Being non-living, viruses do not undergo cell division. Following…

The idea

Viruses lack ribosomes, enzymes for protein synthesis, and ATP production, so they cannot divide. Replication begins when viral attachment proteins bind to complementary receptor molecules on the host cell-surface membrane. This specificity means a virus infects only cells with the correct receptor, like HIV binding CD4 on helper T cells. Viral nucleic acid then enters the cell via injection or engulfment. Inside, the host's machinery replicates the nucleic acid. In retroviruses, reverse transcriptase first makes DNA from viral RNA. The host's ribosomes translate viral mRNA into viral proteins, which are assembled into new particles and released.

What earns the marks

• Attachment proteins on the virus bind to complementary receptors on the host cell.
• Viral nucleic acid enters the cell by injection or engulfment.
• The nucleic acid is replicated inside the cell, or reverse transcriptase makes DNA from viral RNA.
• The host cell produces viral proteins, such as capsid proteins or enzymes.
• New virus particles are assembled and released by lysis or budding.

Common mistakes and how to put them right

• Describing the virus as dividing, growing, or undergoing mitosis.

• Writing that a receptor protein on the virus binds the cell.

  Put it right: The virus carries the attachment protein and the host carries the receptor.

• Claiming the virus makes its own proteins, when the host cell's ribosomes do.

• Listing steps out of order, such as stating proteins are made before the nucleic acid enters the cell.

Exam tips

• Describe viral replication as a clear, chronological sequence of events from attachment to release.
• Use the term 'nucleic acid' rather than 'DNA' when the virus is unspecified, as it could contain either DNA or RNA.