Structure of Viruses: Capsid, Envelope and Genome Types

A virus is not a cell. It is a tiny infectious particle — 20 to 300 nanometres across — made of a nucleic acid genome wrapped in a protein coat, sometimes with an outer lipid envelope. Viruses cannot reproduce on their own; they must hijack a living cell.

Understanding viral structure matters because it explains why antibiotics do not work on viruses, how vaccines and antiviral drugs are designed, and why enveloped viruses are killed by soap. Structure directly determines how a virus infects and how we can stop it.

Key Concepts

The core is the genome: either DNA or RNA, single- or double-stranded, linear or circular — never both DNA and RNA together. Around it is the capsid, a protein shell built from subunits called capsomeres. Capsid shapes include helical (rod, e.g., tobacco mosaic virus), icosahedral (roughly spherical, e.g., adenovirus), and complex (e.g., bacteriophage with head and tail).

Some viruses have an envelope — a lipid bilayer stolen from the host cell membrane, studded with viral spike glycoproteins (e.g., spike protein of SARS-CoV-2, haemagglutinin of influenza) that attach to host receptors. Enveloped viruses are fragile outside the body; non-enveloped (naked) viruses are tougher.

• Virion: the complete infectious particle outside a cell.
• Capsomere: individual protein subunit of the capsid.
• Peplomers: spike projections on the envelope.

How It Works

1. Spike or capsid proteins bind specific receptors on a host cell — this decides which species and tissues can be infected.
2. The virion enters by endocytosis or membrane fusion (if enveloped).
3. The capsid uncoats, releasing the genome into the cytoplasm or nucleus.
4. Host machinery is commandeered to copy the genome and make viral proteins.
5. New capsids assemble around genomes; enveloped viruses bud out, taking membrane with them.

Worked Example

Compare sizes. A typical bacterium is about 2,000 nm long. An influenza virion is about 120 nm diameter; a rhinovirus (common cold) is about 30 nm. If a bacterium were the size of a human (1.7 m), the influenza virus would be roughly 1.7 × (120/2000) = 0.102 m — about 10 cm, like a grapefruit — and rhinovirus about 2.5 cm, like a grape. This scaling shows why viruses pass through filters that trap bacteria, a discovery that first revealed their existence.

Common Misconceptions

• Misconception: Viruses are living cells. Correction: Outside a host they show no metabolism, growth, or reproduction; biologists classify them as obligate intracellular parasites at the border of life.
• Misconception: All viruses have envelopes. Correction: Many, like poliovirus and adenovirus, are naked capsids and resist detergents better than enveloped ones.
• Misconception: A virus contains both DNA and RNA. Correction: Each virus species carries only one type of nucleic acid as its genome.

Real-World Applications

Vaccines use weakened virions, inactivated particles, or just capsid/spike proteins to train immunity. Antiviral drugs target viral structures — e.g., neuraminidase inhibitors block influenza release. In biotechnology, bacteriophages and modified viruses serve as vectors to deliver genes in gene therapy. Diagnostic tests detect viral antigens (capsid proteins) or genomes by PCR.

A virus is simply a DNA or RNA genome in a protein capsid, sometimes wrapped in a spiked lipid envelope. This minimal structure explains its parasitic replication, its vulnerability to soap if enveloped, and the targets for vaccines and antivirals.