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Unit 5: Scientific Article for use with Question 7
Paragraph
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Task
The article gives three sources of variation – are there really three?
There are two. Upbringing is a part of our environment.
Why does the organism have a name in two parts?
Genus species are part of a hierarchical classification system. They are used to ensure that
every species has a unique name.
Can you place protozoa in the modern classification system?
Well, you don't need to do this further than recognising that phylogenetic classification
using molecular differences places these organisms in the eukarya.
What features would you expect protozoan cells to have?
Genetic material enclosed in a nucleus and membrane bound organelles.
Write down the 4 stages that lead to changes in allele frequency due to natural selection.
There are many opportunities for a question about natural selection in the passage so,
make sure that you can suggest appropriate causes of variation, suitable struggles for
existence, features that may result in differential survival by some individuals and a
resulting change to allele frequencies in the population.
What is a pathogen? Which two pathogens should you be an expert on?
A pathogen causes disease. It is one side of a symbiotic relationship called parasitism
where there is a winner and a loser. You should be an expert on TB and HIV by now.
Which of these pathogens carries out replication in a manner that is most like the
description given in the paragraph?
The TB bacterium is taken into macrophages in a vacuole and it can reproduce in this. HIV
inserts its RNA and a couple of useful enzymes into T helper lymphocytes so, on balance I
think that it is more like TB.
Suggest the advantage of shedding oocysts in faeces. What type of adaptation is this?
This seems like a good way to get the parasite back to the soil stage in its rather
complicated life cycle. While it will involve physiological and anatomical adaptations, I am
tending towards behavioural. It was probably a dumb question.
Give 4 different ways in which sexual reproduction leads to greater genetic variation.
The parasite uses the cat for the sexual phase of its life cycle. It spends a lot of its time in
the soil and other animals as haploid, vegetative parasites but carries out fertilisation to
spend a short time as diploid in the cat’s intestine before meiosis returns it to the haploid
phase again. So, fertilisation, crossing over, independent assortment and yes that is only
3! I probably had mutation in my head but this is not a consequence of sex just of cell
division.
Explain the advantage to the pathogen of having a sexual phase to its life history.
The sexual phase may increase the genetic diversity within the parasite gene pool but only
if two parasites with at least some different alleles fuse together. Some organisms benefit
from being diploid as this allows recessive alleles that are less-favourable at that time to
hide behind a dominant allele. However, since the parasite lives most of its life in haploid
form, this will not be of particular benefit. However, the sexual phase in the cat releases a
resistant Oocyst which provides effective transfer via the soil into other animals.
Explain why the muscle and brain are places that are relatively safe from discovery and
action from the body’s immune system.
When the parasite is in cysts inside cells it is less “visible” to immune system cells. The
cysts are protected inside their enclosing vesicle in the cells and so proteins from it are
less likely to be presented to lymphocytes by the host cells. Also, the brain is distanced
from the blood system being at one side of the blood-brain barrier.
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Give the advantage of having a “tough cell wall” around each cyst and suggest why the
phrase is not particularly precise.
Well, it isn’t a cell wall in the sense of a bacterial, fungal or plant cell wall. It is a vesicle,
strengthened with proteins that discourage the host cell from destroying it with
lysosomes.
Find a diagram that summarises the parasite’s life cycle. Annotate it with suggestions for
the advantages / functions of each stage.
Life cycles aren’t explicitly on spec. so you may just be interested in how it moves from
various different animals (including humans) via soil to cats and back again.
Describe the nature of the rat and mouse behaviour where they avoid areas with cat urine
(clue: is it likely to be a reflex? What should you know about reflexes?)
Remind yourself about the Stimulus > receptor > motor neurone > relay > sensory
neurone > effector > response pathway. It is involuntary and fast.
Suggest an advantage to the parasite of having two different hosts.
They do different jobs, rapid reproduction increasing the numbers and sexual
reproduction increasing variation. Perhaps it is best not depending on a single host by
being too specialised; this way leads to extinction when the specific host becomes extinct.
Use the information in this paragraph to describe the process by which ancestors of the
parasite may have developed to become less pathogenic to mice.
Those variants that caused greater disease in mice would be less likely to end up in a cat.
Their alleles would be less likely to be recombined by the sexual stage and so less
variation might make them more susceptible to environmental change e.g. a more
effective host immune system in a host. This wouldn’t prevent the dead animal releasing
the cysts in its cells into soil as they decay however. It seems to have all ways covered.
Use the information in this paragraph to describe the process by which ancestors of the
parasite may have developed to cause the specific behaviour changes to only cats.
Those parasites that changed behaviour of mice to other predators would be less likely to
be eaten by cats (falling prey to other predators) and so may not enter the sexual stage in
the life cycle and may be in an animal that is less prone to distributing its faeces so widely.
