Viruses (Cambridge International AS Biology)

Key Features of Viruses

Spec point — 1.2.7 · state that all viruses are non-cellular structures with a nucleic acid core (either DNA or RNA) and a capsid made of protein, and that some viruses have an outer envelope made of phospholipids

• Viruses are non-cellular particles that infect living cells

  • Note that viruses are not cells, and they are not considered to be living organisms, so are referred to as 'particles'

• They are much smaller than prokaryotic cells, with a diameter of 20-300 nm

• Structurally they have

  • A nucleic acid core made of either DNA or RNA
  • A protein coat called a capsid

• Some viruses have an outer layer called an envelope; this forms from the membrane phospholipids of the host cell in which they were produced

• Viruses can only reproduce by infecting living cells and using their protein-building machinery to produce new viral particles

  • Viruses use attachment proteins on their surface to bind to and infect their host cells

Basic virus structure includes a protein capsid and a nucleic acid core

Exam Hint Never describe viruses as cells or living organisms in exam answers.

Examiners often include this as a trap — viruses are non-cellular particles that rely on host cells to reproduce. Make sure to refer to them as “virus particles”, not “cells”, and state that they cannot carry out life processes independently.