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AS Biology > T2.2 - Viruses > Flashcards

T2.2 - Viruses Flashcards

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1
Q

What are virus particles made of?

A

Nucleic Acids (DNA/RNA)
Proteins (Structural e.g. capsid / enzymes/ attachment)
(in some cases inside of a lipid envelope)

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2
Q

What is the capsid made of?

A

Small protein subunits called capsomeres

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3
Q

State the meaning of the term obligate intracellular parasite

A

Obligate intracellular parasites contain a genome, but can only reproduce within the host cell.

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4
Q

Explain why viruses are dependent on living cells.

A

They are unable to replicate independently, because they do not have appropriate organelles or enzymes

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5
Q

List the four groups of virus as classified by the Baltimore system

A

dsDNA viruses,
negative ssRNA viruses
positive ssRNA viruses
ssRNA retroviruses

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6
Q

What is classification of a virus by the Baltimore system based on?

A

The molecule it uses as genetic material (DNA or RNA)
Whether the genetic material is single or double stranded
The steps the virus uses to make an mRNA

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7
Q

Give an example of a dsDNA virus

A

λ (lambda) phage

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8
Q

Describe the action of dsDNA (double stranded linear DNA) in λ
(lambda) phage

A

dsDNA is transcribed to form mRNA, which is translated to make new viral proteins

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9
Q

Describe the structure of a dsDNA virus / λ (lambda) phage

A

tail fibres made of attachment proteins
double stranded linear DNA
Inside protein capsid (coat)

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10
Q

Describe the steps in the lytic cycle of lambda phage

A
  1. Attachment using viral attachment proteins which target proteins in the host cell membrane
  2. Insertion of dsDNA and degradation of host DNA using enzymes
  3. Replication and transcription of dsDNA into mRNA using host cell replication enzymes
  4. Synthesis of viral proteins (capsomeres and assembly enzymes) by translation using host cell ribosomes
  5. Assembly of mature viruses
  6. Release via lysis
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11
Q

Why does the lytic cycle cause disease in the host organism?

A

Lysis of cell membranes kills the cell.
If enough cells are lysed, it will stop the infected tissue functioning properly.

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12
Q

Describe the steps in the lysogenic cycle of λ (lambda) phage

A
  1. Integration: Viral DNA integrates into host DNA, becoming a
  2. Latency: The prophage remains (dormant) within the host cell due to a viral repressor protein that prevents its translation.
  3. Cell division: The host cell divides as normal, copying prophage into daughter cells.
  4. Induction: If exposed to UV or chemicals the prophage is released out of host DNA
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13
Q

Contrast the lytic cycle with the lysogenic cycle of λ (lambda) phage

A

In the lytic cycle viral DNA destroys host cell DNA, and destroys the cell via lysis, but not in the lysogenic cycle.
In the lytic cycle the virus replications and produces progent phages, wheras in the lysogenic cycle it does not.
The lytic cycle produces symptoms of infection, whereas the lysogenic cycle does not.

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14
Q

State an advantage of the lytic cycle

A

Replication of new viruses is fast

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15
Q

Explain a disadvantage of the lytic cycle

A

The host is almost immediately killed, preventing the viral genome from passing on to the next generation of host cells.

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16
Q

Explain an advantage of the lysogenic cycle / latency

A

Many more viruses can be made because the viral genome is passed onto future generations of host cells

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17
Q

Explain an disadvantage of the lysogenic cycle / latency

A

Replication takes longer because it is dependent on the host cell’s replication

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18
Q

State what is meant by the term latent

A

The state of the non-virulent virus within the host cell

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19
Q

State what is meant by the term lysogeny

A

The period when a virus is part of the reproducing host cell, but does not affect it adversely

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20
Q

State the meaning of the term provirus (or prophage)

A

The DNA that is inserted into the host genome during the lysogenic pathway of viral replication

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21
Q

Give an example of a negative ssRNA virus

A

Ebola virus / Influenza

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22
Q

Describe the action of negative ssRNA (single stranded) virus in Ebola

A

Negative antisense strand cannot be directly translated so it is transcribed into mRNA
by viral RNA replicase first, then translated to make new viral proteins

