Oxbridge Interviews

Neurotransmitters are endogenous (made in the body) chemicals that allow neurons to communicate with each other throughout the body. An excitatory neurotransmitter promotes the transmission of an action potential, while an inhibitory neurotransmitter prevents this. An example of a structure in the brain that makes use of both types of neurotransmitter, is the basal ganglia. This structure is responsible for controlling movement- where the direct pathway activates movement, while the indirect pathway inhibits movement. GABA (this is an example given at A Level) is the inhibitory neurotransmitter, while glutamate is excitatory. A balance between signalling via both neurotransmitters, is essential for coordinating correct movements. Hence, pathologies associated with this structure can lead to neurodegenerative disorders that cause muscle tremors and imbalance, such as Parkinson’s disease.

You may have to apply your understanding of neurotransmitters, and discuss your way through a similar unfamiliar medical example, like this too, with the interviewer.

Remember that acetylcholine is released into the synaptic cleft, and acts on receptors on skeletal muscle, to allow sodium ions to influx, and transmit an action potential. The general mechanism for how acetylcholine is made, released and acts on the postsynaptic cell, can be applied to unfamiliar neurotransmitters too. For example, dopamine signalling is important in the basal ganglia too (following on from the previous example), and problems with dopamine signalling can impair movement and lead to the symptoms of Parkinson’s. Note that dopamine is both a hormone and a neurotransmitter. Just like ATP synthase is an enzyme, but serves as a transmembrane protein in aerobic respiration too. 

One of the causes of cell death are due to cell inflammation. A damaged cell will release chemicals called histamines, which cause the blood capillaries around it, to widen, such that more blood flows through it, which leads to swelling and redness. These cells also release chemicals called cytokines, which attract phagocytes, so these can come over and kill any pathogens that may enter through exposed skin. For example, in tuberculosis, white blood cells, such as phagocytes are recruited by damaged lung cells, and will arrive and build a wall around the bacteria, to engulf and destroy it ultimately. Not just against bacteria, but in autoimmune disease, the body may recruit white blood cells, B-lymphocytes, that produce antibodies against its self-antigens and cause inflammation and cell death of these self cells. Such autoimmune diseases include Rheumatoid Arthritis and Crohn’s Disease, that you may have to think about, this way. 

Huntingdon’s disease is the opposite of Parkinson’s disease. A lack of dopamine signalling causes the symptoms of Parkinson’s, while excessive signalling leads to hyperkinetic movement and Huntingdon’s. 

To treat either, for Parkinson’s, the obvious decision is to increase dopamine levels in the body, artificially. You could artificially inject dopamine into the body; however, dopamine has difficulty crossing a barrier protecting the brain. This is the blood brain barrier. Hence, to get around this, you could consider the pathway through which dopamine is synthesized, and perhaps send a precursor into the body, which can cross the blood brain barrier, and later be converted into dopamine, by enzymes in the brain. If you research the biosynthetic pathway of dopamine, this would be L-DOPA that crosses the barrier, and then is converted into dopamine by DOPA decarboxylase. The details and names in this example are not important, but the understanding around using precursors in biosynthetic pathways, solubility and conversion here, are key for when applying to an unfamiliar but similar situation given by an interviewer.

To treat Huntingdon’s, you could consider the converse. You want to downregulate dopamine levels, so you might consider interrupting the synthesis or release of dopamine. For example, tetrabenazine, is a drug that blocks a transporter (called VMAT2) found in neurons that secrete hormones such as serotonin, dopamine etc. This stops VMAT2 from transporting the hormones into vesicles in the first place, so they cannot be secreted into the synaptic cleft via exocytosis, inhibiting signalling. The idea here is not recalling these particular names, but recognising possible points of intervention, which include blocking transporters, vesicles or even enzymes that are meant to synthesise dopamine. Blocking enzymes may remind you of competitive inhibition.

Another example of how competitive inhibition can be used when deducing drug targets in medicine, is with a classic everyday drug- ibuprofen. ibuprofen is a cyclooxygenase (COX) enzyme inhibitor, so works by blocking this enzyme. Otherwise, COX enzymes are responsible from forming a hormone called prostaglandins from arachidonic acid. High levels of prostaglandins cause the uterus to contract during menstruation, and result in ‘period pain’. Hence ibuprofen blocks COX, to prevent this downstream mechanism to relieve pain. 

These are just a few examples, but for any similar cases, you may need to think in this way, to determine where drugs may target, to treat diseases/ conditions. 

Toxins are harmful substances released by bacteria, that cause damage to host cells. Endogenous toxins can also be produced by the body, for example MPP+ is the toxic metabolite of MPTP, which is usually excreted from the brain and body, just like you will have learnt about urea which is a by-product of protein hydrolysis, that is excreted via the kidneys. MPTP is a lipophilic substance and this property means it is able to cross the phospholipid bilayer of glial cells, and be hydrolysed into MPP+, which is a neurotoxin. This can be absorbed by dopaminergic cells. Now MPP+ is able to block the electron transport chain in the inner mitochondrial membrane. From this information, you can reason that this inhibits oxidative phosphorylation and ATP synthesis, which impacts cellular functions, and ultimately leads to dopaminergic cell death, and thus reduced dopamine signalling. This is one of many contributors to Parkinson’s disease. This is just an example however. They key takeaway from this is being guided to the knowledge that MPP+ impacts the electron transport chain. You should then be able to use A Level knowledge to suggest how it impacts ATP synthesis, and the consequences this will have on the cell, and how this leads to cell death, and the symptoms of a given disease- in this case Parkinson’s. It’s a chain reaction way of thinking, essentially. 

