Viruses: Structure and Types Explained for Students

A virus is an infectious particle of genetic material wrapped in a protein coat. Viruses are acellular: no cells, no cytoplasm, no machinery to make energy or proteins. They can only reproduce inside the living cells of a host, which is why they are called obligate intracellular parasites.

Viruses matter because they cause many important diseases of humans, animals and plants, from the common cold and influenza to tobacco mosaic disease of plants. Studying viruses also revealed how genes work, and modified viruses are now used in vaccines and gene therapy.

Key Concepts

Every virus has nucleic acid (DNA or RNA, single- or double-stranded) and a capsid, a protein shell of repeating capsomers protecting it. Together they form the nucleocapsid. Some viruses, like influenza and HIV, also carry an envelope, a lipid membrane with protein spikes for attachment.

• DNA viruses: e.g. adenovirus, herpesvirus, poxvirus (double-stranded DNA); parvovirus (single-stranded DNA).
• RNA viruses: e.g. tobacco mosaic virus, influenza virus, coronavirus (single-stranded RNA); reovirus (double-stranded RNA).
• Retrovirus: an RNA virus, like HIV, that uses the enzyme reverse transcriptase to make DNA from its RNA inside the host cell.
• Shapes: helical or rod-shaped (TMV), icosahedral or roughly spherical (adenovirus, poliovirus), complex (bacteriophage with head and tail).
• Size: most viruses are 20 to 300 nanometres; parvovirus at about 20 nm is among the smallest, poxvirus at about 300 nm among the largest.

How It Works

A virus infects a cell in a fixed sequence of steps. First, the virus attaches to specific receptor molecules on the host cell surface, which limits each virus to certain cell types. Second, it enters the cell and releases its nucleic acid. Third, it hijacks host ribosomes to replicate its genome and synthesize viral proteins. Fourth, new particles assemble from the copied parts. Finally, the new viruses are released, to infect neighbouring cells.

Worked Example

Viruses are far smaller than bacteria. An E. coli bacterium is about 2 micrometres long, and tobacco mosaic virus (TMV) is a rod about 300 nanometres long. How many TMV particles placed end to end would span one E. coli cell?

1. Convert the bacterium's length to nanometres: 2 micrometres = 2000 nanometres.
2. Divide by the virus length: 2000 / 300 = 6.67.

Answer: about 7 TMV particles end to end equal the length of one bacterium. This shows why viruses can only be seen with an electron microscope.

Common Misconceptions

• Misconception: Viruses are living organisms. Correction: Outside a host cell a virus shows no metabolism, growth or reproduction, so most biologists do not consider viruses truly alive; they sit at the boundary between living and non-living.
• Misconception: Antibiotics cure viral infections. Correction: Antibiotics attack bacterial structures like cell walls; viruses have none of these, so antibiotics do not work against viruses.
• Misconception: All viruses cause disease. Correction: Many viruses infect hosts harmlessly, and bacteriophages that infect bacteria are even used as tools in medicine and research.

Real-World Applications

• Vaccines against polio, measles, rabies and COVID-19 train the immune system using weakened or inactivated viruses or viral proteins.
• Gene therapy uses modified, harmless viruses as delivery vehicles to carry correct genes into patients' cells.
• Phage therapy and viral research tools help fight bacterial infections and study how genes are controlled.

A virus is a tiny acellular particle of DNA or RNA in a protein capsid, sometimes with an envelope, that reproduces only inside host cells. Viruses are classified by their nucleic acid type and shape, discovered through the work of Ivanovsky, Beijerinck and Stanley. They cause major diseases but also power vaccines, gene therapy and biological research.