A2.3 Viruses. Practice questions with markscheme.

• D: DNA damage is the signal that induces the prophage to excise and switch to the lytic cycle, producing new phages that lyse the cell; A is wrong because no phage particles exist in a lysogenic cell; B is wrong because dead cells cannot synthesize phage components; C describes a process that does not occur;

• D: lytic: attach, inject nucleic acid, take over host machinery to copy genome and make capsid proteins, assemble virions, lyse the cell; integration and dormancy describe lysogeny;

• both change the antigens on the surface of the virus, so both alter the way the virus is recognized;
• both allow the virus to escape antibodies produced in response to earlier infections or vaccines;
• in both, the change is heritable and is passed on to the descendants of the virus;
• drift is caused by the accumulation of point mutations during replication, whereas shift is caused by reassortment of whole genome segments between two strains infecting the same host cell;
• drift produces small, gradual changes in the antigens, whereas shift produces a large and sudden change;
• drift causes seasonal epidemics and the need to reformulate the vaccine each year, whereas shift can produce a strain to which almost no one is immune, causing a pandemic;

Each point must be an explicitly paired statement. Award converse.

• both have a nucleic acid genome enclosed in a protein capsid / both lack cytoplasm and metabolism;
• phage genome is (double-stranded) DNA, whereas influenza's is (segmented, single-stranded) RNA;
• influenza is enveloped (host-derived membrane with glycoproteins), whereas the phage is naked (no envelope);
• the phage has a complex head–tail structure (with tail fibres for attachment), whereas influenza is (roughly) spherical with surface spikes (haemagglutinin/neuraminidase);

At least one similarity AND one difference for full marks. Award converse.

• progressive/escape hypothesis: viruses arose from fragments of cellular nucleic acid (mobile genetic elements) that gained the ability to leave the cell;
• regressive/reduction hypothesis: viruses are remnants of (parasitic) cells that lost most of their structures;
• virus-first hypothesis: viruses (or their replicators) predate/co-evolved with the first cells;
• uncertainty: viruses leave no fossils / evolve rapidly, and (their diversity suggests) several independent origins, so evidence is indirect, OWTTE;

Any two hypotheses plus one reason.

• I, glycoprotein / (attachment) spike;
• II, (phospholipid) envelope / host-derived membrane;
• III, capsid / protein coat;
• IV, nucleic acid / genetic material (RNA or DNA);

For IV accept RNA or DNA; do not accept "chromosome".

• for: viruses have genetic material (DNA or RNA) and can evolve/adapt by mutation and natural selection;
• for: they reproduce (make many copies) and show heredity, so pass on characteristics;
• against: they have no cytoplasm, ribosomes or metabolism of their own;
• against: they cannot reproduce independently — they are obligate intracellular parasites that require a host cell's machinery;
• against: outside a host they are inert particles (can even be crystallized), carrying out no functions of life;
• judgement: because they lack independent metabolism and reproduction, most biologists place them outside the usual definition of life / 'at the edge of life', though this depends on how life is defined, OWTTE;

A reasoned judgement referring to the criteria for life is required for the top band.