Viruses · Paper 2 — Free Biology HL Practice Questions

Influenza A virus is an enveloped virus with a segmented single-stranded RNA genome. The virus binds to sialic acid receptors on host respiratory epithelial cells via its haemagglutinin (HA) glycoprotein. After endocytosis, the low pH of the endosome triggers fusion of the viral envelope with the endosomal membrane, releasing the viral RNA segments into the cytoplasm. The viral RNA-dependent RNA polymerase (RdRp) replicates the viral genome and transcribes mRNA. New viral particles assemble at the host cell plasma membrane, where neuraminidase (NA) cleaves sialic acid to release progeny virions. A researcher treats infected cell cultures with oseltamivir, a competitive inhibitor of neuraminidase. After 24 hours, the number of viral particles released into the culture medium and the number remaining attached to the host cell surface are measured.

A laboratory culture of Escherichia coli is infected with bacteriophage T4. The phage injects its double-stranded DNA into the host cell, which then replicates the phage genome and synthesises phage proteins. New phage particles are assembled, and the host cell is lysed by phage-encoded lysozyme, releasing approximately 200 progeny phages per infected cell. This the lytic cycle. The culture initially contains $5.0 \times 10^{8}$ bacterial cells and is infected at a multiplicity of infection (MOI) of $0.1$ (1 phage per 10 bacterial cells).

A culture of $\text{CD4}^+$ T-cells was infected with HIV at time $= 0\,\text{h}$. The concentration of free virions in the culture medium was measured every $6\,\text{h}$ for $48\,\text{h}$. At $t = 12\,\text{h}$ the culture was divided into three treatment groups: - Group A: no drug (control) - Group B: treated with zidovudine (AZT), a nucleoside analogue that inhibits reverse transcriptase - Group C: treated with raltegravir, an inhibitor of viral integrase Viral concentrations (thousands of virions $\text{mL}^{-1}$) are shown below. Time (h) — 0 — 6 — 12 — 18 — 24 — 36 — 48 Group A — 0.0 — 0.5 — 2.0 — 8.0 — 32.0 — 512.0 — 8192.0 Group B — 0.0 — 0.5 — 2.0 — 2.1 — 2.2 — 2.4 — 2.6 Group C — 0.0 — 0.5 — 2.0 — 1.9 — 1.7 — 1.4 — 1.1