2.3.9 Viral Particle Replication

• 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)