A2.3 Viruses

Question 1

origin of viruses?

Answer 1

likely have multiple origins with any shared features due to convergent evolution

Question 2

what are the shared features of viruses?

Answer 2

small size(need to be smaller than host cells so they can enter and they lack cytoplasm etc), fixed size(no growth so cant increase in size), nucleic acid as genetic material(use the universal genetic code bc their proteins are synthesized by the nucleic acid-to-polypeptide translation mechanisms of their host cell), capsid made of protein(self-assembly of repeating subunits of the capsid gives viruses a symmetrical structure), no cytoplasm and few or no enzymes(rely on the metabolism of their host)

Question 3

what are the 2 things that make viruses diverse in structure?

Answer 3

genetic diversity(no genes occur in all viruses can be DNA(linear/circular)/RNA(positive-sense and used directly as mRNA or negative-sense and need to be transcribed before translation)or single/double stranded; enveloped/non-enveloped

Question 4

when do some viruses become enveloped in the membrane?

Answer 4

during lysis, with phospholipids from the plasma membrane of the host cell and proteins, mostly glycoproteins from the virus itself. the membrane helps the enveloped virus to make contact with a host cell and infect it

Question 5

are animal viruses mostly enveloped or non-enveloped?

Answer 5

enveloped

Question 6

are plant viruses and bacteriophages enveloped or non-enveloped?

Answer 6

mostly non-enveloped

Question 7

bacteriophage lambda: type

Answer 7

bacteriophage-DNA viruses that use prokaryotes as hosts

Question 8

bacteriophage lambda: enveloped?

Answer 8

non-enveloped

Question 9

bacteriophage lambda: genetic material

Answer 9

1 double-stranded DNA molecule with 32 genes

Question 10

bacteriophage lambda: features

Answer 10

can follow either a lytic cycle in which it bursts and kills the host or a lysogenic cycle with its DNA inserted into the host cell DNA, so it is passed on to daughter cells when the host divides

Question 11

bacteriophage lambda: host

Answer 11

E.coli-a gut bacterium

Question 12

COVID-19: type

Answer 12

Corona virus-RNA viruses with crown shape and animal hosts

Question 13

COVID-19: enveloped?

Answer 13

enveloped

Question 14

COVID-19: genetic material

Answer 14

1 single-stranded positive-sense RNA molecule with 16 genes

Question 15

COVID-19: features

Answer 15

caused a pandemic, disease was zoonotic bc the virus spread to humans from another species likely a bat

Question 16

COVID-19: host

Answer 16

Epithelium cells in the airways and lungs of humans

Question 17

HIV: type

Answer 17

retrovirus-viruses that convert RNA to DNA after entry to host

Question 18

HIV: enveloped?

Answer 18

enveloped

Question 19

HIV: genetic material

Answer 19

2 copies of a single-stranded positive-sense RNA molecule with 9 genes

Question 20

HIV: features

Answer 20

virus contains reverse transcriptase which makes a double-stranded copy of the viral RNA genome, which is integrated into the host cell’s chromosomes

Question 21

HIV: host

Answer 21

T-helper cells in the human immune system

Question 22

how does bacteriophage lambda bind to its host?(lytic cycle)

Answer 22

binds to its host E.coli using proteins at the tip of its tail, then injects its DNA into the host cell through the tubular tail, viral DNA has single-stranded ends, which link by base pairing to convert the molecule from a linear to a circular form

Question 23

summarise the lytic cycle

Answer 23

attachment to a host cell using tail fibres; DNA entry via tail and pores in plasma membrane; DNA replication; synthesis of viral proteins using mRNA transcribed from viral DNA; assembly of new viruses with DNA inside a protein coat; lysis-bursting to release the new viruses

Question 24

difference between the lytic cycle and lysogenic cycle

Answer 24

the virus attaches to a host cell and injects its DNA but instead of replication, the virus’s DNA becomes integrated into the host cell’s DNA molecule, it stays there undetected and inactive

Question 25

summarise the lysogenic cycle

Answer 25

attachment to host cell using tail fibres; DNA entry via tail and pores in plasma membrane; integration of viral DNA into the circular bacterial chromosome; lysogenic cycle of bacteriophage lambda

Question 26

what is the viral DNA called?

Answer 26

it’s called prophage while it remains integrated in the bacterial DNA-it is temperate in this state bc it doesn’t kill its host and causes minimal harm

Question 27

why is the viral DNA lysogenic?

Answer 27

bc it could change to the lytic state and then cause lysis, the stimulus for this can come from inside or outside the bacterial cell

Question 28

what is the evidence for there being several origins of viruses?

Answer 28

obligate parasites(need host to replicate); use same universal genetic code as living; very diverse in structure(suggests multiple origins from living cells)=all suggest cells evolved before viruses

Question 29

what are the 3 main reasons viruses can show rapid rates of evolution?

Answer 29

very short generation times<1 in lytic cycle; high mutation rates, esp in RNA viruses; intense natural selection due to host organisms evolving defences such as antibodies for destroying viruses

Question 30

what is influenza caused by?

Answer 30

a rapidly evolving RNA virus-uses RNA replicase to replicate its genetic material, the enzyme doesnt proofread or correct errors leading to a high mutation rate

Question 31

how many RNA molecules does influenza have?

Answer 31

8 separate ones, if a host cell is invaded by 2 diff strains of the virus, a new strain can be formed with the combo of RNA from the 2 strains

Question 32

what is another thing that triggers the evolution of influenza?

Answer 32

transmission of flu between humans and other species

Question 33

what happens because of the rapid evolution of the flu virus?

Answer 33

annual vaccinations are needed to give immunity to new strains

Question 34

which virus has the highest known mutation rate of any virus and what does it do?

Answer 34

HIV-it is a retrovirus that uses reverse transcriptase to convert its single-stranded RNA genome to DNA, the enzyme doesnt proofread or correct errors

Question 35

what is another thing that causes mutations in HIV?

Answer 35

cytieine deaminase, an enzyme made by the host that converts cytosine to uracil, mutations will produce many genetically diff strains helping the virus to evade the immune system and become resistant to antiretroviral drugs

Question 36

is HIV chronic or curable?

Answer 36

chronic