Structure of the human immunodeficiency virus (HIV) and its replication in…

The idea

HIV is a retrovirus containing two RNA strands and reverse transcriptase within a protein capsid. This is surrounded by a lipid envelope derived from the previous host's membrane, studded with attachment proteins (gp120) and fusion proteins (gp41). Replication starts when gp120 binds to a CD4 receptor on a helper T cell. The envelope fuses with the host membrane, releasing RNA and enzymes into the cytoplasm. Reverse transcriptase synthesises a DNA copy from viral RNA, which is inserted into the host's DNA. Following transcription, the host's ribosomes translate viral mRNA into viral proteins. New particles assemble and bud from the cell, acquiring their lipid envelope and destroying the helper T cell.

What earns the marks

• Attachment proteins on the virus bind to specific receptors (CD4) on the helper T cell.
• The viral lipid envelope fuses with the cell-surface membrane, releasing viral RNA and enzymes into the cell.
• Reverse transcriptase synthesises a complementary DNA strand from the viral RNA template.
• The newly formed viral DNA is inserted into the host cell's DNA.
• Host cell ribosomes are used to translate viral mRNA into viral proteins (e.g., capsid proteins or enzymes).
• New viruses are assembled and released by budding, acquiring their lipid envelope from the host cell membrane.

Common mistakes and how to put them right

• Calling the virus's attachment protein a receptor.

  Put it right: Receptors are located on the host cell membrane, whereas the virus possesses attachment proteins.

• Stating that HIV contains DNA or replicates by mitosis.

  Put it right: HIV is an RNA retrovirus that relies entirely on host cell machinery to replicate, not cell division.

• Saying the virus makes its own proteins.

  Put it right: Viruses lack ribosomes, so they must use the host cell's ribosomes for translation.

• Describing HIV as replicating in B cells.

  Put it right: HIV specifically targets and replicates within helper T cells.

Exam tips

• When describing viral replication, structure your answer chronologically: attachment, entry, reverse transcription, integration, protein synthesis, assembly, and release.
• Always name 'reverse transcriptase' when discussing HIV replication; this specific enzyme distinguishes retroviruses from other viral replication cycles.
• If asked to label an HIV diagram, ensure you correctly identify the attachment proteins on the exterior and the RNA and reverse transcriptase within the core.