Think back to your understanding of DNA mutations, such as addition and deletion, which cause frameshift, and substitution which can cause a point mutation if it is not degenerate. These can result in non-functional proteins, including enzymes or transporters, which can impact cellular processes. They may give you an example of a specific mutation that impacts a particular protein, to lead to a certain disease, that you may need to think about. For example, in cystic fibrosis, a mutation in the CFTR gene, alters the structure of this transporter, which affects chloride ion movement into and out of cells. You may then be required to consider how this may lead to symptoms, such as sticky mucus. The lack of chloride transport, means a low water potential is not created. As such, water would be unable to move down its water potential gradient, from higher to lower water potential, so the mucus becomes viscous and stickier, than runny. You could of course be given some other example, and be required to apply A Level knowledge, to make suggestions, in this way. 

You may consider chromosome mutations such as: translocation, duplication, deletion, insertion and inversion, from A Level knowledge, and have to relate this to a relevant genetic disorder in which one of these events have taken place. You may also recall chromosome non-disjunction, which is where homologous chromosomes or sister chromatids fail to separate. Having more than two chromosomes in a pair is called polysomy, and this occurs in Down’s Syndrome- trisomy 21 (three copies of chromosome 21). Having less than two chromosomes in a pair is called monosomy, and this occurs in Turner’s Syndrome. This is monosomy of sex chromosomes, where the only one chromosome present is an X.

You will recall how oestrogen, which is a steroid hormone, can regulate transcription, as it acts as a transcription factor. Oestrogen is lipid-soluble so will diffuse across the phospholipid bilayer and bind to the oestrogen receptor to form a complex. This complex then enters the nucleus and binds to DNA, to modulate transcription of a gene. Now cells that are in hypoxic (lack of oxygen) conditions, may find themselves in essentially anaerobic conditions, with a lack of oxygen. A hot topic for medicine, is the discovery of the hypoxia-inducible factor (HIF), by Kaelin, Ratcliffe and Semenza, who received the 2019 Nobel Prize in Physiology or Medicine for this. This factor also behaves as a transcription factor, which binds to DNA and regulates transcription in response to these decreased oxygen levels, and has been noted for its role in cancer, because it activates transcription of genes involved in a process called angiogenesis. This is the formation of new blood vessels, around the cell, in attempt to increase oxygen supply to the cell, to allow the cancerous cell to divide and proliferate. You might also make a link to aerobic respiration, and how a lack of oxygen can lead to a lack of the final electron acceptor, and hence lack of ATP synthesis, which could contribute to cell death; however, this effect is not immediate, as cells can rely on anaerobic respiration for a little while first. 

GLP-1 agonists, like Ozempic and Exenatide, are heavily used to manage Type 2 Diabetes and Obesity. This is a hot topic for interviews. The interviewer may test the pharmacology side of things, and get you to try and apply your understanding of the adenylate cyclase cascade’ from A Level Biology (remember that hormone binds to receptor 🡪 this activates the enzyme adenylate cyclase 🡪 the enzyme catalyses the following reaction: ATP 🡪 cyclic AMP (cAMP) 🡪 this activates a protein called protein kinase A. This goes on to phosphorylate and thus activate further proteins or enzymes etc.). In general, any kinase does phosphorylation (while the opposite, which is a phosphatase, would do dephosphorylation). In the same way, GLP-1 acts as the hormone and the receptor it actually binds to is known as a GPCR (G-protein coupled receptor). There are three types: Gs (stimulatory), Gi (inhibitory) and Gq. The GLP-1 specifically acts on the Gs version of the receptor, to activate this cascade and stimulate phosphorylation of insulin, so that it is activated and released. This idea of phosphorylating molecules, to make them more reactive, is a key concept across medical biology and further examples will show where else phosphorylation is important for reactivity/ activation too. 

Once again, this is a good example of how drugs function on cells to lead to responses, and most drugs or hormones function via GPCRs this way- either through Gs, Gi or Gq, and your main focus is likely to be Gs, as this is the particular version that functions through the adenylate cyclase cascade from A Level, so of all three, you are highly likely to be given a drug that works through this pathway, to apply your understanding to.

While the brain is a key part of A Level Biology, you may have come across the blood brain barrier (BBB) as part of wider reading, and as aforementioned. The BBB is a network of blood vessels and tissue with closely spaced cells. The barrier forms a tight seal to protect the brain from harmful substances, but it also prevents most drugs from getting to brain tissue. This is a hot topic for interviews, as it is a major challenge in the way of drug discovery and pharmacology, due to inaccessibility of the brain. For example, recent medical research is looking into using nanoparticles to deliver drugs to the brain. From GCSE Chemistry, you may also recall the short section on nanotechnology and buckminsterfullerene, which can be used to cage and deliver drugs.

