Q.Describe the structure, genetic material and mode of replication of viruses with reference to the tobacco mosaic virus and a bacteriophage, and clearly distinguish viruses from viroids and from prions.

Prions, infectious protein-only particles with no nucleic acid, were discovered by Stanley B. Prusiner.

When classifying life forms, biologists encounter a few entities that blur the line between living and non-living. Viruses have nucleic acid wrapped in protein; viroids (discovered by T.O. Diener) are even simpler — free RNA with no protein coat. Prions go a step further: they are purely proteinaceous infectious particles with NO nucleic acid at all. A prion is a misfolded version of a normal cellular protein; on contact, it converts the normal protein into the same misfolded, disease-causing form, allowing the abnormality to 'spread' without any genetic material being involved. This unusual mechanism causes neurodegenerative diseases such as scrapie in sheep, mad cow disease (BSE), and Creutzfeldt-Jakob disease in humans. Stanley B. Prusiner proposed and established this protein-only in …

Viruses that infect bacteria are called bacteriophages (literally 'bacteria eaters').

Viruses are named according to the host they infect. A bacteriophage (often shortened to 'phage') attaches to the surface of a bacterial cell, injects its nucleic acid into it, and uses the bacterium's own machinery to replicate, eventually causing the bacterial cell to burst (lysis) and release new viral particles. The T-phages (e.g., T4 bacteriophage, which infects E. coli) are classic examples studied in molecular biology.

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Viruses are non-cellular infectious agents whose defining feature is a genome of either DNA OR RNA (never both), packaged in a protein coat; they lack their own metabolism, enzymes, and are obligate (not facultative) parasites.

Examine each option against what is actually known about viruses:

1. "Possess their own metabolic system" — FALSE. Viruses have no ribosomes, no enzymes for respiration or protein synthesis, and no cytoplasm. They are completely dependent on the host cell's metabolic machinery to replicate.
2. "They are facultative parasites" — FALSE. A facultative parasite CAN survive and multiply outside a host if needed. Viruses cannot multiply outside a living host cell at all — they are OBLIGATE intracellular parasites.
3. "They contain DNA or RNA" — TRUE. Every virus has a nucleic acid genome that is EITHER DNA or RNA (single- or double-stranded), never both together in the same virion. This is one of the defining criteria used to classify viruses (e.g. Baltimore classification). …

Viruses are called a 'biological puzzle' because they show no signs of life (cannot metabolise, grow or reproduce) outside a living host cell, yet display full properties of life once inside one.

Among the four groups listed, only viruses sit right on the border between living and non-living:

• Outside a host — a virus is essentially an inert particle (a nucleic acid core wrapped in a protein coat); it can be crystallised like a chemical substance (as famously shown for Tobacco Mosaic Virus by W. M. Stanley), shows no metabolism, no growth, and cannot reproduce on its own.
• Inside a host cell — the same virus behaves entirely like a living entity: it hijacks the host's machinery to replicate its genetic material, produce new virus particles, and undergoes mutation/evolution. …

Vaccine development against SARS is difficult mainly because the virus's RNA genome mutates rapidly, continuously changing its surface antigens (antigenic variation), so immunity built against one strain may not protect against the next.

Vaccines work by training the immune system to recognize specific viral surface proteins (antigens). RNA viruses like the SARS coronavirus replicate using an RNA-dependent RNA polymerase that lacks efficient proofreading, so errors (mutations) accumulate much faster than in DNA-based organisms. This continuous change in the virus's surface antigens (antigenic drift) means antibodies raised against an earlier form of the virus may fail to recognize a later, mutated form - making a single, durable vaccine much harder to design.

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Viruses are termed a biological puzzle/paradox because outside a host they behave like non-living chemical crystals, while inside a host cell they replicate and behave like living organisms.

A virus is an obligate intracellular parasite made of a nucleic acid core (DNA or RNA) enclosed in a protein coat (capsid). Outside a host cell, a virus is metabolically inert - it shows no respiration, growth, or response to stimuli, and some viruses (like Tobacco Mosaic Virus, first crystallized by W. M. Stanley) can even be crystallized like a chemical substance. Yet once inside a living host cell, the virus hijacks the host's machinery to replicate, mutate, and behave exactly like a living entity. This dual, borderline nature - neither fully living nor fully non-living - is why viruses are described as a …