GEO-metry at the Brussels Institute of Natural Sciences

This July, I spent a week in Brussels - a capital well-known for its striking silver Atomium structure that stands out as a futuristic symbol on the city skyline. Beyond the Atomium, Brussels is home to a richer science scene: including its Institute of Natural Sciences. This museum is most visited for its impressive collection of dinosaur remains and early hominid ancestors, yet hidden in the basement is a dazzling exhibition of mineral specimens that revealed the complex molecular geometry behind the facets and vertices of crystal topology. 

One thing that I noticed as I walked around the exhibition was the fact that so many of the samples - disguised in a full array of colours, shapes and sizes - were all ultimately the same mineral: quartz. Quartz is the second most abundant mineral in the Earth's crust after feldspar, but this does little to account for the fact that every variety of quartz is so strikingly unique - even the majority of sand grains on the beach are, in reality, miniature quartz crystals. So what makes each quartz deposit so different?

To understand this, we must first distinguish between the theoretical crystal form of quartz and the true crystal habits it exhibits in real life as a result of variations in its formation. The form of quartz is a model of how the 'perfect' crystal should look in its pure form, unaffected by any chemical or physical factors. 

Quartz also goes by the name of silica, or silicon dioxide, with a molecular formula of SiO2. Since silicon is tetravalent like carbon, it also takes on a similar molecular shape when it is in a lattice. Each oxygen atom in the crystalline lattice is shared between two silicon atoms, so that each silicon atom has a coordination number of four, meaning that there are four oxygen atoms around each silicon despite the overall empirical formula being SiO2. The compound therefore takes on a tetrahedral arrangement with a bond angle of 109.5 degrees between each Si-O covalent bond. This shape produces a non-planar hexagon which can be visualised in this simplified representation of a section of the structure that I generated using MolView 3D. 

The agglomeration of the tetrahedral units produces a hexagonal prism shape. This has 4 distinct axes in its geometry: 3 are of equal length and on the same plane and 120 degrees apart from each other, with a fourth perpendicular to this from the point of intersection and at a different length. As crystallisation occurs (in ideal conditions), the molecules are added on the same plane as existing molecules, causing the formation of these neat edges and faces instead of a curved or irregular structure. 

Another element to consider is the actual process of crystallisation, or nucleation. At a high temperature and pressure, the silica is able to dissolve. Then, as the solution cools, it enters a state known as metastable supersaturation in which the concentration of the solution is higher than the solubility yet the mineral still remains in solution. This is considered metastable because the solution is currently in a local minimum energy - not the lowest possible energy state (achieved by overcoming the nucleation barrier) but still a stable intermediate. Upon further cooling, the boundary of this metastable state is reached and as it is overcome, crystallisation occurs. A true theoretical crystal achieves this homogenously without a scaffold of foreign particles. In reality, it is much faster and easier to form a nucleus on a heterogenous base of an existing impurity (link how a snowflake forms its crystal structure on the nucleus of a dust particle in the atmosphere). Nucleation - in spite of the initial energy barrier that must be overcome in crystallisation - is energetically favourable and facilitates further crystal growth. Because supersaturation makes it easier to form nuclei, the crystals formed in a highly supersaturated condition are smaller than those formed under less supersaturation because nucleation is favoured over growth of existing crystals. Likewise, for low supersaturation, the crystals will be larger because growth is more favourable than forming an entirely new crystal.

The habit of a crystal is how it actually forms if the conditions and specific differences of formation are taken into account. It is the variation in habit of quartz which results in the formation of several different kinds of appearance and structure that are visible in nature. 

Structural deformations to crystals often include interstitial defects (when extra atoms are present in a space that they are not typically found) and the inverse, vacancy defects. These often occur at the same time within the structure. For example, the Frenkel defect is marked by the movement of an atom from one place to another, causing an interstitial defect in the destination and a vacancy defect in the position it used to occupy. 

Impurities are the result of unwanted chemicals that are present in the structure during formation. However, in the commercial world these are rarely unwanted changes - in fact, the wide spectrum of quartz colours is desired and manufacturers often treat certain kinds of quartz with heat to cause them to change colour. An example of this is the production of citrine from amethyst: amethyst is a variety of quartz given its striking purple colour by the presence of iron (IV) ions within its lattice. Transition metals such as iron are able to absorb and reflect certain wavelengths of light, giving them bright colours. The particular wavelength of light that is emitted is relative to the oxidation state the ion is in, and changes as the ion is either oxidised or reduced. In the formation of golden citrine from amethyst, high temperatures are used to reduce the iron (IV) ions to iron (III) ions, which causes a shift in the wavelength of light reflected, thus changing the mineral colour. This is a widely-used manufacturing process because citrine is so rare in nature, since it requires the natural amethyst to undergo similar naturally-occurring high levels of heat. 

