Revision notes for Edexcel A A Level Biology Brain chemicals, personalised medicine and GM organisms. Open the guide for explanations and worked examples. Written against the Edexcel A A Level Biology (9BN0) specification, so the content matches what's examinable rather than general Biology background.
Brain chemicals, personalised medicine and GM organisms
What you'll learn
- How neurotransmitters allow neurones to communicate at synapses.
- How drugs can alter synaptic transmission and help treat disorders linked to brain chemicals.
- Why genetic variation can affect how well a medicine works for a person.
- How genetically modified organisms can be used to produce medicines.
The starting point: genes, proteins and cell signalling
A lot of this topic comes down to one idea: proteins do specific jobs. In the nervous system, proteins act as receptors, ion channels, enzymes and transporters. In medicine, proteins may be the drug target, the drug itself, or the enzyme that breaks the drug down.
Definition
Gene and allele
A gene is a length of DNA that codes for a polypeptide or functional RNA. An allele is a different version of the same gene, often caused by a difference in the DNA base sequence.
Small DNA differences can alter a protein’s amino acid sequence, shape, abundance or activity. That matters because drugs usually work by binding to proteins.
Key Idea
Why genetics matters for medicine
If a person’s alleles affect a drug target, a drug transporter or a drug-metabolising enzyme, the same medicine may work well in one person but poorly, or dangerously, in another.
Brain chemicals: communication at synapses
A synapse is a junction between two neurones, or between a neurone and an effector such as a muscle cell. Most synapses in the brain are chemical synapses, meaning the signal crosses the gap using a chemical messenger.
Definition
Neurotransmitter
A neurotransmitter is a chemical released from a presynaptic neurone that diffuses across the synaptic cleft and binds to receptors on a postsynaptic cell, changing its activity.
At a typical synapse:
- An action potential arrives at the presynaptic neurone terminal.
- Voltage-gated calcium ion channels open.
- Calcium ions enter the presynaptic terminal.
- Vesicles containing neurotransmitter move to and fuse with the presynaptic membrane.
- Neurotransmitter diffuses across the synaptic cleft.
- Neurotransmitter binds to complementary receptors on the postsynaptic membrane.
- Ion channels open or close, changing the postsynaptic membrane potential.
- The neurotransmitter is removed by enzyme breakdown, reuptake into the presynaptic neurone, or diffusion away.
Excitatory and inhibitory effects
Some neurotransmitters make a postsynaptic neurone more likely to fire an action potential. These are described as excitatory. Others make it less likely to fire. These are inhibitory.
The same basic synapse mechanism can therefore increase or decrease activity in a neural pathway.
Common Mistake
Assuming all neurotransmitters excite neurones
Do not write “neurotransmitters stimulate the next neurone” as a blanket statement. Some neurotransmitters are inhibitory, so the effect depends on the receptor and ion channels involved.
How drugs affect synaptic transmission
Many drugs work because they change what happens at synapses. They may:
- increase neurotransmitter release
- mimic the neurotransmitter at the receptor
- block the receptor
- inhibit enzyme breakdown of the neurotransmitter
- inhibit reuptake, so neurotransmitter remains in the synaptic cleft for longer
Definition
Agonist and antagonist
An agonist binds to a receptor and activates it, mimicking the normal neurotransmitter. An antagonist binds to a receptor but does not activate it, blocking the normal neurotransmitter from binding.
For example, some antidepressants are selective serotonin reuptake inhibitors, or SSRIs. Serotonin is a neurotransmitter involved in mood, sleep and other brain functions. SSRIs reduce the reuptake of serotonin into the presynaptic neurone.
Example
Predicting the effect of blocking reuptake
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Identify the normal process: serotonin is released into the synaptic cleft, binds to postsynaptic receptors, then is removed partly by reuptake into the presynaptic neurone.
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Apply the drug action: if reuptake transporters are inhibited, serotonin stays in the synaptic cleft for longer.