A relevant example in relation to Parkinson’s disease, is some research into using gold nanoparticles to cage and deliver GLP-1 agonists, such a Exenatide and Ozempic. Although GLP-1 is mostly used to manage Type 2 diabetes and obesity (as aforementioned), it has been found to improve dopamine cell function in Parkinson’s models. Nanoparticles are designed so that they specifically bind to some molecule in the BBB, and deliver the drug by transcytosis. This may remind you of the idea of forming complementary antibody-antigen complexes or enzyme-substrate complexes at A Level. This idea of complementary protein structures is important. 

For example, P-selectin (positively charged protein) is found on cancer blood vessels in the BBB, and fucoidan nanoparticles (these are made of sulfate polysaccharides, which you will recall from A Level Chemistry, have negative charge due to SO42-, so they attract to the positively charged P selectin better) are used to deliver drugs as they complementarily bind to P-selectin. In one of my own research projects, I explored this idea too through studying nanoparticles that could potentially deliver a chemotherapy drug (called temozolomide) into the brain, to treat a brain cancer (glioblastoma multiforme), and looked for particular protein targets in the blood brain barrier, to address this. The main issue with this particular drug, was how quickly it degraded in the physiological environment of the body and hence nanoparticle encapsulation is a possible solution, to extend its half-life. Drug half-life is another pharmacological issue you could consider when discussing drugs and efficacy. Hence, you could be introduced to the latest research and innovative methods in drug discovery, and required to suggest how you think the method may work, and why it is has been well-designed etc. 

The research explained above is linked below, for interest or reference: 1 2 3

Once again, this is a good example of how drugs function on cells to lead to responses, and most drugs or hormones function via GPCRs this way- either through Gs, Gi or Gq, and your main focus is likely to be Gs, as this is the particular version that functions through the adenylate cyclase cascade from A Level, so of all three, you are highly likely to be given a drug that works through this pathway, to apply your understanding to.

You may have come across the case in A Level Biology, where early scientists thought that our genetic material was in the form of protein, and that DNA was far too simple in structure to be our genetic material. You may want to recall the structure of DNA (one of four nitrogenous bases- ATCG, deoxyribose sugar and phosphate per nucleotide. Remember that there are two hydrogen bonds between AT and three hydrogen bonds between CG). 

You could be then introduced to the Hershey and Chase Experiment, which elucidated that DNA is our genetic material and not protein. The reason why this experiment is likely, is because the concept behind this experiment, links closely to the viral replication process you study at A Level, in immunity, and the idea of radioactive labelling, which is key in medical imaging. Recall that a virus structure consists of a protein capsid, and viruses inject their RNA (if it is a retrovirus like HIV) or DNA into the host cell. The protein capsid is made of cysteine or methionine amino acids, and hence contains sulfur. Recall that sulfur-containing amino acids are able to form disulfide bridges.

In this experiment, the virus was cultured in a radioactive sulfur medium, so that it took up radioactive sulfur into its capsid. A virus that specifically infects bacteria cells, is called a bacteriophage. Hence, the bacteriophage was made to infect the host bacteria cell, and where the source of radioactivity was tracked. It was found that radioactivity did not come from the bacteria, but from the bacteriophage, hence the capsid had not been transferred to the host cell. So protein was not the genetic material. 

Likewise, another parallel experiment was conducted, wherein the virus was cultured in radioactive phosphate medium, so radioactive phosphate was taken up into the viral RNA/ DNA. When the bacteriophage infected a host cell, it was found that radioactivity came from within the bacteria, and not the bacteriophage anymore, so RNA/ DNA had been transferred, and was the genetic material. Remember, if it is RNA, then this is duplicated into DNA by the enzyme reverse transcriptase. 

On the topic of immunity and viruses, considering the mechanism of a virus, and how it infects host cells, and replicates itself using the host cell machinery to cause the host cell to burst, may lead you to consider how they can be used in cancer treatment, as a form of immunotherapy. Oncolytic viruses are an area of research, wherein viruses can be modified to have specific attachment proteins, that allow viruses to attach to the tumour markers on cancerous cells, invade these, and cause them to burst, in order to destroy the tumour. While this exploits the mechanism of how viruses work, to treat cancer, there are of course factors to consider, such as attack from the immune system, and potential solutions can be considered, for example giving the patient immunosuppressant drugs, while this therapy takes place. There are still risks of such viruses being able to bind to other healthy host cells and causing side effects and damage to the body, hence this is only an area of research at the moment. This research explained above is linked below, for interest or reference:

https://www.cancerresearch.org/treatment-types/oncolytic-virus-therapy

Previously, we have seen how radioactivity is important in tracking the movement of molecules, in the example of the Hershey and Chase Experiment. Radioactive labelling is essential in medical imaging too. Technically this draws back on AQA GCSE Physics knowledge, wherein radioactive iodine can be used to asses thyroid function. The thyroid gland uses iodine, for its function, and hence we can inject radioactive iodine into a patient’s body, and track the movement of radioactivity to elucidate as to whether the thyroid is functioning well or not. If the thyroid gland is not working properly, it will be unable to take up iodine, and hence radioactivity will be detected from one place only. However, when iodine is taken up and used by the thyroid as normal, and the products (made from the radioactive iodine) are released back into the body, radioactivity will be detected from across the body, and this suggests the thyroid is working fine. This idea of tracing radioactivity in the body can be applied to other organs, to cleverly assess their functions, and hence is  fundamental concept to understand and know how to apply. 