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23
Q

Describe the structure of the Ebola virus

A

●Filamentous shape
● negative ssRNA and RNA replicase
● inside protein capsid made of many capsomeres
● surrounded by lipid envelop containing glycoprotein spikes

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24
Q

Describe the replication cycle of a negative ssRNA virus

A
  1. Attachment of glycoproteins to cell surface receptors
  2. Endocytosis releases ssRNA and viral enzymes into host cell.
  3. Replication and transcrition of the negative antisense ssRNA strand into mRNA using viral replication enzymes.
  4. Synthesis of viral proteins via translation (capsomeres and assembly enzymes) at host cell ribosomes.
  5. Viral proteins are packed into vesicles by RER/Golgi
  6. Assembly of capsid around ssRNA.
  7. Release via budding forming new envelope around the capsid forming new virion.
25
Q

Give an example of a positive ssRNA virus

A

Tobacco mosaic virus

26
Q

Describe the action of positive ssRNA (single stranded) virus in Tobacco mosaic virus

A

Positive sense strand acts as mRNA and is directly translated into proteins at the ribosomes

27
Q

Describe the replication cycle of TMV

A
  1. TMV inserted into plant cell through wound via vector. Replication of positivemssRNA using viral replication enzymes.
  2. Positive sense strand acts as mRNA and is directly translated into proteins (capsomeres and assembly enzymes) via translation using the host cell’s ribosomes.
  3. Replication of positive ssRNA using viral replication enzymes.
  4. Assembly of proteins and viral RNA inside capsid.
  5. Virions spread to other plant cells through plasmodesmata
28
Q

Describe the replication cycle of a positive ssRNA virus in animals

A
  1. Attachment of glycoproteins to cell surface receptors
  2. Endocytosis releases ssRNA and viral enzymes into host cell (except TMV which enters plant cell through wound).
  3. Replication of positive ssRNA using viral replication enzymes.
  4. Positive sense strand acts as mRNA and is directly translated into proteins (capsomeres and assembly enzymes) via translation using the host cell’s ribosomes.
  5. Viral proteins packed into vesicles by RER/Golgi
  6. Assembly of proteins and viral RNA inside capsid.
  7. Release via budding forming new envelope around the capsid forming new virion.
29
Q

Describe the structure of fthe Tobacco mosaic virus

A

Positive ssRNA and RNA polymerase
inside protein capsid made of many capsomeres
Structural and attachment proteins

30
Q

How does Tobacco mosaic virus infect plant cells?

A

As they infect PLANT cells, they cannot enter via endocytosis due to cell wall).
They have to enter through a wound, usually via a such as (their stylet pireces the cells when they feed)

31
Q

How do plant viruses spread between plant cells?

A

via the plasmodesmata (cytoplasmic bridges between cells).

32
Q

Give an example of an ssRNA retrovirus

A

HIV (Human Immunodeficiency Virus)

33
Q

Describe the action of retroviral ssRNA

A

The viral enzyme reverse transcribes ssRNA into DNA, which enters nucleus and is intergrated into host cell DNA to form a provirus, it then gets transcribed into mRNA.

34
Q

Describe the structure of HIV

A

Spherical shape
2 ssRNA strands, reverse transcriptase
inside protein capsid made of many capsomeres
surrounded by lipid envelope containing glycoprotien spikes

35
Q

Describe how HIV particles are able to enter T helper cells.

A

● {glycoproteins / GP120} on the surface of the virus bind to (CD4) receptors on the surface of the T helper cells
● viral envelope fuses with cell membrane of T helper cell releasing viral RNA into the cell

36
Q

Explain why a mutation in the gene coding for CD4 receptors may make someone immune to HIV infection

A

● CD4 receptors are the receptor that HIV binds to via a glycoprotein (GP120)
● a mutation may result in a different amino acid put into the polypeptide chain when CD4 receptors are made, changing their shape
● this means HIV’s glycoprotein is unable to bind with the CD4 receptor
● so viral RNA cannot enter the cell

37
Q

Explain why the destruction of T helper cells causes the symptoms of AIDS.

