viruses

Question 1

what structural features are common to viruses? (6 marks)

Answer 1

small, fixed size, capsid made of protein, nucleic acid as genetic material, no cytoplasm, no/ few enzymes

Question 2

why have scientists deduced that viruses have multiple evolutionary origins? (2 marks)

Answer 2

diversity of genetic material and enveloped vs non-enveloped viruses

Question 3

give 5 examples of variation/ diversity in viruses?

Answer 3

shape of cuspid, RNA vs DNA and how it is replicated, envelope, attachment proteins, lifecycle (lytic/lysogenic)

Question 4

shape of genetic material?

Answer 4

linear or circular

Question 5

what different ways can single-stranded RNA be replicated?

Answer 5

positive sense RNA used directly as mRNA

negative sense RNA transcribed -> mRNA

retroviruses: make double stranded copies of their RNA, negative sense RNA transcribed to produced mRNA

Question 6

explain why some viruses have an envelope and some don’t?

Answer 6

some viruses bud out of the host cell (common in animal cells)

take some host cell membrane as envelope (viral proteins come from virus itself tho)

viruses causing cell lysis burst out of host so do not have envelope

Question 7

bacteriophage lambda as an example of diversity

Answer 7

capsid

one double stranded DNA in head

follows either lytic or lysogenic pathway

tail tube which injects DNA into host cell

tip of tail can pierce wall of bacterium

non-enveloped

host: ecoli

Question 8

covid-19 as an example of diversity

Answer 8

one +ve RNA

spike proteins bind to host

enveloped

host: human cells maybe animal

Question 9

HIV as an example of diversity

Answer 9

a retrovirus

enveloped

reverse transcriptase > double stranded DNA

Question 10

what is the lytic lifecycle of a virus?

Answer 10

The virus reproduces inside host. The new virus are released from the host cell, host dies. Viruses spread through body of host

Question 11

what is the lysogenic lifecycle of a virus?

Answer 11

viral DNA is integrated into the host DNA. When host cell replicates, Viral DNA does too.

Question 12

do temperate viruses cause harm to host?

Answer 12

while in lysogenic cycle, does not lyse so minimal harm. can enter lytic which is harmful

Question 13

what does viruses being obligate parasites suggest about its evolution?

Answer 13

cannot survive w/o host so must hv evolved after cells

Question 14

what is the progressive hypothesis?

Answer 14

viruses evolved in a searies of steps from simpler structures

e.g. retrotransposons found in some cells share some characteristics of retrovirus

Question 15

what is the regressive hypothesis?

Answer 15

virus evolved from cells, which gradually lost cell components

eg some bacterial cells like chlamydia are parasitic and hv lost ability to perform some metabolic functions w/o host cell + hv similar lifecycles to viruses

Question 16

what is convergent evolution?

Answer 16

independent evolution of similar features (analogous traits) in unrelated viral lineages, driven be similar environmental pressures

Question 17

why is evolution of viruses rapid?

Answer 17

very short generation time > many gens produced quick

high mutation rate > genetic variation

immune system > natural selection

Question 18

influenza virus as an example of rapid evolution

Answer 18

single strand RNA replicated by enzyme RNA, no proof-reading function > mutation

segmented genome, new strain in host if invaded by some combined RNA molecules

Question 19

HIV virus as an example of rapid evolution

Answer 19

RNA converted to DNA w/ reverse transcriptase - no proof reading > mutation

highest mutation rate of any virus

can mutate and become resistant to the drug so combination of drugs required