This example might also make you consider barium meals from A Level Chemistry. A barium meal is a form of medical tracer and consists of insoluble barium sulfate. You will recall the trend in solubility of Group 2 sulfates, down Group 2. Usually, X-rays are good for detecting solid materials, like bones, but aren’t very good for detecting soft tissues, like those in the gastrointestinal tract. Hence, insoluble barium meals, can be consumed by a patient, and coat the oesophagus, stomach and intestines, so that when a patient undergoes an X-ray, the coated organs can now reflect the x-rays and soft organ structure now appears on the X-ray. 

A common trip up, is to ask students what opens up the DNA strand, for transcription of the template strand to occur. Since DNA helicase is used to break the hydrogen bonds in DNA, and open up the DNA strands for DNA replication, it is common to think DNA helicase is also responsible for opening up DNA for transcription to take place. However, you are taught that actually the RNA polymerase itself, that opens up DNA for transcription. 

When understanding transcription at a higher level, what actually happens is, RNA polymerase binds to general transcription factors (called TFIIs). A transcription factor is a protein that binds to DNA, to help promote the transcription of a particular gene. You can get general transcription factors that must bind to DNA to promote transcription of any gene, at  the most basic level. Then you can get gene-specific transcription factors, that are present to speed up the transcription of a particular gene, essential to a certain type of cell. When RNA polymerase first binds to general transcription factors (before transcription starts), we call this a ‘pre-initiation complex’. One of the general transcription factors is called TFII H. I often say H for Hannah, helps to remember this! This general transcription factor actually acts as both a helicase and a kinase- it is basically a  bifunctional enzyme. This means that TFII H in the pre-initiation complex, is actually responsible for opening up the DNA, and not RNA polymerase. And the kinase activity of this general transcription factor, is responsible for phosphorylating RNA polymerase, to make RNA polymerase more active and separate from the complex, and start moving across the DNA template to make mRNA. Remember from the adenylate cyclase cascade at A Level (and as aforementioned), that kinases phosphorylate things, and remember that phosphorylation often makes things more reactive.

Recall from A Level, that tumour suppressor genes encode proteins that help to slow cell growth, while proto-oncogenes encode proteins that speed up cell growth. As such, it is important to have a balance between both types of gene expression, to prevent cancerous growth. A mutation in either gene can affect this balance, and lead to cancer. For example, a mutation in tumour suppressor genes, can cause proto-oncogenes to take over, and hence lead to net fast cell growth. A mutation in a proto-oncogene can exacerbate its level of expression, such that we know call this an oncogene. It is cancerous because it is hyper-expressed, and leads to fast cell growth and division, thus cancer. 

Small interfering RNAs (siRNAs) are small strands of RNA, a few nucleotides long, which are endogenously produced (made in the body) and are responsible for controlling gene expression. It is an epigenetic form of control of gene expression, which means it does not directly cause mutation or change in base sequence, to affect gene expression, but externally modifies gene expression. They do this by combining with an enzyme to form a complex, and the siRNA binds to the complementary section of mRNA, and the bound enzyme will cleave the mRNA transcript, so that it cannot be further transcribed. As such, this form of control is important in regulating which genes are expressed in a cell, and which are not. You have certain siRNA upregulated in your cells, which block oncogene expression, and have certain siRNA downregulated which would usually block tumour suppressor genes, as we want these genes to be activated, to slow cell growth. In cancer cells, the levels of these siRNA will be the other way, hence oncogenes are being expressed. A specific type of siRNA are microRNAs, which are a hot topic for cancer research at the moment, and something I had myself read into, as part of a personal research project. The link to this area of research is provided below for interest or reference:

https://www.nature.com/articles/sigtrans20154

You can recall the structure of a ribosome from A Level, as being an organelle that is made of the large and small subunit, and specifically, being made of ribosomal RNA (rRNA) and protein. The ribosome itself, is responsible for translating mRNA into protein. Hence, it may lead you to question, what therefore makes the protein of a ribosome, if a ribosome does not yet exist? The answer to this lies in understanding the nature of rRNA, which is explained through the RNA World Hypothesis. rRNA is essentially a ribozyme (you may have come across this term at A Level too), as it can catalyse peptide bonds between amino acids, itself, to form a polypeptide. Hence, early ribosomes consisted of the rRNA itself, and could catalyse the formation of very simple proteins. These proteins would have then incorporated into the structure of a ribosome, and today, these evolved ribosomes are capable of catalysing more complex proteins. This is an interesting question that stems off A Level knowledge, and Oxbridge Interviews are all about thinking outside of the box, and answering these bigger questions requiring more thought and suggestion. 

You will recall how oestrogen, which is a steroid hormone, can regulate transcription, as it acts as a transcription factor. Oestrogen is lipid-soluble so will diffuse across the phospholipid bilayer and bind to the oestrogen receptor to form a complex. This complex then enters the nucleus and binds to DNA, to modulate transcription of a gene. This idea of steroid hormones being able to regulate gene expression can thus be applied with other steroid hormones, you could be asked about. For example, a good one to know is aldosterone. This is a steroid hormone, that increases reabsorption of Na+ in the proximal tubule of the kidney (you will have studied the kidneys and selective reabsorption at A Level). As such, aldosterone must enter the epithelial cells lining the proximal tubule, and increase transcription of something that helps to reabsorb sodium. This could include sodium channels and transporters. This is a medical example, that works in a similar way to basic example of oestrogen given at A Level. You could be given a different steroid hormone, that works similarly to think about, and decide the transcription of what proteins this may upregulate, depending on what organ system or function it relates to.

