ANSWER]

Viruses were discovered through Ivanowsky's filtration experiments (1892), Beijerinck's naming of the infectious agent (1898), and Stanley's crystallisation of TMV as a chemical, largely protein substance (1935); structurally, every virus is built from a nucleic acid core (DNA or RNA, never both) enclosed by a capsomere-built protein capsid showing helical, icosahedral or complex symmetry, sometimes with an additional lipid envelope.

• Covering only the historical discovery or only the structural organisation, when the question explicitly asks for both together.
• Mixing up the contributions and dates of Ivanowsky, Beijerinck and Stanley, or omitting one of the three key discoveries.