• Viruses?are?non-cellular infectious particles?that straddle the boundary between ‘living’ and ‘non-living’
• They are?much smaller?than?prokaryotic cells
  • They are typically between 20 and 300 nm in diameter
  • For example, the Human Immunodeficiency Virus (HIV) is about 100 nm (0.1 μm) in diameter

   

• Viruses are relatively?simple in structure
  • Unlike prokaryotic cells, they?do not have?a plasma membrane, cytoplasm or ribosomes

   

• Structurally they have:
  • A?nucleic acid core?(their genomes are either DNA or RNA, and can be single or double-stranded)
  • A?protein coat?known as a?‘capsid’
  • Some viruses have an outer layer known as an?‘envelope’?formed usually from the membrane-phospholipids of a cell they were made in

   

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

Viral replication

• Being non-living, viruses?do not undergo cell division
• All viruses are?parasitic, meaning they can?only reproduce?by infecting living cells (known as?host cells)
• Viruses replicate by?injecting their nucleic acid?into a host cell:
  • First, a virus uses?attachment proteins?on its surface to bind to?complementary receptor proteins?on the surface of a host cell
  • The virus then?injects?its?DNA?or?RNA?into the host cell
  • The host cell then uses its nucleic acid and protein-building machinery (ribosomes) to produce?new viral particles

   

• Eventually, the new viral particles are?released. This may occur when:
  • The host cell?bursts?open, releasing all the new viral particles at once
  • Viral particles leave individually through the host cell membrane via a process known as?‘budding’, often taking a section of the membrane with them (this is the ‘envelope’ that surrounds some viruses)

   

• The exiting of viruses?damages?their host cells, causing?disease
  • For example, HIV infects cells of the immune system (such as helper T cells and macrophages)
  • When the newly replicated HIV particles exit, these host cells are destroyed
  • Over time, the immune system becomes severely compromised and this eventually leads to Acquired Immunodeficiency Syndrome (AIDS)

   

The process of viral replication - in this case, the virus is a bacteriophage (a virus that infects specific prokaryotic host cells)