A2.3 Viruses

Question 1

What are the characteristics of viruses?

Answer 1

1. Enclosed by a capsid
2. Fixed small size
3. Presence of DNA / RNA (single or double stranded)
4. Sometimes have an envelope (outside of capsid)
5. No cytoplasm
6. Obligatory intracellular parasites

Question 2

What are the hypotheses for the origins of viruses?

Answer 2

1. Virus first hypothesis: viruses originated before cells.
2. Regressive hypothesis: Viruses were once small cells that became parasites of bigger cells, eventually shedding small structures no longer needed via gene reduction.
3. Escape hypothesis: DNA / RNA escaped from larger cells and became surrounded by an outer boundary.

Question 3

Explain the effect of antigenic shift

Answer 3

1. 2 viruses infect the same cell, causing DNA / RNA to recombine.
2. Major changes in a relatively short period of time to the surface proteins of the virus.
3. This causes the virus strain to be unrecognisable to the immune system.

Question 4

Describe the effect of antigenic drift

Answer 4

1. Virus undergoes incremental changes in genetic material over time.
2. Slowly producing changes in the surface proteins of the virus.
3. Accumulated changes eventually prevents the immune system from recognising the virus.