I know about this, I have a cat!
In which lobe of the cerebral hemispheres is the amygdala found?
Temporal lobe.
Describe two ways by which the functions of the amygdala in mammals may have been
identified in that region.
fMRI, PET scanning, damage leading to loss of function, direct electrical stimulation.
Suggest how the parasite may have two genes that are related to mammalian genes.
Common ancestry, mutation of multiple copies of the same gene, retrovirus gene.
What type of substance is dopamine?
Neurotransmitter.
Explain how the reward pathways mentioned are implicated in the reinforcement of
activities and addiction.
Animal does an activity, results in production of dopamine in an area of the brain which
results in higher frequency of impulses in reward pathways, these pathways are used for
survival behaviours such as responses to foods and they reinforce actions that are an
advantage – although clearly only to the parasite in this case!
Which brain disorder have you learned about that is also connected with dopamine?
Parkinson’s
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Explain why the parasite produces more severe symptoms with individuals with a
weakened immune system.
The parasite will be able to infect more cells and increase its population in the body if the
immune system is unable to defend the body through the development of active
immunity.
Which components of the immune system are likely to be key to a response to infection
by the parasite?
T helper cells. Macrophages.
Give two reasons why the effect on a foetus is likely to be significantly greater.
The immune system is undeveloped and damage to cells in developing organs is more
likely to lead to more severe symptoms. Mum’s immune system is also depressed, so the
foetus is not rejected.
Suggest which other infection may be expected to result in more severe symptoms from
this parasite and explain why.
HIV as it reduces the number of T helper lymphocytes.
Explain the difference between correlation and cause.
Correlation describes a link between two factors where the change in one is matched with
a change in the other. A causal relationship is where a biological mechanism explains the
link.
Suggest two alternative explanations other than that the parasite causing changes to the
brain that may lead to schizophrenia.
Genetic, drugs, social isolation, pre-natal damage to the brain.
The pathogen is thought to gain entry to a brain cell by forcing itself through the cell
membrane. This results in the parasite surrounded by cell membrane in the cytoplasm.
What properties of the host’s cell membrane allow it to enter and form a vesicle in this
way?
It is fluid due to the phospholipid bilayer.
What is the advantage to the parasite of being surrounded by the host cell’s membrane?
It forms a barrier to the host’s defence mechanisms. It may allow the parasite to ‘hide’
from the cell.
Suggest how the parasite may further protect itself inside the vesicle / vacuole.
Production of proteins that further reinforce the membrane. Produce proteins that
interfere with the host cell’s metabolism.
Suggest a cell organelle whose function may be inhibited by the parasite, reducing the
cell’s capacity to attack it.
Lysosome. Golgi apparatus.
Suggest two different ways that the parasite could interfere with the functioning of a
brain cell so that the amount of dopamine it produces is altered.
Production of proteins that either act as negative transcription factors or through
complementary attachment, disable transcription factors. They may also affect the
production of transport vesicles that deliver the transmitter.
The pathogen may eventually lead to the death of cells by apoptosis. What is this process?
Programmed cell death in response to a harmful factor e.g. mutation, entry of a pathogen.
Explain how the process acts as a defensive mechanism in the event of significant changes
to a cell.
A number of metabolic pathways are activated that lead to the death of the cell and
therefore the parasite.
Explain how the three Type strains mentioned may have arisen.
Mutation of specific alleles. Isolation.
Suggest why all three strains produce the same behavioural effect on mice.
Common ancestor had evolved this behavioural effect.
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What is the conclusion that was developed based on the information in paragraphs 12 to
14?
Changes to mouse behaviour is caused by the parasite before it forms cysts.
Use your knowledge of brain cell structure to suggest 3 different ways in which the “hard
wiring” mentioned could have taken place leading to the behaviour being permanently
maintained in the mice.
May involve changes to synapses (e.g. transmitter release, calcium or sodium
permeability, receptor protein production, reuptake or hydrolytic enzyme activity) or
possibly development of myelin sheath around axons.
What are the benefits and risks associated with using mice as models to help us
understand human diseases?
Benefits: mammal so some similarity with human chemistry, various ‘models’ already
developed by GM, fewer ethical concerns compared with primates.
Risks: biochemistry is not sufficiently like that of humans, welfare concerns, economic
concerns related to the profitability of any resulting treatments.
What are the ethical problems that result from this type of use of animals in research?
The balance between inevitable suffering and the possible beneficial outcomes, concerns
from various animal welfare and activist groups.
Suggest the nature of the “genetic tools” mentioned.
Genetically modified mice with additional ‘human’ genes or genes that have been
‘knocked out’ / silenced to produce a feature that can be used in the study.
