Week 4: Worksheet

Question 1

2 reasons why most RNA virus genomes are smaller that DNA virus Genomes

Answer 1

(i) RNA polymers are much less stable chemically than are DNA polymers, and the probability
that damage will occur in a single RNA genome increases with length, thereby decreasing genetic
stability. Furthermore, the presence of a complementary strand in viruses with DNA genomes allows
single-strand damages to be repaired via copying of the undamaged strand. (ii) Most RNA genomes
lack a proofreading endonuclease and are therefore replicated with considerably lower fidelity than
those comprising DNA. A noteworthy exception are members of the coronavirus family, which
encode a proofreading endonuclease and are able to maintain a ~30,000-nucleotide (+) strand RNA
genome.

Question 2

What is a quasispecies in virology? Use this concept to explain potential bottlenecks.

Answer 2

Virus populations exist as dynamic collections of non-identical but related replicons called
quasispecies. Although the genome sequences cluster around a consensus or average for a given
Name: BIOL 416/516 Worksheet_Week 4
virus population, every genome can be different from every other in the population. Furthermore,
the population comprises a group of interactive variants, and such diversity is critical for survival. A
bottleneck occurs when virus reproduction is restricted to a small number of members in the
population. Such restriction exerts extreme selective pressure and results in loss of diversity,
accumulation of unselected mutations, or both.