A2.3 VirusesIB Biology HL: Revision notes

Relatively few features are shared by all viruses:

• small, fixed size;
• nucleic acid (DNA or RNA) as genetic material;
• a capsid made of protein;
• no cytoplasm;
• few or no enzymes.

Because they lack cytoplasm, ribosomes and metabolism, viruses rely on a host cell for energy supply, nutrition, protein synthesis and other life functions.

Viruses are highly diverse in shape and structure. Genetic material may be RNA or DNA, single- or double-stranded. Some are enveloped in host cell membrane (gained as they bud out) and others are not enveloped.

• Bacteriophage lambda: double-stranded DNA, protein head and tail, not enveloped; infects E. coli.
• Coronaviruses (e.g. SARS-CoV-2): single-stranded RNA, enveloped with spike proteins.
• HIV: single-stranded RNA and reverse transcriptase, enveloped; a retrovirus.

1. Attachment of the phage tail to receptors on E. coli.
2. Injection of phage DNA; the capsid stays outside.
3. Replication of phage DNA and synthesis of phage proteins using the host's ribosomes, nucleotides, amino acids and ATP.
4. Assembly of new phages.
5. Lysis: phage-coded enzymes break the cell open, releasing many new phages (often around 100).

The injected phage DNA is integrated into the bacterial chromosome as a prophage. A phage repressor keeps the lytic genes switched off. The prophage is replicated with the host chromosome and passed to daughter cells for many generations; the lysogenic cell is immune to infection by more lambda. Stress such as DNA damage (e.g. UV light) inactivates the repressor, the prophage excises and the lytic cycle begins.

The diversity of viruses suggests several possible origins, from different organisms. Viruses share an extreme form of obligate parasitism, so their common structural features could be the result of convergent evolution rather than common ancestry. The genetic code is shared between viruses and living organisms, which suggests viruses originated from cellular life.

Reasons: high mutation rate (RNA copied without proofreading), short generation time, huge numbers of virus particles, and strong selection by host immunity and drugs.

• Influenza: antigenic drift (gradual change in surface antigens) and antigenic shift (reassortment of genome segments between strains).
• HIV: reverse transcriptase errors give about one mutation per genome copied.

Consequences for treatment: influenza vaccines must be updated regularly; single anti-HIV drugs quickly select resistance, so combination therapy is used; no HIV vaccine exists.