Finally, another kind of deformation is twinning. This is when multiple crystals join together as they form. The Brussels Institute has handily colour coded the faces in the diagram below of a quartz that shows how multiple quartz crystals can form together and still retain the consistency of their planes. Even though this diagram shows quartz crystals all facing the same direction attached by the red-coded faces, twinning can also happen as another crystal joins to one of the green or yellow-coded faces - during which the crystal will grow at a sharp angle to create an interesting (and much spikier) structure. 

Above is a quartz crystal in my collection - you can easily see how its faces at the perfectly clear tip match up with the shaded diagram in the exhibition and the hexagonal model presented earlier. This proves just how predictable a natural phenomenon can be with the use of geometric and thermodynamic rules. The other end of the crystal, however, has become crushed and deformed over many years of weathering and wear into a unique and disorderly shape. Using mathematics can only do so much to predict how they will form: as I observed each specimen in the Institute's collection, I realised that we must instead learn to translate the intricate stories about the past written directly on the facets of each individual crystal. 

References:

 

All accessed 22nd September 2024

https://www.unearthedstore.com/blogs/geology-unearthed/how-do-crystals-form-into-different-shapes?srsltid=AfmBOoru01fMjNVAAtup8PlD9HHJOs6kmzet4s5pqo4xen0BOzXW9Bpi

https://www.mt.com/gb/en/home/applications/L1_AutoChem_Applications/L2_Crystallization/Supersaturation_Application.html

https://en.wikipedia.org/wiki/Crystal#Defects,_impurities,_and_twinning

https://en.wikipedia.org/wiki/Crystal_growth

How the Solar Eclipse Changed your Perception of Colour

 Monday 8th April marked a rare astronomical event across Western America - the first total solar eclipse since 2017. Despite living in England, where the solar eclipse was not visible, I still managed to view photographs taken by a friend who witnessed it at the time that captured the white halo of light hugging the dark silhouette of the passing moon. Although the beauty of the phenomenon was transmitted through the images, my friend spoke of another - somewhat supernatural - effect sparked by the eclipse that could not be captured by even the most high-resolution lens: a silvery-blue cast had descended over the scene, tainting the surroundings with its dusty tinge. 

This effect is known as the Purkinje shift. To explain it, we must examine the anatomy of both the human eye, and the composition of the sunlight that reaches Earth. 

The eye contains two types of specialised cells tasked with translating light input into an electrical output, a language understood by the brain that allows us to perceive the colour, shadow, and contour of the world around us. These cells are called rods, which detect light and dark, and cones, which pick up colour signals. Cones are able to detect the longer wavelengths of red and green, and the shorter, blue wavelengths; rods receive signals over a smaller wavelength range that encompasses the longer portion of blue light and short portions of red and green. However, the optimal wavelength for rod vision covers non of these zones - this is the reason that scotopic vision tends to be perceived in minimal colour. 

The structure of the rods and cones are relatively similar - a region of isks which detect the ultraviolet light, followed by a mitochondrial cytoplasm zone and nucleus region linked by a cilial bridge, ending in a synaptic area that transfers the resulting signal. Ultraviolet light at a certain wavelength is detected by proteins called opsonins, which exhibit cis-trans isomerism upon the incidence of this light. These proteins exist alongside a side-chain of retinal which can absorb a photon, leading to the activation of transducin as the next step in the protein cascade. This is bound to guanosine diphosphate, which differs from adenosine diphosphate by only its nitrogenous base, This is changed into guanosine triphosphate, which causes the activation of a phosphodiesterase enzyme which hydrolyses cyclic guanosine monophosphate, lowering its overall concentration in the region of the cell. The falling concentration results in the closure of cation channels, so cations build up next to the membrane between the disks and cell body. This leads to the setup of a charge difference: an electrical signal, which passes over the terminal synapse through the optic nerve to the occipital lobe of the brain for further processing. 

