a2.3- viruses

Question 1

structural features common to viruses [8]

Answer 1

• non-living (no metabolic machinery of their own)
• small
• fixed size
• nucleic acid (DNA or RNA) as genetic material
• capsid made of protein
• no cytoplasm
• no/few enzymes
• relies on a host cell to replicate

Question 2

lytic cycle mechanism

Answer 2

1. attaches
   - binds to receptor that allows it to dock there
   - inject genetic material
2. makes copies of the virus
3. sm copies- cell membrane explodes
4. infect other organisms

Question 3

lysogenic cycle mechanism

Answer 3

1. attachment
2. entry of genetic material
3. integration
4. cell division
   - DNA becomes integrated into the bacterial DNA molecule
   - daughter cell continues the process
   - usually naked
   - temperate (wont try to kill the host yet)

Question 4

lysogenic cycle example

Answer 4

bacteriophage lambda of E. Coli

Question 5

convergent evolution

Answer 5

• progressive hypothesis
• originated from simpler genetic elements
• gradually acquired the ability to infect and replicate within host cells

Question 6

evidence of convergent evolution

Answer 6

virus like components in some cells
eg. retrotransposon/ circular plasmid is simple and can be synthesised

Question 7

divergent evolution

Answer 7

• regressive hypothesis
• originated from more complex cellular organisms that underwent a process of degeneration

Question 8

evidence for divergent evolution

Answer 8

• some viruses share genome similar to cellular organisms
• show variation in complexity and self-reliance

Question 9

reasons for rapid evolution [4]

Answer 9

1. short generation time
2. high mutation rates- often do not proofread
3. increases rate of survival: evade defence mechanism of the host cell
4. high natural selection pressure

Question 10

rapid evolution examples [2]

Answer 10

1. influenza
   - RNA replicase doesnt proofread + often makes errors
2. HIV
   - reverse transcriptase (rna -> dna) tricks us to use it

Question 11

antigenic drift

Answer 11

as more mutations occur- cant recognise the virus anymore

Question 12

antigenic shift

Answer 12

animal → human

Question 13

consequences for treating diseases caused by rapidly evolving viruses

Answer 13

develop immunity to medicine- wont die

Question 14

what virus cant do lysogenic cycle

Answer 14

T7 virus

Question 15

eg of small and simple virus

Answer 15

polio virus

Question 16

eg of large and complex virus

Answer 16

smallpox

Question 17

expectation for cells in bacteria

Answer 17

• to be self reliant
  EXCEPTION: parasitic bacteria, replicates inside a host cell

Question 18

diversity of structure in viruses [2]

Answer 18

1. diversity of genetic material
2. enveloped / non-enveloped

Question 19

eg of dna virus [2]

Answer 19

1. smallpox
2. hepatitis B

Question 20

eg of rna virus [2]

Answer 20

1. corona virus
2. ebola