The most common modification to the mRNA transcript, that happens after transcription, is splicing, as covered at A Level. This is to reduce the size of the mRNA transcript, so that it can easily pass out of the nucleus, and by removing the introns, it makes the mRNA easier to read at the ribosome, during translation. However, there are further post-transcriptional modifications to think about. As the mRNA transcript moves out of the nucleus and into the cytoplasm, we have to consider how the mRNA transcript will remain stable, and will not be attacked by endonucleases (enzymes that can break RNA or DNA down, starting from the edges). To protect DNA, we could suggest putting ‘caps’ of nucleotides at the ends of the mRNA transcript, so that these are attacked, but the actual coding region within remains protected, until we get to the ribosome etc. This idea is called RNA capping and happens at the 5’ end of DNA, and polyadenylation is the version that happens at the 3’ end of DNA. It involves adding extra bases to either side of the mRNA transcript, to protect the coding mRNA within. 

The topic of splicing can also be linked to the use of gene technology, to deduce how splicing was discovered. DNA hybridisation is a genetic technique used at A Level, to assess the degree of genetic similarity between DNA sequences. Hence, the strand of DNA that was transcribed, was hybridised with the mRNA transcript produced, and where we had gaps and the strands were not fully overlapping, suggested introns had been removed and splicing had taken place. This is known as Robert and Sharp’s Experiment, and could be a basic experimental design you could also propose, based off your A Level understanding of base complementarity and DNA hybridisation. The idea here is to get you thinking about basic experimental design that can evolve from your understanding of genetic techniques from A Level. 

This is a very hot topic for its links to gene expression, that is worth reading up on, and is covered at A Level. Not all exam boards do cover this (for example OCR A does not), hence I recommend reading on this one a little, if you haven’t come across this before. A common starting point to this route of enquiry is often: DNA in the nucleus of a cell is 2m long. How is it able to fit inside the nucleus of a cell that is not even visible to the naked eye? The answer lies in our understanding of chromatin.  DNA is wrapped around proteins called histone proteins. DNA wraps around 2-ish times, per histone octamer, to form a nucleosome. There are gaps and stretches of exposed DNA in between nucleosomes, as DNA goes from histone to histone. The nucleosomes are then condensed together to form chromatin, which ultimately condenses into chromosomes. 

You will also have covered examples of epigenetic modifications, including DNA methylation and histone acetylation, at A Level, and how these can modulate transcription. Once again, OCR A, for example, does not cover this, but this is an interesting field in cancer research at the moment, so worth having basic knowledge around. 

DNA methylation is where methyl groups (-CH3) are attached to DNA. Most commonly, it attaches to cytosine bases. This makes DNA less accessible, as it gets in the way of transcription factors of transcriptional machinery (like RNA Polymerase) so the gene cannot be transcribed. 

Histone acetylation is all to do with the charge of histone proteins. Histones are positively charged proteins around which negatively charged DNA is wrapped, to form chromatin. Acetylating histones (adding -COCH3) will reduce their positive change, so they bind DNA less strongly. When this happens, transcriptional factors can access the DNA easier, so the gene is switched on. Vice versa for deacetylation, as the enzyme histone deacetylase helps remove the acetyl group and cause the opposite effects.

While DNA methylation inactivates gene expression through getting in the way of transcriptional machinery, it also inactivates gene expression through a second way. DNA methyl binding proteins can bind to methylated DNA and recruit transcriptional corepressors, such as the histone deacetylase, and lead to inhibition of transcription this way. 

However, DNA methylation can also increase transcription, if methylation occurs in a different position on DNA. If it happens on the side where DNA meets histone, it can weaken the attraction between the histone protein and DNA, allowing DNA to uncoil from the nucleosome structure, and so transcription machinery can easily access DNA and promote transcription. 

At A Level, you would have learnt how DNA replication consists of unwinding DNA, so that complementary nucleotides can bind to the template strands and DNA polymerase can catalyse the formation of phosphodiester bonds, to form DNA. You may have heard of continuous and discontinuous strand formation during DNA replication. This is all to do with the fact that DNA polymerase can only work in the 5’ to 3’ direction. You will know that DNA strands are anti-parallel, and hence if DNA polymerase only works in one direction, it makes you wonder, how replication of the other (discontinuous) strand really happens. 

This is to do with Okazaki fragments. RNA primers bind to several positions of the discontinuous DNA strand, and DNA polymerase is able to work backwards in the 5’ to 3’ direction, off these to form Okazaki fragments. RNase then removes the primer, and DNA polymerase fills these gaps. DNA ligase then joins each fragment together. However, the very last primer that is removed, will now be filled, as DNA polymerase has nothing ahead of this, to continue building down from. This is what is known as the end-replication problem. You could potentially be talked through this process, until you reach and realise this end-replication problem, and then be asked to suggest how to solve this. The answer lies in the synthesis of telomeres. Telomerase is an enzyme that binds to the very end of this discontinuous DNA strand and contains an RNA strand in its active site. DNA polymerase is able to build the remaining gap on the discontinuous strand, by reading the RNA strand in the active site of the telomerase. This extends the DNA length. The DNA polymerase can then work off this telomere, and fill the gap on the discontinuous strand. The idea here, is to be able to consider complementarity of bases once again, and suggest how this can help to ‘lengthen’ DNA.  