Photopic vision lies in the range of the cone wavelengths, and these are the cells which are thus activated more during the day. However, during an eclipse, a new type of vision distinct from either scotopic at night and photopic in the day is employed: mesopic vision. This vision range lies between scotopic and photopic and therefore uses both rod and cone output simultaneously. As covered earlier, the rod range falls over blue, red and green areas, yet predominantly lies overlapping with the blue wavelength field. Thus during an eclipse, when both rods and cones are being fired at the same time, the rod output tends to align with the blue wavelength output and the main signals received are towards the blue end of the spectrum: the Purkinje shift explained biologically.

Additionally, this phenomenon can be explained using the physics of the light that reaches our planet from the sun. Direct sunlight composes all visible (and some invisible) wavelengths of light and these create the white light of daytime. The explanation for the sky's blue wash is simple: blue wavelengths are shorter and therefore more easily scattered by the randomly moving air molecules. It is this scattered light which reaches our eyes as a reflection, as red and green light is more easily transmitted to make sunlit objects look more yellow, rather like a child's drawing of a bright, dandelion sun in the corner of a blue page. On the other hand, during an eclipse, we do not receive direct light - instead all light that passes to us reaches the Earth indirectly as the direct light has been shielded by the moon. And this indirect light - the same as the blue of the sky - consists of the most easily scattered blue wavelengths. With a reduction in the amount of red and green light reaching the ground, my friend noticed a slight blue shroud to the landscape: a physical product of our unique anatomy and the diversity of spectrum wavelength that can only be admired in person, with no records existing as no camera has yet been able to perfectly replicate the intricacies (and natural flaws - wavelength 'blind spots') in our rod and cone cells that allow this change to be detected.

When Politics and Particles Collide

 Although quantum mechanics is typically mislabelled as a very recent concept, it actually dates back to 1900 - an entire 53 preceding the landmark discovery of the double helix DNA structure made by Franklin, Crick and Watson. Its initial reputation as a branch of science set to revolutionise and rebel against the classical principles of physics, yet over time quantum mechanics has become deeply integrated into the fundamental laws of life. For example, the device that you are currently reading this on would not be able to exist without the quantum principles of superconductors and electronics. 

Having outlined its importance, it is still crucial to understand what 'quantum' actually means in the context of science. Energy can be seen not as a continuous transferral, but as something passed on in indivisible chunks labelled 'quanta'. These quanta take many forms, but the most widely-recognised is the photon, which acts as a vector of light. Quantum mechanics deals with the interactions between these quanta and their environment, on an imperceptibly small scale of subatomic particles and fields of charge. 

The electron plays a critical role in both the understanding and the complexity of quantum mechanics, as a result of the theory that electrons exhibit properties not exclusively like particles, but also oftentimes as waves in a state similar to energy itself. These waves are of interest as they show some quantum properties. These include the ability to 'exist' in more than one place at once; this is due to the fact that the location of an electron in its orbital at any given time can be reduced to a probability rather than a certain coordinate. It is thought that while an electron is not being observed, the probability mechanisms work in a way that the electron could feasibly exist in multiple spaces within its orbital. This changes as the electron is measured, or theoretically observed, however, since its existence in two places at once is not definite and is instead based on abstract probability and as soon as it is defined as in one place, there is now a 100% probability of it occurring in this location at the exact time of observation.

This principle is applicable in theory, but on a larger, multi-particle scale system, the randomness of these probabilities have the tendency to cancel out and the overall disorder minimises the effect of any quantum events such as tunnelling in which particles seem to be able to 'jump' an energy barrier without overcoming it and instead skipping directly through. While this can happen on a smaller level, it is relatively impossible for a whole human being to experience this tunnelling effect as this would require the alignment and coordinated tunnelling of so many subatomic particles that the chances of this happening are far lower than either of us ever winning the lottery. 

One of the pioneers of this theory was Pascual Jordan, a German-born theoretical physicist who published a groundbreaking research paper on the matter in 1932, titled 'Quantum Mechanics and the Fundamental Problems of Biology and Psychology'. However, this paper was shocking in a more unexpected way - while the scientific concepts he presented were factual and researched, he presented his findings rather controversially. 