A

● a lack of T helper cells reduces cytokine production
● therefore reducing { cloning / activation } of B cells
● which reduces antibody production
● so there is an increased risk of opportunistic infections

38
Q

Name the enzymes found in HIV

A

● Reverse transcriptase
● Integrase
● HIV Protease

39
Q

Describe how HIV replicates

A
  1. Attatchemtn via gycoprotiens to CD4 receptors on T helper cells
  2. Fusion with cell membrane releases ssRNA and viral enzymes into host cell.
  3. Reverse transcripttase transcribes ssRNA into DNA.
  4. Viral DNA intergrates into host genome using intergrase
  5. Proviral DNA transcribed to produce mRNA along with host DNA.
  6. mRNA is translated to produce joined viral protiens
  7. Viral protease cuts multiprotein chain down into enzymes, capsomeres (proteolysis)
  8. Capsid assembles around enzymes and viral RNA
  9. Budding forms a new envelope around the capsid and releases new virion.
40
Q

List the four groups of virus as classified by the Baltimore system and give an example of each.

A

dsDNA viruses - λ (lambda) phage
negative ssRNA viruses - Ebola virus
positive ssRNA viruses - Tobacco mosaic virus
ssRNA retroviruses - HIV

41
Q

Contrast the action of the (+)ssRNA in the common cold virus with the RNA found in HIV

A

● reverse transcriptase in HIV, no reverse transcriptase in cold virus
● DNA formed in HIV, no DNA formed in cold virus
● provirus formed in HIV infection, no provirus formed in cold virus

42
Q

Explain how antiviral drugs work

A

Viruses are not living cells and so antivirals must work by inhibiting virus replication by:
● targetting the receptors by which viruses recognise / attach to the host cells
● targetting the enzymes that help replicate or transcribe the viral DNA or RNA
● inhibiting the protease enzymes thhat enable new virus particles to bud from host membranes

43
Q

Explain why ‘cocktails’ of drugs are given to HIV patients

A

● Different drugs target different steps in replication cycle, so using multiple drugs is more effective at reducing HIV replication
● HIV has a high mutation rate, sand some mutations may provide resistance to a drug.
● However the chance of a HIV becoming resistant to multiple drugs at the same time is very small because using multiple drugs kills the virus before it gets a chance to mutate.

44
Q

Explain how a mutation in the gene that codes for reverse
transcriptase could result in resistance to the non-competitive reverse
transcriptase inhibitor Nevirapine

A

● A change in the base sequence results in a change in amino acid sequence of
the polypeptide,
which when folding into reverse transcriptase could result in a different shape of
the enzyme.
● This may result in Nevirapine not being able to bind to reverse transcriptase at its
usual (allosteric) site,
therefore it would not block it from working.

45
Q

Explain how viral disease outbreaks are controlled

A

Viral diseases are very difficult to treat.
Therefore, disease control focuses on mass vaccination and preventing spread of
the virus.

46
Q

State the meaning of the term virulent microgranism

A

A bacterium / virus / fungus / protist that is disease-causing

47
Q

How is Ebola virus transmitted?

A

Ebola is spread to humans from animal faeces, urine, blood and meat of infected
animals
It spreads between humans by direct contact of the skin / mucous membranes with
an infected person’s bodily fluids (e.g. blood, faeces, and other secretions)

48
Q

Mortality rate for Ebola ranges from 50-90%. What factors effect this
mortality rate?

A

● Strain of the virus
● Health of the infected person
● Speed infected people get detected
● Speed of receiving health care

49
Q

Explain the main ways in which the spread of an infectious disease can be controlled

A

● Mass vaccination programme: stops transmission between people as makes
people immune so they do not contract the disease if they encounter an infected
person.
● Early detection of disease: reduces case numbers as infected people are put
into isolation before transmitting the disease.
● Nursing in isolation prevents spread among family and wider community.
● Practicing good hygine e.g. regular handwashing prevents, careful disposal of
infected bodily wastes, and disinfection of surfaces: prevents person-to-person
transmission and transmission from contaminated surfaces.
● steriling or disposal of equipment and bedding after use Stops infection via bodily fluids left on equipment / bedding.
● Extreme barrier nursing health workers wearing PPE including gloves,
facemasks, full body suits, goggles and gloves to prevent contact with any bodily
fluids.
● Identify contacts ensures people who have been in contact with infected people are treated and/or isolated rapidly if they show signs of the disease.