As you will know from A Level, red blood cells do not have a nucleus, in order to create more space to carry and transport maximum oxygen. However, red blood cells also contain hemoglobin, which is a protein. This may lead you to question as to how red blood cells are able to make hemoglobin, without DNA present in a nucleus. The answer lies in thinking about the red blood cell’s life cycle. Premature red blood cells actually begin with a nucleus, before it is discarded. Hence hemoglobin is synthesised early on, and after discarding the nucleus, red blood cells lack the ability to make further hemoglobin, hence have a limited lifespan of around 120 days. This is a thinking-style question again, and you should be able to think about this link, as you will have come across therapeutic cell cloning, in the stem cell topic at A Level.

Therapeutic cell cloning is also known as somatic cell nuclear transfer, where an egg cell (ovum) is taken, and its nucleus is discarded. A somatic cell from the patient’s body is taken, and the nucleus is removed and then transferred to the egg cell. The egg cell is stimulated to divide by electric shock, to produce embryonic stem cells, which can now differentiate into the require cell types, but will contain DNA that is compatible with the patient. This prevents immune attack and there is therefore no need for the patient to take long term immunosuppressant drugs. Stem cell therapy, in general, is a very significant topic in regenerative medicine for example for bone marrow transplants or its potential in treating paralysis, so is worth reading around ahead of interviews too. 

You will recall the process of ultrafiltration, from the kidney topic at A Level. It can also be called glomerular filtration. The glomerulus is a capillary knot, at which the blood is filtered into the renal tubules. Blood enters the glomerular capillaries via afferent arterioles (which bring blood from the renal artery) and then exits via the efferent arterioles, and into the renal vein. Each glomerulus is associated with a single renal tubule. The epithelial cells composing this renal tubule, also form the Bowman’s capsule. This envelopes the glomerulus, and we call this space, the Bowman’s space. Overall, the glomerulus, Bowman’s capsule and renal tubules, form the functional unit of the kidney, which we know as the nephron.

The efferent arteriole tends to be constricted more than the afferent arteriole. This builds up pressure in the glomerular capillaries, to drive ultrafiltration into Bowman’s space. There could be mention to GFR (glomerular filtration rate), which is covered in some A Level exam boards. You may consider factors like surface area, permeability, gradients and vasoconstriction, which can all impact the rate of filtration (similar to the factors that affect the rate of diffusion) Remember smaller molecules: glucose, amino acids, water, urea, ions can pass through, while larger molecules like red blood cells and proteins, cannot pass through. It is important to remember that all glucose must later be reabsorbed.  Urine should have very little or no glucose at best. If glucose levels are high in urine, this could either indicate a problem with the kidney or excessive blood glucose levels, hence the kidney is not reabsorbing all glucose back and is trying to remove some glucose via urine (glycosuria). Therefore, this could indicate diabetes. This is tested via Glucose-Urine Tests. 

The Bowman’s capsule has several layers, that the fluid must pass through. This includes: endothelial cells of the capillary (fenestrated) and the negatively charged endothelial basement membrane. Thus, charge, size, shape are all factors that can control which types of molecule can pass through the capsule. You are not expected to know about the charges of the layers of the Bowman’s capsule, and this could be a learning point, during the interview, wherein you could be asked to interpret given data to deduce that the basement membrane must favour positively charged, smaller-sized molecules. For example, an experiment could involve infusing dextrans of different charges (these are polysaccharides that are anionic, cationic or neutral) into an animal, and recording the rate at which they were filtered at the glomerulus. This data could be presented as a graph, and you would find that cationic dextrans were filtered most for a given time, from which you could suggest that the capsule layers are most likely to be negative. 

Pacinian corpuscles are a type of touch receptor located in skin. It contains stretch-mediated sodium channels in the cell surface membrane. When you press on skin, deformation occurs, and these receptors are stretched and the sodium channels open, allowing the rapid influx of sodium ions into the cell. This causes depolarisation and results in the generation of an action potential. The key mechanism to take away from this A Level example, is how stretch receptors function, as this knowledge can then be applied to unfamiliar medical examples. 

For example, when the effective circulating volume (volume of blood) is very high, this stretches the atria in the heart, and hence distends the stretch receptors in the atria, leading to the release of a peptide, called the atrial natriuretic peptide (ANP), which intends to block sodium reabsorption, to prevent the formation of a water potential gradient, so further water is not reabsorbed from the kidneys, as blood volume is already high. 

Another example is when high blood pressure can stretch the arterioles in the kidney, and cause distension of stretch receptors here. As ions enter the cell and cause depolarisation, this change in membrane potential/ voltage, opens voltage-gated ion channels (VGCs), such as calcium VGCs. As such, calcium ions influx and cause muscle contraction to slow blood flow. This is known as the Bayliss effect. 