Describe one method by which the mice may have been modified.
Identify gene, extract gene, restriction enzymes, multiply e.g. by bacterial plasmid,
insertion into vector, addition of ‘marker’ gene, modification of mouse genome.
Suggest the mechanism within the mice that was modified to prevent the production of
cysts.
e.g. production of proteins that lead to endocytosis.
Explain the evidence that suggests that free-range farming of livestock may lead to a
higher rate of Toxoplasma infection than more intensive, indoor rearing.
Free-range livestock are more likely to contract the parasite from soil and so transmit it.
e.g. meat containing cysts.
Suggest why this may be an acceptable increase in risk.
The balance between animal welfare and human health. Low-level, asymptomatic in most.
When researchers replicated the effect produced by the wasp species, describe the key
part of the experimental design that would provide valid evidence that they have worked
out the mechanism.
Their experiment would be validated by a suitable control.
Suggest what the “antidote injection” may contain and explain how it would release the
cockroaches from their zombie state.
It could be an enzyme that only hydrolyses the substance in the sting or a protein with a
complementary shape to the active ingredient in the sting.
Why is it only the female wasp that is able to sting the cockroach?
Because the female lays eggs. There is no benefit to the male other than if it used the
cockroach for food and like many insects, it is the larvae that do the feeding and growing.
Once the single egg hatches, the wasp larva inside the cockroach eats the insect’s organs
in a particular order. Suggest the advantage of this behaviour.
To keep it alive for as long as possible and therefore extend the feeding period. Nice!
One of the compounds in the wasp’s sting is also produced by some plant species. What is
the advantage to a plant in producing this compound?
It may discourage the plant from being eaten by herbivores – a physiological adaptation.
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This compound may be one that could have clinical benefits. Outline the process which
would be used to test this compound with a view to its use as a medical treatment for
cardiac arrest.
Tested for effectiveness on target molecules, model cells and model animals; on lab
animals to test for safety and suggest a starting dose for clinical trials; phase 1 trials on a
few volunteers to check dosage and safety; phase 2 clinical trial with a randomised, case
controlled design comparing the new drug with current alternative or placebo; phase 3
clinical trials with larger population of patients to check for safety, long-term side effects
and effectiveness.
Using what you have learned of the methods used to investigate the process of the
development of vision in mammals, give a suitable way that the target of the second sting
could be identified.
Radioactive or fluorescent tracers that indicate the pathway to the brain.
Use your knowledge of synapse function to suggest one way that the toxin in that sting
could “block a key chemical messenger”.
Block receptors or bind to transmitter to prevent binding.
Describe the innate escape response in the cockroach that is prevented from taking place.
Stimulus: wasp, Receptors: light sensitive cells in eyes; sensory neurone; central nervous
system; motor neurone; muscle in limbs; Response: run away.
In humans, which two parts of the brain are responsible for the coordination of muscle
groups associated with walking?
Cerebellum and motor cortex in frontal lobe of cerebrum.
Explain how the wasp venom is able to allow the cockroach to walk but not attempt to
escape from the wasp.
Control of movement follows a different set of pathways from the escape response.
What can you deduce about the shape of the compound in the wasp venom and the
compound the researchers used to provide evidence of the venom’s mechanism of
action?
Complimentary shape.
The changes to the behaviour of carpenter ants infected by Ophiocordyceps fungus are an
example of an extended phenotype. What is a phenotype?
The observable characteristics of an organism.
Suggest which organism’s phenotype is being observed.
Ophiocordyceps (not the ant’s).
Describe in outline the process that produces several different species of Ophiocordyceps,
all with the life cycle described in the passage.
Isolation of populations e.g. ecological, geographical > different environments provide
selective pressure or genetic drift over time i.e. insect hosts with differing behaviours or
habitats > natural selection > development of sufficient differences so that fungi cannot
reproduce successfully with other populations.
Explain why information published in a journal such as BMC Ecology is likely to be
considered as reliable.
Peer review process.
Give one other way in which scientists may report the results of their studies.
At conferences bringing together subject specialists.
Give two features that separate fungi from other eukaryotic organisms.
Chitin cell wall, molecular genetics.
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Suggest an advantage in the fungus’ effects, causing the host ant to bite down on a leaf at
around noon and in a roughly North-western direction.
Fixed at a height that will distribute spores effectively.
Suggest how a so-called “clock” gene could produce an effect that is timed in the way
described.
Genes produce protein product (e.g. transcription factor) that increases in concentration
until it is at a critical level that causes the effect after a period of time.
What is the role of mitochondria in muscle tissue?
Oxidative phosphorylation / production of ATP / complete oxidation of glucose.
Use your knowledge of the muscle contraction cycle to explain why fewer mitochondria
will produce the effect described in the passage.