50
Q

Explain why it was particularly difficult to contain the spread of the Ebola virus in West Africa in 2014.

A

● No mass vaccination programme: there was no fully tested vaccine available
at the time of the 2014 Ebola outbreak so thousands of people were infected.
● no early detection of disease: it took a long time to detect the disease and set
up effective testing regimes.
● Difficult to nurse in isolation as sealed isolation units are rarely available in
developing countries such as in West Africa, so their poor health infrastructure
meant those infected were being cared for by family members, who consequently
caught the disease.
● Burial rituals were a major transmission route at the start of the epidemic as
bodily fluids from deceased people came in contact with family members as they
washed them during burial rituals.
● Use of unsterilised equipment one of the main transmission routes for Ebola
at the beginning of the epidemic was through unsterilised needles used in an
antenatal clinic.
● High population density in citirs with poor sanitation meant the disease
could spread quickly witin a commmunity

51
Q

Describe the methods that were used to prevent the spread of Ebola.

A

● Testing regims were eventually set up to detect the disease early.
● Eduction on burial practices meant communities accepted that these had to
change, and bodies were sealed in plastic bags and buried immediately after
death.
● Healthcare workers and burial teams wore full including gloves, facemasks, full body suits, goggles and gloves to prevent contact with any bodily fluids.
● Infected people were isolated in mobile field hospitals and treated by
healthcare workers from NGOs
● Equipment and bedding in isolation wards was sterillisted or disposed of
● Identification of contacts ensured people who had been in contact with
infected people were treated and/or isolated rapidly if they showed signs of the
disease.
● Increased border controls put in place.

52
Q

What features of a new drug are determined during tests and trials
before a doctor can prescribe them?

A

Safety / Side-effects
Efficacy (effectiveness)
Dosage

53
Q

State what is meant by the term side-effect

A

Unintended effect of a medicine, which may be harmful.

54
Q

Outline the stages in testing a new drug

A

Early phase research- using computer modelling to model potential effects of the drug
Preclinical trials- drug is tested on human tissues in a laboratory, before being tested on animals
Clinical trials:
- Phase 1 involves a small group of healthy individuals to identify any
side-effects.
- Phase 2 will be done on a larger group of ill patients to determine the efficacy
and dosage of the drug. Usually involves placebos
- Phase 3 involves comparing the drug to existing drugs to see if it works any
better. A large number of patients are split into two groups, each of which receives
either the new drug or the existing one.

55
Q

What is a placebo?

A

A treatment that looks exactly like the drug but contains no active ingredients
Used to eliminate the possibility of the placebo effect

56
Q

What is the placebo effect?

A

This is where a patient will show improvements in their health due to the that they are receiving the drug.

57
Q

Explain the role that a placebo has in producing valid conclusions.

A

A placebo provides a control group for comparison, therefore ensuring that the
active ingredient / drug is causing the effect and not a psychological effect

58
Q

Explain the role that a double blind trial has in producing valid
conclusions.

A

Double blind means that neither the patient or the doctor knows who has been
given the actual drug, therefore it removes bias from the trial

59
Q

Evaluate the ethical implications of using untested drugs/vaccines
during epidemics.

A

Reasons for -
● disease has high mortality
● new drug/vaccine is unlikely to affect other people
● may help develop the drug/vaccine for other patients
● during severe epidemics / pandemics thousands of lives are at risk
● new untested drugs may be the only treatment available

Reasons against -
● unknown side effects
● patient may not be able to provide informed consent
● who decides who can be treated if the drug/vaccine is in limited supply
Conclusion
If disease has high mortality, e.g. Ebola, then benefits outweigh risks

AS Biology (7 decks)

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