Structure and diversity of viruses

Cells (Unity and diversity) · Viruses · note 1 of 3

Spec A2.3.1, A2.3.2

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Structure and diversity of virusesSpec A2.3.1, A2.3.2

In short

Viruses share relatively few features: a small, fixed size, nucleic acid (DNA or RNA) as genetic material, a capsid made of protein, no cytoplasm, and few or no enzymes. Otherwise they are highly diverse: their genetic material may be RNA or DNA, single- or double-stranded, and some are enveloped in host cell membrane while others are not.

Viruses are not cells. Relatively few features are shared by all viruses:

A small, fixed size: most are about 20–300 nm across, far smaller than bacteria, and they do not grow.
Nucleic acid as genetic material: either DNA or RNA.
A capsid made of protein subunits, which protects the nucleic acid.
No cytoplasm.
Few or no enzymes, so they depend on a host cell for metabolism.

Diversity of structure

Viruses are highly diverse in shape and structure. Their genetic material may be RNA or DNA, and either can be single- or double-stranded. Some viruses are enveloped in membrane taken from the host cell; others are not enveloped.

Three example viruses
Feature	Bacteriophage lambda	Coronaviruses (e.g. SARS-CoV-2)	HIV
Host	The bacterium Escherichia coli	Mammals and birds; SARS-CoV-2 infects cells lining the human airways	Humans, mainly helper T cells
Genetic material	Double-stranded DNA	Single-stranded RNA	Single-stranded RNA (two copies)
Capsid	Icosahedral head on a long, flexible tail; the tail tip binds the host	Helical capsid wrapped around the RNA	Cone-shaped capsid
Envelope	Not enveloped	Enveloped, with spike glycoproteins that bind host cell receptors	Enveloped, with glycoproteins that bind receptors on helper T cells
Enzymes carried	None	None	Reverse transcriptase, integrase and protease

An envelope is a piece of host plasma membrane picked up as the virus buds out of the host cell, with viral glycoproteins inserted into it.

Three viruses not to scale: bacteriophage lambda with an icosahedral protein head containing double-stranded DNA, a long flexible tail and a tip fibre, not enveloped; a coronavirus with an envelope of host membrane, spike glycoproteins and a helical capsid around single-stranded RNA; and HIV with an envelope, glycoproteins, a cone-shaped capsid containing two single-stranded RNA molecules and reverse transcriptase. — Viruses are diverse: DNA or RNA, and enveloped or not

Exam tip:

Guiding question: how can viruses exist with so few genes? They need no genes for ribosomes, ATP production or most metabolism, because the host cell supplies all of these.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What do all viruses have in common?

All viruses have a small, fixed size, nucleic acid (either DNA or RNA) as their genetic material, a capsid made of protein, no cytoplasm, and few or no enzymes. Beyond these shared features they are highly diverse in shape, in the type of nucleic acid they carry and in whether they have an envelope.

What is the difference between the lytic and lysogenic cycle?

In the lytic cycle a virus uses the host cell to make many new viruses quickly and then bursts the cell. In the lysogenic cycle the viral DNA integrates into the host chromosome as a prophage, so the host survives and copies the viral DNA each time it divides, until induction triggers the lytic cycle.

Why does the flu vaccine change every year?

The flu vaccine changes every year because influenza viruses evolve rapidly. Their RNA genome mutates often, gradually changing the surface antigens haemagglutinin and neuraminidase (antigenic drift), so antibodies made against earlier strains no longer bind well. The vaccine is reformulated each year to match the strains expected to circulate.

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