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Predict the synaptic effect: serotonin has more opportunity to bind to receptors, so serotonin signalling at that synapse is increased or prolonged.
Tip
Use a cause-and-effect chain
For drug questions, link drug action → neurotransmitter level in synaptic cleft → receptor binding → postsynaptic effect → symptom or side effect.
Brain chemical imbalances and illness
The phrase “chemical imbalance” is useful, but it can be oversimplified. Brain disorders usually involve complex interactions between genes, neurones, receptors, circuits and environment. However, changes in neurotransmitter signalling are clearly involved in several conditions.
Dopamine and Parkinson’s disease
Dopamine is a neurotransmitter involved in movement, motivation and reward pathways. In Parkinson’s disease, dopaminergic neurones in a brain region called the substantia nigra degenerate. This reduces dopamine signalling in motor control pathways, contributing to symptoms such as tremor, muscle rigidity and slow movement.
A common treatment is L-DOPA, a precursor molecule that can be converted into dopamine in the brain.
Definition
Blood-brain barrier
The blood-brain barrier is a selective barrier formed by capillary endothelial cells and associated cells in the brain. It restricts which substances can pass from the blood into brain tissue.
Dopamine itself does not cross the blood-brain barrier effectively, but L-DOPA can. Once inside the brain, L-DOPA is converted into dopamine, increasing dopaminergic signalling.
Example
Explaining why L-DOPA is used instead of dopamine
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Parkinson’s disease involves reduced dopamine signalling because dopaminergic neurones degenerate, so increasing dopamine activity can reduce some motor symptoms.
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Dopamine is not given directly because it does not cross the blood-brain barrier effectively.
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L-DOPA is used because it can cross into the brain and then be converted into dopamine, increasing stimulation of dopamine receptors in relevant pathways.
Serotonin and depression
Serotonin is involved in mood regulation, sleep, appetite and other functions. Some treatments for depression alter serotonin signalling, for example by inhibiting reuptake.
Be careful: depression should not be explained simply as “not enough serotonin”. The biology is more complex, and changes in receptor sensitivity and neural networks may help explain why antidepressants can take time to improve symptoms.
Common Mistake
Avoid overclaiming causation
If a medicine affects serotonin and improves symptoms, that does not prove the whole condition is caused only by low serotonin. In exams, phrase this as “serotonin signalling is involved in depression” rather than giving an oversimplified cause.
Personalised medicine
Traditionally, many medicines were prescribed using a “one size fits all” approach: choose a standard drug and adjust if it does not work. Personalised medicine aims to choose the most suitable treatment for an individual from the start.
Definition
Personalised medicine
Personalised medicine uses information about a person’s genotype, phenotype and clinical data to guide prevention, diagnosis or treatment, including drug choice and dose.
A closely related term is pharmacogenomics: the study of how genetic variation affects a person’s response to medicines.
A common type of genetic variation is a single nucleotide polymorphism, or SNP. This is a difference at one DNA base position in the genome. SNPs may affect proteins such as:
- drug-metabolising enzymes
- membrane transporters that move drugs into or out of cells
- receptors that drugs bind to
- proteins in disease pathways
Why the same drug can affect people differently
Imagine a drug is broken down by a liver enzyme. If someone has an allele producing a very active version of that enzyme, the drug may be broken down quickly, giving a lower blood concentration and a weaker effect. If someone has an allele producing a less active enzyme, the drug may remain at a higher concentration for longer, increasing the risk of side effects.
Example
Using genotype data to guide a drug choice
A trial tests a drug in two genotype groups. In the CC group, 48 out of 64 patients respond. In the CT or TT group, 22 out of 88 patients respond.
- Calculate the response rate for the CC group.
- Calculate the response rate for the CT or TT group.
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Compare the groups: the CC group has a much higher response rate, so a genetic test could help identify patients more likely to benefit from this drug.
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Evaluate the conclusion: this does not mean every CC patient will respond or every CT/TT patient will fail; it gives a probability that should be combined with clinical judgement and monitoring.