From A Level, you will recall that the pituitary gland is divided into the posterior and anterior pituitary. ADH is produced in the hypothalamus, and transported to the pituitary gland, and is specifically released from the posterior pituitary. ADH acts as a switch. It can either cause dilution or concentration of urine, depending on whether it is released or not. When osmoreceptors in the hypothalamus detect low levels of water in the blood, ADH is released and causes the insertion of water channels (called aquaporins) into the collecting duct of the nephron, to encourage more water reabsorption into the blood. 

The osmoreceptors sense as little as 1% changes in water potential in the body. You may consider why the resolution is so little- it is to do with your knowledge of water potential and osmosis. Since animal cells lack a cell wall, too much or too little water outside cells can lead to cell crenation and edema or cell turgidity. This can ultimately lead to organ damage and organ failure, if cells die. 

As you have studied the adenylate cyclase cascade at A Level, you can actually apply this understanding to understand how ADH actually causes the insertion of aquaporin channels, if prompted to suggest how this really happens. The ADH can bind to a G-protein coupled receptor, and it is Gs version again (as aforementioned). This activates adenylate cyclase, which produced cAMP, which activates a kinase. We know that phosphorylation makes things active, so you can potentially consider that the kinase phosphorylates some kind of transcription factor (specifically the CREB transcription factor), to promote the transcription of a gene that leads to aquaporin production. It could also phosphorylate the cytoskeletal elements to drag the vesicle in which the synthesised aquaporin is stored, to the membrane so that the vesicle can fuse here and insert the aquaporin channels into the cell membrane. 

In diabetes insipidus, the body tends to produce high levels of dilute urine, even if the body does not necessarily have higher water content. Thinking about how ADH controls water reabsorption, you may need to make suggestions for what may have gone wrong in this condition. Possibilities include thinking about the failure of the posterior pituitary in making or releasing ADH (this is called neurogenic diabetes insipidus, as it is to do with problems in the pituitary gland in the brain) or you could consider that even when ADH is present, there is an issue with the aquaporins, via which water is reabsorbed into the blood. There could be a mutation in the aquaporin protein, or issues in the trafficking of the vesicle containing the aquaporins, to the membrane, hence they have not been inserted into the membrane properly, preventing water reabsorption (this is called nephrogenic diabetes insipidus, as it is to do with problems in the kidney/ nephron). For any given or similar condition, you may have to make suggestions, in a similar way, for what could possibly have gone wrong, using the basic knowledge you already have or are prompted to. 

You will recall from A Level, that the sodium-potassium ATPase is responsible for generating the resting membrane potential, through moving 3 Na+ ions out of the cell, for every 2 K+ ions into the cell. Likewise you can find these ATPases in the epithelial cells lining the nephrons in a kidney too. They are essential for moving Na+ ions out the epithelial cell and into the tissue fluid surrounding cells, and then into blood, to establish a sodium gradient, so that sodium ions from the renal tubules can enter the epithelial cells, and then be reabsorbed into the blood. You will have already studied ion reabsorption from the proximal tubule, during selective reabsorption, and this is a key link you can make between topics studied at A Level. A further link you could consider is how an ATPase requires ATP to function. As such, you can expect the rate of oxygen consumption to be high, since oxygen is needed as the final electron acceptor in aerobic respiration, when synthesising ATP. 

You can further develop on your understanding of carriers and channels, as transporter proteins, if you were shown a specific type of transporter, for example GLUT (glucose transporter) and SGLT (sodium-glucose linked transporter) are types of transport proteins, important in the small intestine and kidneys, for reabsorption. An example question could also be that in the proximal tubule of the kidney, you find two isoforms of the SGLT transporter. This is SGLT 1 and SGLT 2.  SGLT 1 is found across most of the proximal tubule epithelial cells, where 1 sodium ion moves for every 1 glucose. But at the end of the proximal tubule,  we find SGLT 2, where 2 sodium ions move for every 1 glucose. To understand this change in stoichiometry, you might want to think along the lines of moving more sodium, to generate a larger gradient, to give more power for moving any last molecules of glucose into the blood and out of the proximal tubule, to prevent any glucose passing out in urine. 

Recall from A Level Chemistry, that zwitterions are ions that contains two functional groups, so have both positive and negative charges. They are hence electrically neutral, as charges cancel to zero. A zwitterion forms when the overall pH of the molecule is zero- this is the isoelectric point. Amino acids exist as zwitterions, and can become both acidic or basic, depending on the surrounding conditions.

In acidic conditions (low pH), the lone pair on oxygen, on the negative end of the zwitterion, is likely to accept a H atom, forming a positive (acidic) end to the amino acid, on the NH3+ side only.

In basic conditions (high pH) the H atom on the NH3+ is likely to be lost, and reacts with OH- in solution to form H2O, leaving a basic (negative) end to the molecule on the COO- side only.

For this reason, amino acids exist ion cationic, anionic or neutral form. 

Due to the different charges on amino acids, we would expect to have different transporter proteins for amino acids, for reabsorption from the kidneys. Mutations can alter the tertiary structure of a transporter protein, and as such can impact the reabsorption of particular amino acids. For example, a mutation in a cationic transporter for cysteine, can lead to cysteine build up in the renal tubules, as cysteine is a cationic amino acid. As such, cysteine can accumulate and form kidney stones which can block ureters/ bladder. This specific condition is known as cystinuria. 

