Viral Particle Replication (AQA A Level Biology)

Viruses Do Not Undergo Cell Division

• 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)