On each cycle of interaction between actin and myosin resulting in shortening of the
muscle cells as the proteins overlap more, ATP releases the myosin ‘heads’ from binding
sites on actin and ‘resets’ the position of the head molecules to then bind to actin further
along the length of the protein so that the proteins further overlap in the next cycle.
What is the role of the sarcoplasmic reticulum in muscle tissue?
To act as a store and rapid source of calcium ions.
Explain how loss of this organelle affects muscle contraction.
Calcium ions are not stored in the sarcoplasmic reticulum, increasing the concentration in
the cytoplasm of muscle cells resulting in continuous cycling of the actin and myosin
interactions and shortening of the muscle fibres.
Which type of organism causes lockjaw in humans?
Bacterium. Tetanus.
In what way is the same effect caused in a different way from the so-called “lockjaw” in
the ants?
The toxin affects the neurones controlling muscle contraction.
Describe how the individual cells of the “yeast” stage of the fungus carry out nutrition.
Absorption of molecules from the fluids surrounding the insect’s organs.
Suggest how the cells cause their effects on the brain and muscle cells if they do not enter
the cells.
Produce compounds that diffuse into the affected organs from the insect’s body fluids.
Insects have an ‘open’ blood system; the organs are bathed in a fluid that contains
nutrients.
Suggest a mechanism by which the movement of water in or out of the fungal cells results
in spores being fired into the soil below.
Water potential of cells decreases (less free water) > water enters by osmosis through
partially permeable membranes > volume increases > cell walls resist expansion >
increasing pressure > cell walls break > contents are projected by forces due to the sudden
release of pressure.
It isn't clear from the passage if the fungus has a sexual stage in its life cycle or if the spore
production is asexual. Give two advantages for the reproduction of the fungus being
entirely asexual and one disadvantage.
Reproduction may be faster, have greater numbers for the same resource inputs and all
individuals are a clone; adapted to their environment. However, little variation is
produced and so a change in the environment may cause extinction.
If there is a sexual stage to the life cycle, it is likely that the fungus is haploid for much of
its life cycle. What does haploid mean.
Each cell has one copy of each chromosome and therefore one copy of each gene.
If spores from two different individual fungi of the same species are picked up by the
same foraging ant, describe how this may lead to variation in the spores that are then
produced.
May have a sexual phase in which haploid cells combine and then divide by meiosis to
produce a new generation.
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The majority of ants in a colony are unable to reproduce. Explain how their role in the
colony ensures that their genes are successfully passed into the next generation.
They share many of the same genes as the queen and males that can reproduce. They
assist the survival of these individuals and therefore their own alleles.
In what way can the relationship between the Ophiocordyceps fungus, the ant and the
hyper parasitic fungus that feeds on the Ophiocordyceps fungus be considered as an
“arms race”?
A change in the biology of one will affect the environment of the other leading to a
different selective pressure and drive natural selection to increase allele frequency for
features that result in a change to the other species. This then provides a change to the
environment that is likely to result in a change in allele frequency in the other organism.
Suggest why the Ophiocordyceps fungus can never win this “arms race”.
If it causes the extinction of the specific ant species it uses, it will become extinct as it is
unable to infect other ant species. Its biology is so specific to that ant species that it is
unable to adapt quickly to survive outside of its host.
Give 3 measures of biodiversity.
Species richness (number of different species), species evenness (relative population sizes
of different species), genetic diversity.
Give two DNA based methods used to identify two organisms as different species.
Genetic fingerprinting, DNA hybridization, sequence of specific gene.
What evidence might suggest that the zombifying fungus developed before many of the
continents split apart?
Its current geographical distribution – over a wide area, on different continents.
Suggest an advantage to the fungal hyperparasite of preventing the zombie-ant fungus
from ejecting its spores.
It reduces the ability of the fungus to disperse and infect ants and therefore reduces ant
mortality which reduces the risk of extinction of specific ant species so, ensuring that the
fungus has a host and the hyperparasite also has its host.
Which features of an ecosystem are demonstrated by the relationships in the paragraph?
Its biotic environment.
Explain why monoculture agriculture is considered by many to be unsustainable.
Loss of biodiversity and damage to soil fertility means that use of the forests for timber or
short term agriculture reduces the potential of the area to produce these useful resources
in the future.
Give 3 challenges to global food security from climate change.
Rising average temperatures will affect organisms whose enzymes are adapted for the
current range of temperatures; increasing desertification leads to a reduction of land area
for agriculture; the need for irrigation in warmer climates will affect water security in the
area and also lead to increased evaporation and increased salinity of soils; rising sea levels
may reduce usable coastal farmland; migration of communities may put pressure on
existing agricultural systems.