A2.3 Viruses (HL)

In a bacteriophage lytic cycle, the virus must first attach to specific receptor sites on the bacterial cell surface before any nucleic acid can enter the host.

Attachment (ii) is a prerequisite for injection (i). Without receptor binding, the phage cannot penetrate the cell wall to deliver its genome.

Once the viral genome is inside, host machinery replicates viral nucleic acid and synthesises capsid proteins. Only then can new phage particles be assembled (iii).

After assembly, the host cell is lysed (iv) by viral enzymes (lysozyme), releasing the new phage particles. The correct sequence is (ii) → (i) → (iii) → (iv).

We need the correct chronological order of four stages in a bacteriophage lytic infection cycle.

Options starting with (i) injection are immediately incorrect because a phage cannot inject its nucleic acid before it has attached to the host cell surface. This eliminates (i) → (ii) → (iii) → (iv) and (i) → (iii) → (ii) → (iv).

The sequence (ii) → (iii) → (i) → (iv) places assembly before injection, which is impossible — viral components cannot be assembled before the viral genome has entered the cell.

The remaining option (ii) → (i) → (iii) → (iv) correctly places attachment before injection, injection before assembly, and assembly before lysis.

The central dogma states that nucleic acids carry genetic information that is ultimately translated into proteins. Viral RNA serves as messenger RNA (or is converted to it) to direct synthesis of viral proteins.

When viral RNA enters a host cell, host ribosomes recognise it and translate it, producing viral structural proteins (capsid components) and enzymes (such as RNA-dependent RNA polymerase) needed for replication.

The function of viral RNA is informational — it carries the genetic code. It is not itself a structural protein, nor does it directly degrade host DNA.

The correct answer is: 'To encode instructions for the production of viral proteins.' This aligns with the established role of nucleic acids as carriers of genetic information.

We need to identify the primary role of viral RNA once it is inside a host cell being translated by ribosomes.

Capsid components are proteins (capsomeres), not RNA. RNA does not form the structural protein shell of the virus.

While some viruses do disrupt host cell activity, RNA itself is not an enzyme that degrades DNA. Any degradation would be carried out by viral proteins, not the RNA molecule directly.

Host DNA polymerase replicates host DNA; viral RNA does not function as a primer in this process. This option confuses viral RNA with primer sequences used in DNA replication.

The correct role is encoding instructions for viral protein production — this is the function of any mRNA (or equivalent) in a cell.