Common Mistake
Treating genetic results as certain predictions
A genotype can alter risk or probability, but it rarely gives a guaranteed outcome. Environment, age, other genes, dose and interactions with other medicines can all affect drug response.
Genetically modified organisms and medicines
A genetically modified organism, or GM organism, has had its genetic material altered using biotechnology. In medicine, GM microorganisms, plants or animal cells can be used to produce useful molecules, especially proteins.
Definition
Genetically modified organism
A genetically modified organism is an organism whose DNA has been deliberately changed, often by inserting a gene from the same or a different species.
Examples of medicines produced using genetic modification include human insulin, growth hormone, clotting factors and some antibodies or vaccine components.
Making a human protein using bacteria
Bacteria are useful because they reproduce quickly and can be grown in fermenters under controlled conditions. A typical process is:
- Identify the human gene coding for the required protein.
- Use a DNA-cutting enzyme called a restriction enzyme to cut DNA at specific base sequences.
- Insert the gene into a small circular bacterial DNA molecule called a plasmid.
- Use DNA ligase to join the DNA fragments, forming recombinant DNA.
- Transfer the recombinant plasmid into bacteria. This is called transformation.
- Select successfully transformed bacteria, often using a marker gene.
- Culture the bacteria in a fermenter.
- Extract and purify the protein product.
Definition
Vector
A vector is something used to transfer genetic material into a cell. In bacterial genetic modification, a plasmid often acts as the vector.
Why cDNA may be needed
Human genes contain introns, which are non-coding sections removed from mRNA during RNA processing. Bacteria cannot remove introns in the same way as human cells. For this reason, scientists often use complementary DNA, or cDNA, made from mature mRNA using reverse transcriptase. This cDNA contains the coding sequence without introns.
Key Idea
Personalised medicine versus GM production
Personalised medicine uses genetic information about the patient to choose treatment. GM organisms use altered DNA in a production organism to make a useful medicine.
Fermenters and product yield
When GM bacteria are used to make a medicine, conditions must be controlled to maximise yield and reduce contamination. Important variables include temperature, pH, nutrient concentration, oxygen availability, stirring rate and sterility.
Example
Calculating purified protein yield
A GM bacterial culture produces a therapeutic protein at a concentration of 0.25 kg m⁻³ in a fermenter volume of 2.0 m³. The purification process recovers 80% of the protein.
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Convert the percentage recovery into a fraction: 80% becomes 0.80.
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Use mass equals concentration multiplied by volume, then multiply by the recovery fraction.
- Cancel the volume units and calculate the final mass.
Benefits, risks and ethics
Personalised medicine can reduce ineffective prescribing and lower the risk of adverse drug reactions. It can also make clinical trials more efficient by targeting patients most likely to respond.
GM organisms can produce large quantities of human proteins without extracting them from human or animal tissues. This can improve purity, reliability and safety.
However, there are important concerns:
- genetic privacy and consent
- unequal access to expensive genetic tests or personalised treatments
- possible discrimination based on genetic information
- containment of GM organisms
- transfer of antibiotic resistance marker genes
- environmental effects if GM organisms escape
- careful regulation of clinical trials and medicine approval
Exam technique
In the exam
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For synapse questions, always link the drug’s action to neurotransmitter concentration, receptor binding and the postsynaptic effect.
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For personalised medicine, use cautious language: genetic variation affects probability of response, not guaranteed success or failure.
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For GM organism questions, name the key tools precisely: restriction enzymes cut DNA, DNA ligase joins DNA, plasmids can act as vectors.
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When given numerical data, calculate percentages or yields clearly and include units where relevant.
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Include both benefits and risks when evaluating personalised medicine or GM medicine production.
Self review
Check yourself
- How could an antagonist at a postsynaptic receptor reduce synaptic transmission?
- Why can L-DOPA be useful in Parkinson’s disease when dopamine itself is not given directly?
- How can a SNP affect whether a patient responds well to a medicine?
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