A2.3 Viruses (HL)

Question 1

Structural features common to viruses

Answer 1

non living
non-cellular
multiple origins (too many differences for single common ancestor)
20-300nm
no growth
DNA or RNA
protein capsid: symmetrical, repeating units on the outside
no cytoplasm and few enzymes
rely on host to do everything

Question 2

diversity of structure in viruses

Answer 2

1. genetic material: DNA or RNA, use it in different ways
2. envelope: enveloped vs non-enveloped, enveloped is the cell membrane that a virus has burst through

Question 3

Lytic cycle of a virus

Answer 3

1. The phage attaches to the surface of host
2. the viral DNA enters the host cell
3. Phage DNA replicates and phage proteins are made
4. New phage particles are assembled
5. The cell lyses, releasing the newly made phages
   common with plants and animals cause nearby cells to infect after lysis
   balance needed:
   - too virulent - run out of hosts
   - not virulent enough - immune system

Question 4

lysis

Answer 4

the cell bursts

Question 5

Lysogenic cycle

Answer 5

Difference:
- instead of transcription and translation of the viral DNA, DnA becomes part of host (integration)
- when host divides, copies genetic material
- does not kill its host
- undetectable
- certain stimuli can cause virus to switch to lytic cycle

Question 6

Evidence for several origins of viruses

Answer 6

universal genetic code suggests a common ancestor, but diversity of genetic material and structure suggests multiple origins of viruses

Question 7

hypothesis about viruses

Answer 7

progressive: viruses evolved from modified cell components
regressive: viruses evolved from the loss of cell components

Question 8

rapid evolution in viruses

Answer 8

because:
- short generation time
- lots of mutations (variation) particularly RNA viruses
- Natural selection favors viruses with genetic traits that help evade detection/ destruction

Question 9

Influenza virus

Answer 9

RNA virus with 8 separate molecules (high mutation rate)
Transmission between species can produce new strains
Antigens (proteins on virus surfaced used for attaching to host cell) can change form new combinations that result in new strains
Can be infected multiple times (by different strains) and need frequent vaccinations

Question 10

HIV

Answer 10

RNA is reverse transcribed into DNA without proofreading (more mutations)
Enzymes made by host also cause mutations
a person can infect by multiple strains at the same time, which can combine to make even more new strains
evades the immune system
can become drug resistant