Volume 28, Number 1, September 2015
Activity
Viruses student worksheet
Martin Rowland
1 Viruses are often described as:
(a) non-cellular
(b) obligate intracellular parasites
Use your knowledge of biology to suggest why each of these terms is appropriate.
2 Construct a table of your own design to summarise the five types of virus covered in the
presentation. For each, show the presence or absence of an outer envelope, the nature of the nucleic
acid core and the nature of the cells infected.
3 We have used the term ‘type’ to describe viruses.
(a) Explain why it is not possible to refer to a species of virus.
(b) Suggest a system that could be used to classify the different types of virus.
4 The influenza virus has a fragmented genome. Explain what this means.
5 Explain why viruses are specific in the host cells they infect.
6 Cold sores around the mouth are caused by the herpes simplex virus (HSV). The sores typically
heal within 2–3 weeks. Suggest why some people are prone to repeated outbreaks of cold sores in the
same area of the mouth.
7 Read the following passage.
Guinea is one of the countries in West Africa in which there has been an Ebola epidemic.
To date, there have been 3786 cases and 2520 deaths reported in Guinea.
In July 2015, a peer-reviewed paper reported that a new vaccine against Ebola had been
successfully trialled in Guinea. The trial had involved over 4000 people and was described
as remarkable because it had been completed in just 12 months, whereas clinical trials
normally take more than 10 years.
The usual method of testing a vaccine involves recruiting volunteers from a population at
risk and, using a double-blind design, vaccinating half of them with the trial vaccine and
giving the other half a placebo. In the Guinea trial, a ‘ring’ design was used. In this design,
when researchers discovered an outbreak of Ebola, they visited the affected village and
vaccinated all the willing family, friends and neighbours of the sick. Children and pregnant
women were excluded from the vaccination.
The results showed that, 10 days after they had been vaccinated, none of the villagers
developed symptoms of Ebola infection.
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5
10
14
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Use information from the passage and your own knowledge to answer the following questions.
(a) Explain the importance of a scientific paper being ‘peer-reviewed’ (line 3).
(b) The trial had involved over 4000 people (line 4). Explain the importance of using this number of
people in the trial.
(c) In Guinea, the trial was completed in a much faster time than is usual for clinical trials (lines 5–6).
(i) Give two reasons why clinical trials normally take more than 10 years.
(ii) Suggest why it was decided to complete the trials in Guinea so quickly.
(d) What is a ‘double-blind’ design (line 8)?
(e) Suggest two features of a placebo that would be appropriate in this double-blind investigation
(line 9).
(f) Suggest why children and pregnant women were excluded from the trial (lines 11–12).
(g) The researchers tested people for signs of Ebola infection 10 days after vaccination (lines 13
and 14). Suggest why they chose a period of 10 days.
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www.hoddereducation.co.uk/biologicalsciencesreview
Answers
1 (a) Cells, whether prokaryotic or eukaryotic, possess a cell-surface membrane and cytoplasm
containing ribosomes. Viruses lack these structures.
(b) A parasite is an organism that lives in, or on, another organism causing it harm but itself deriving
benefit. Although not organisms, viruses ‘live’ in a host and cause harm to it. Since viruses are
replicated inside cells of the host, they are intracellular parasites. The term ‘obligate’ tells us that being
an intracellular parasite is the only way in which viruses can be replicated (there is no alternative).
2 A possible table is shown below.
Virus
Envelope
Nucleic acid core
Cells infected
Ebola virus
Absent
ssRNA
Many different human cells,
including liver cells,
macrophages, fibroblasts and
dendritic cells
Human immunodeficiency
virus
Present
ssRNA
T helper cells and
macrophages
Influenza virus
Present
ssRNA
Epithelial cells of nose, throat
and lungs of humans
λ phage
Absent
dsDNA
Escherichia coli
Tobacco mosaic virus
absent
ssRNA
Plants, especially tobacco
plants
(a) A classical definition of a species is a group of organisms that have the potential to interbreed and
produce fertile offspring. Viruses are not living organisms and are not capable of reproducing.
(b) A sensible suggestion would be to use the shape of the virion and any of the features in the table
above.
4 The genome is the total gene complement. The genome of the influenza virus is divided into eight
short strands of RNA, hence fragmented.
5 Glycoproteins on the surface of the virion are complementary only to those on one type of cell. Only
if there is a collision between complementary glycoproteins at an appropriate angle and the
appropriate speed can a virus attach to the surface of any cell.
6 Following infection, the HSV remains dormant (in the facial nerves) in the same area of the mouth in
which the cold sore appeared. Reactivation of the virus (often caused by physiological stress) results
in cold sores in the same area.
7 Suitable answers will include the following points.
(a) ‘Peer-reviewed’ means that the report of the research has been evaluated by other scientists in the
same field of research. Readers can, therefore, be confident that the methodology was valid, the data
are reliable and the conclusions are based on appropriate statistical analysis of the data.
(b) Using a large number of people ensures that any anomalous data have a small impact on the
outcome of the research.
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(c) Suggestions might include the following:
(i) Trials are first carried out in the laboratory using, for example, cell culture. They are then
carried out on animals, which involves application for a licence. If successful, they are then carried out
on a small group of volunteers, often under hospital conditions. Finally, trials are extended to a large
scale following approval from government health departments.
(ii) There had been a large number of deaths from Ebola infection (over 2500 in Guinea and
many more in other West African countries). The authorities (including the WHO and the Guinean
government) decided the risk of the vaccine was less than the risk of further deaths from the Ebola
virus.
(d) Neither the person administering the treatment nor the person receiving it knows whether it
involves the active drug/vaccine or the placebo.
(e) It would have the same colour/consistency/viscosity/diluent/flavour (it was actually injected but
students cannot deduce this from the passage) as the vaccine.
(f) The effect of the vaccine was unknown. Neither a fetus nor a child is able to make an adult
decision/is legally responsible, so it would be unethical to expose them to the vaccine.
(g) Ten days is long enough for symptoms of Ebola infection to appear and is long enough for the
primary immune response to occur.
This resource is part of BIOLOGICAL SCIENCES REVIEW , a magazine written for A-level students by subject experts.
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