You can link this idea to enzymes and denaturing/ mutations from A Level, and how a change in the active site of an enzyme, prevents the formation of enzyme-substrate complexes, and can slow down the rate of metabolic reactions, due to the formation of damage or non-functional enzymes, and have adverse effects on the body too.

You will remember that the main function of the Loop of Henle is to produce a low water potential in the medulla of the kidney. It does this by acting as a countercurrent multiplier to produce concentration gradients. The descending limb is permeable to water and so as filtrate flows down this part of the limb, its water potential decreases. The filtrate’s water potential is at its lowest, at the bottom of the loop. The ascending limb is impermeable to water, but allows the movement of Na+ and Cl- out of the filtrate, so the water potential of the filtrate rises again. This process allows the kidney to produce urine that is more concentrated than the blood. You may want to remember that another way of referring to water potential is also ‘osmolarity’. This is the number of particles of solute per litre of solution. 

To build onto this knowledge, you can be introduced to a transporter, called the sodium-potassium-chloride transporter (NKCC) which is found in the ascending limb, and via which these ions move into the medulla of the kidney. You could be asked to reflect on your knowledge of what happens at the Loop of Henle, to suggest what might happen if a drug, such as furosemide, was to inhibit NKCC. In this case, it would prevent the movement of ions out of the renal tubule, and this prevents water reabsorption from the descending limb, so the countercurrent multiplier is unable to function. The kidneys cannot concentrate the urine, and water remains in the tubules, instead of the body. You could then be guided to think about what kind of medical conditions, this could treat. You would think about conditions where you have excessive fluid buildup in the body, and want to prevent further water reabsorption. This would include conditions like oedema or high blood pressure. 

On the topic of the Loop of Henle, and its role in establishing a low water potential in the medulla, you could be prompted to consider why too much water does not exit the medullary cells and crenate them (Surel this would be damaging to the kidney, but clearly our kidneys continue to function fine). Your thought process should align with the definitions of isotonic (water potential of cell is same as solution), hypertonic (water potential of solution is lower than cell) and hypotonic (water potential of solution is higher than cell). To prevent water from exiting medullary cells, the cell must maintain their water potential level, to whatever the medullary space falls too. To do this, they must contain some kind of salt or osmolytes within them. To be specific, medullary cells contain inert osmolytes such as sorbitol and inositol, to maintain an isotonic environment and prevent crenation. 

You may recall from A Level, that the pH of blood is around 7.4. pH 7 is neutral, as you will recall, and tends to be the optimum pH for most body enzymes. Link between pH and enzymes is key. Of course, there are a few exceptions, like pepsin is a protease in the stomach, which thus has an acidic optimum pH of 2). 

A variation of the Henderson Hasselbach equation that you may have come across in A Level Chemistry, shows that pH is dependent on [HCO3-] in the body and PCO2. This suggests that both the lungs (removing CO2) and kidneys (reabsorbing HCO3- ions) are involved in maintain blood plasma pH. Remember that CO2 dissolves in the water in blood to form carbonic acid, and can further dissociate into H+ and HCO3- ions (CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-). You may be told that this reaction is actually catalysed by an enzyme called carbonic anhydrase. Overall, this decreases blood pH and makes it more acidic and less favourable for body enzymes. You may also recall the Bohr Effect and its impact on hemoglobin’s affinity for oxygen. Remember that HCO3- ions are basic, and therefore must be useful for neutralising acids found in the blood, in a neutralisation reaction (acid + base 🡪 salt + water), to maintain blood plasma pH at 7.4.

HCO3- is reabsorbed from the kidneys, just like all other ions, and based off the equation mentioned previously, you would be able to suggest that HCO3 can combine with H+, form H2CO3 and this reversible reaction can be further catalysed by carbonic anhydrase, to form CO2 and H2O. These substances can diffuse across the cell membrane as you have learnt at A Level, and enter the bloodstream, where the reverse reaction occurs again, to re-form HCO3-. This will create a lower water potential in the blood too, and as such, water can also be reabsorbed into the blood, from the kidneys, down its water potential gradient.

To link this to a medical example, glaucoma, is an ophthalmic condition, wherein the optic nerve (which connects eye to brain) becomes damaged, due to excessive fluid building up in the aqueous humour (front part) of the eye. Thinking back to what we have looked at in terms of reabsorbing HCO3- and therefore water, you might suggest that we need to intervene with water reabsorption, to prevent excessive fluid building up. When considering the mechanism by which HCO3- is reabsorbed, that we have already covered, you would now be looking for some point of intervention, to prevent HCO3- from moving out of the kidneys, so that water does not follow, reducing intraocular pressure. Given the hint of the enzyme, carbonic anhydrase, this may lead you to think back to competitive inhibition of enzymes, at A Level, and as aforementioned, how blocking enzymes are a good target for drugs. Therefore, you could suggest the use of carbonic anhydrase inhibitors, for the treatment of glaucoma. One example includes acetazolamide- but again, the details are not key, the understanding behind how we chain-reason to the sort of drug we need, after discussing the conditions, and applying our A Level knowledge, is key.