CIE A Level Biology复习笔记1.2.5 Viruses

Key Features of Viruses

  • Viruses are non-cellular infectious particles that straddle the boundary between ‘living’ and ‘non-living’
  • They are relatively simple in structure; much smaller than prokaryotic cells (with diameters between 20 and 300 nm)
  • Structurally they have:
    • A nucleic acid core (their genomes are either DNA or RNA, and can be single or double-stranded)
    • A protein coat called a ‘capsid’
  • Some viruses have an outer layer called an envelope formed usually from the membrane-phospholipids of a cell they were made in
  • All viruses are parasitic in that they can only reproduce by infecting living cells and using their protein-building machinery (ribosomes) to produce new viral particles


Viruses are not cellular like prokaryotes and eukaryotes – this is just one example of a virus structure