topic 2.2

Question 1

what are viruses?

Answer 1

Viruses are non-living structures - arrangements of genetic material and protein that work by invading other living cells and taking over their biochemistry to make more viruses.

Question 1

are viruses living or non-living?

Answer 1

non living as they also have no mechanism or cytoplasm and can’t self-replicate.

Question 2

why are viruses sometimes classed as living organisms?

Answer 2

as they can reproduce and they change and evolve in a adaptive way

Question 3

why are viruses difficult to treat?

Answer 3

As they’re non living -so we cant kill them.

Antivirals must work by inhibiting
virus replication.

The focus of disease control should be on preventing the spread ( eg the 2014 Ebola outbreak in West Africa)

Question 4

how are viruses classified?

Answer 4

according to structure and nucleic acid types

Question 5

name 3 type of viruses with examples

Answer 5

1. DNA virus ( λ (lambda) phage )
2. RNA virus ( tobacco mosaic virus and
   Ebola )
3. RNA retrovirus ( HIV - human immunodeficiency virus)
   The way in which the viral genetic material is used in host cells to make new viruses depends on which type of virus it is.

Question 6

how do viruses attach to host cells?

Answer 6

through specific protein antigens - Virus Attachment Particles (VAPs) that target proteins in host cell’s membrane

Question 7

DNA virus

Answer 7

genetic material - DNA
nucleic acid
the viral DNA acts directly as a template for mRNA transcription + DNA replication

Question 8

RNA virus

Answer 8

genetic material - RNA
70% viruses have RNA genetic material
more likely to mutate than DNA viruses

majority contain single strand of RNA - ssRNA
Positive ssRNA have RNA which can be directly translated into proteins by ribosomes as act directly as mRNA
EG of positive ssRNA - tobacco mosaic viruses

Negative ssRNA viruses cannot be directly translated.
The RNA strand must be transcribed to produce MRNA before its translated at ribosome,
EG of negative ssRNA - Ebola, measles, influenza

Question 9

RNA retroviruses

Answer 9

single strand RNA
Have protein capsid and lipid envelope

single strand of viral RNA directs the synthesis of the enzyme - reverse transcriptase - which makes a DNA molecule corresponding to the viral genome.

This (double stranded) DNA then incorporated into host cell DNA and used as template for new viral proteins + viral RNA genome.
EG of RNA retrovirus - HIV

Question 10

viral structures for RNA virus
a. envelope?
b. capsid?
c. capsomeres?

Answer 10

a. coat around the outside of virus - made from lipids in the host cell

b. the protein coat of a virus

c. the repeating protein units making up the capsid

Question 11

virus reproduction

Answer 11

attach to other living organisms as can only reproduce inside cells
attach host cells
bacteriophages attach to bacteria by injecting genome into it but bulk of viral material remains outside bacteria forming plasmid within bacteria

Question 12

latency/lysogenic pathway

Answer 12

non-virulent (not disease causing) virus injects DNA into host cell DNA as provirus.

Viral DNA replicates each time host cell divides.

Virus produces repressor proteins to inhibit transcription = MRNA not produced.

virus does not affect host cell. at this part when virus is part of the reproducing host cells = virus is latent.

latent virus enters lytic pathway when host cell is damaged or immune system weakens. amount of repressor decreases.

Question 13

lytic pathway

Answer 13

virulent (disease causing) virus injects nucleic acid into host cell cytoplasm.
viral genetic info replicates immediately, independently of host cell DNA

Many viruses are assembled and eventually bursts host cells - releasing loads of new virus to invade other cells
- cell lysis.

Question 14

how can viruses cause disease in cycle?

Answer 14

through the cell lysis of host cell - can cause host cells to release their own lysosomes + digest themselves from inside or by production of toxins than inhibit cell metabolism

Question 15

how can viruses spread from one organism to another?

Answer 15

infected mucus
droplets of saliva
infected blood or faeces
simple contact between infected organisms

Question 16

how did Ebola spread?

Answer 16

Ebola virus is an animal disease.
spread to humans through faeces, urine, blood or meat of infected animals.

then spread from person to person by direct contact of skin, blood, faeces , body fluids from infected person.

Question 17

how do antivirals work?

Answer 17

cant affect actual virus particle itself, instead it targets virus replication.

• target receptors/antigens to stop the virus from recognising host cell and therefore prevent entry
• target the enzymes that help translate or replicate the viral DNA or RNA

Question 18

what is mortality rate and what does it depend on?
Ebola?

Answer 18

measure of number of deaths caused in a given population due to specific cause ( disease )

Depends on strain of virus, health of infected person, how fast they get help from health care

Ebola -highly infectious viral disease causing fever ad internal bleeding and death sometimes
mortality rate - around 50% (25-90%)

Question 19

controlling viral infections - vaccines

Answer 19

when vaccinated - become immune to disease - will not get infected if encounter it

very young and elderly and health care workers usually get vaccinated first in epidemic as there’s rush to deliver vaccines

Question 20

how can the spread of disease be controlled?

Answer 20

• nursing in isolation so they dont come in contact with other ppl that are healthy
• preventing transmission from 1 person to another through regular hand-washing, before + after contact with patients, careful disposal of infected bodily wastes, and frequent disinfecting surfaces
• wear protective clothing by health workers and public, face masks, gloves, goggles, gowns
  -sterilising or disposing of equipment or bedding after use.
• identifying contacts of ppl who’s infected so they can be treated or isolated fast

Question 21

development of new medicine

Answer 21

Pre-clinical testing - done in a laboratory using cells, tissues and live animals.
[Efficacy, toxicity and dosage are tested at this stage]

Clinical trials - use healthy volunteers and then patients.

Very low doses of drug given at start of the clinical trial.

If the drug is found to be safe = further clinical trials are carried out to find the optimum dose for the drug.

In double-blind trials, some patients are given a placebo - double-blind study.

Question 22

Ebola outbreak - how was it dealt with

Answer 22

outbreak in late 2013 in West Africa.
took until mid 2014 for world to recognise severity of it.
WHO looked for ways to fast track vaccines already in development.
the vaccines has not completed human trials yet

Question 23

factors to consider when deciding whether a drug or vaccine should be fast tracked for use in a epidemic

Answer 23

• severity of disease
• availability of other treatments
• effectiveness of standard disease control measure for spread of it
• informing public and getting consent
• freedom of choice

Question 24

ethical implications of using untested drugs

Answer 24

• not ethical to use under any circumstances as they have not completed full human trials
• unexpected side effects which could worsen situation
• informed consent is an issue, depends on level of education to understand drug and how it works
• deciding who gets the drug/vaccine first = difficult. (local people vs health workers)