Biology · Biological Classification · NEET
A virus has ONLY ONE type of nucleic acid - either DNA or RNA, never both together. This is a key point NEET tests. A general rule from NCERT: plant viruses usually have single-stranded RNA (ssRNA), bacteriophages (viruses that attack bacteria) usually have double-stranded DNA (dsDNA), and animal viruses can have ssRNA, dsRNA or dsDNA. So the correct answer to 'DNA or RNA?' is 'one of the two, depending on the virus'.
The capsid is the protein coat that covers the genetic material of a virus. It is made of many small protein sub-units called capsomeres. These capsomeres are arranged in a repeating pattern (helical or polyhedral). Remember: capsid = whole coat, capsomere = one small protein brick of that coat. The capsid protects the nucleic acid inside.
The GENETIC MATERIAL (nucleic acid) is the infectious part, NOT the protein coat. This is a very common NEET trap. The protein coat only protects and helps the virus attach to a host. Once inside the host, it is the nucleic acid that takes over the host machinery and makes new viruses. So if an option says 'the infective constituent in viruses is the protein coat', that statement is WRONG.
Most plant viruses have single-stranded RNA (ssRNA). The classic example is Tobacco Mosaic Virus (TMV), the first virus ever studied (mosaic disease of tobacco). In contrast, most bacteriophages have double-stranded DNA. NEET often asks you to match the virus type with its nucleic acid, so memorise: plant virus -> ssRNA, phage -> dsDNA.
Three names matter for NEET. Dmitri Ivanowsky (1892) found the cause of tobacco mosaic disease was smaller than bacteria (it passed through bacteria-proof filters). M.W. Beijerinck (1898) called it a 'virus' and named the fluid Contagium vivum fluidum (infectious living fluid). W.M. Stanley (1935) showed viruses could be crystallised and the crystals are largely protein. Memory order: Ivanowsky -> Beijerinck -> Stanley (I-B-S).
'Obligate parasite' means a virus can only reproduce inside a living host cell - it cannot multiply on its own. Outside a host, a virus is inert (inactive) and can even exist as a crystal. Once it infects a cell, it hijacks the host's machinery to copy itself, which usually kills the host cell. This is why viruses sit on the border between living and non-living.
Which of the following statements is incorrect?
Statement I: RNA mutates at a faster rate. Statement II: Viruses having RNA genome and shorter life span mutate and evolve faster. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. A single virus has only one kind of nucleic acid - either DNA or RNA, never both at the same time. This single fact is enough to reject any option that says a virus contains both.
A simple virus has a central genetic material (RNA or DNA) surrounded by a protein coat called the capsid, which is built from protein sub-units called capsomeres. Some animal viruses also have an outer lipid envelope. Together the nucleic acid plus capsid is called the nucleocapsid.
Viruses have no cell structure - no cytoplasm, no cell membrane of their own, no organelles, and no ribosomes. Outside a host they exist as inert crystals. Because they lack a cell, they are called non-cellular (acellular) and were left out of Whittaker's five kingdoms.
The capsid is the entire protein coat of the virus. A capsomere is one small protein sub-unit; many capsomeres join together to build the capsid. Think of the capsid as a wall and capsomeres as the individual bricks.
Most bacteriophages (viruses that infect bacteria) have double-stranded DNA (dsDNA). This contrasts with plant viruses, which usually have single-stranded RNA. NEET likes to test this matching.
