Biology · Biological Classification · NEET
They are BOTH, depending on where they are. Outside a host cell a virus is non-living (an inert, crystal-like particle). Inside a living host cell it behaves as living because it can reproduce. Because it fits neither box fully, NCERT places viruses on the BORDERLINE between living and non-living. For a NEET one-mark match, the safe answer is: viruses are non-cellular organisms with an inert crystalline structure outside the host cell.
NCERT says something is truly 'living' only if it has a CELL structure. Viruses are non-cellular — they have no cell, no cytoplasm, no ribosomes, no metabolism of their own. Outside a host they can even be crystallised (turned into crystals), and crystals are made mostly of protein. Non-living things can be crystallised; a normal living cell cannot. That is why, on their own, viruses are treated as non-living.
Inside a living host cell the virus uses the host's machinery (ribosomes, enzymes, raw materials) to make copies of itself. This means it can REPRODUCE and multiply, which is a property of living things. It also carries genetic material (RNA or DNA) that can mutate — another living property. So the SAME virus that was a dead crystal outside becomes 'alive' the moment it enters a host.
Obligate means 'compulsory / no choice'. A virus is an obligate parasite because it CANNOT reproduce on its own — it MUST be inside a living host cell to multiply. This is the key reason for the borderline nature: it has the living property (reproduction) but only by depending completely on a host, never independently.
The NUCLEIC ACID (the genetic material — RNA or DNA) is the infectious part, NOT the protein coat. The protein coat (capsid, made of capsomeres) only protects the genetic material. NEET repeatedly traps students here: 'infective constituent is the protein coat' is a WRONG statement. Remember: the genetic material is infectious.
Virus = nucleic acid (RNA or DNA) WITH a protein coat; borderline living/non-living. Viroid = FREE RNA of low molecular weight, NO protein coat (causes plant diseases like potato spindle tuber disease). Prion = abnormally folded PROTEIN, no nucleic acid (causes mad cow disease / BSE and CJD). NEET mixes these three every year, so keep them separate.
Which of the following statements is incorrect?
Mad cow disease in cattle is caused by an organism which has:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT says viruses are not truly 'living' if we understand living as organisms that have a cell structure. Viruses are non-cellular organisms characterised by an inert crystalline structure outside the living cell.
No. A normal living cell cannot be turned into a crystal. Viruses can be crystallised because they are mostly nucleoprotein, and this crystal-forming ability is a strong reason to treat them as non-living when outside a host.
No. Viruses have no metabolic machinery of their own — no ribosomes, no respiration, no way to make energy. They fully depend on the host cell's machinery, which is why they cannot live independently.
A virus has either RNA or DNA, never both. Plant viruses usually have single-stranded RNA; animal viruses have RNA or double-stranded DNA; bacteriophages are usually double-stranded DNA. This nucleic acid is the infectious part.
Biological Classification asks 1-2 marks on viruses almost every year, usually as 'which statement is correct/incorrect' mixing virus, viroid and prion. Knowing the living/non-living boundary and that the nucleic acid is infectious protects easy marks.