At the time in Germany, something else was brewing: this time period of the early 30s to mid-40s marked the rise and fall of the Nazi empire. And alongside inspirational ideas surrounding quantum biophysics, Jordan fell into the trap of these radical and authoritarian views still condemned globally to this day, gradually succumbing to intensifying political beliefs that infiltrated his papers. In this unsettling line from his paper, the prioritisation and deification of the government was set out clearly: '...absorption of a light quantum in the steering centre of the cell can bring the entire organism to death and dissolution - similarly to the way a successfully executed assault against a leading statesman can set and entire nation into a profound process of dissolution.' (Jordan, 1932)

In comparing the risk of damage to the central 'authority' of a cell, Jordan promoted the argument that submitting to higher power was biological, a somewhat natural and science-defined order of life. Submit to a higher power was exactly what Jordan would consequently go on to do as the year directly following the publication of his striking paper, he joined the Nazi party himself. The reasons for this were largely left up to debate as in his own defence, he frequently claimed that he only joined the party in a bid to prevent Nazi regimes from colliding with the world of science (Dahn, 2023); this is exactly what he ended up doing himself. His papers in the years that followed grew more and more littered with references - implicit and implicit - towards the agendas of the party, as he fell into frequent correspondence with many other individuals more closely linked to Hitler himself. 

Until 1933, Jordan adopted a pseudonym to write under called 'Domeier' that he utilised to conceal his Nazi involvement from the rest of the scientific community, including those that he has collaborated with in the years before. Just eight days before he made his move to join the party, he finally published a propaganda-rich paper under his own name, forever linking himself, and the scientific community he stood for, to Nazi ideology and contemporary politics. In this paper, he urged the University of Rostock to take on a 'militant character' (Dahn, 2023) in a way overtly supportive of the party's regimes. 

Despite some accuracy to his statements - particularly the notion that living organisms are distinct from organic matter in their centralisation of key molecules (such as proteins and DNA) (Al-Khalili and McFadden, 2014) - Jordan's work was ultimately dismissed by his contemporaries and thus rarely referenced in the current scientific world save for in the context of his political involvement. Perhaps the most controversial outcome of this was the criticism received by the men he used to collaborate on research with, Wigner and von Neumann. Many argue that they should have cut ties with him following the surfacing of the scandal, but fail to realise that their own credit for papers was on the line - alongside their lack of clear knowledge regarding the situation as a direct result of the pseudonym he wrote under. The public at the time were physically, if not mentally, subject to many sources of influence and propaganda dictating the way they should think, interact and live and showing active disrespect for Nazi ideologies may have turned many of their alliances against them. 

After all, should we really allow politics to infiltrate the world of science? Perhaps it is best to try and differentiate Jordan's scientific accomplishments from his political shortcomings. However, doing so would completely disregard the fact that he himself was incapable of removing governmental influences from his writings and let explicit biases slip through. Pulling the life out of a scientist's life work is, by definition, impossible, and to learn from and truly appreciate scientific history, we must understand the context in which it was written.

https://pubs.aip.org/physicstoday/article/76/1/44/2877362/Nazis-emigres-and-abstract-mathematicsToday-Jordan 

date accessed: 15/02/2024 

J. Al-Khalili and J. McFadden, Life on the Edge (Bantam Press, 2014)

Die Naturwissenschaften, vol. 20 (1932), pp. 815-2

Right Now, your Body Just Fought a Cancer Cell

While cancer is widely attributed to genetic malfunctions, it also your genes that you have to thank for combatting these faults before damage is caused. The genes that claim this title are the tumour suppressor genes, which do exactly as on the tin: suppress tumours and prevent uncontrolled cell division.

To understand and appreciate the importance of the role of this family of genes, a foundation of knowledge of the cell life cycle is essential. The cell cycle is what allows the organs and tissues of your body to work in perfect harmony, enabling growth and repair to maintain balance throughout the fabric of your body. At the most simple level it is divided into three main stages: interphase, a long period of cell activity and preparation for division; mitosis, the precursor to the split of the mother cell into two identical daughter cells; and cytokinesis, the breathtaking moment when the cytoplasm pinches off into its two distinct successors to continue the ever-ebbing flow of cellular life. This entire process takes on average 24 hours - as the sun rises and sets around us, as do our cells.

During interphase, many critical tasks are undertaken alongside the cell's particular function in anticipation of a successful future division. The first step of the interphase is known as G1 - the first growth - and plays the role of increasing the space within the cell to maximise its capacity for division, while upregulating the synthesis of proteins that aid organelle production. This stage relies on a perfect balance of free nucleotides, amino acids, temperature and nutrients to provide the right conditions to engender life. The DNA only commences replication during the following S (synthesis) stage to form two sister chromatids for every homologous chromosome pair, provided that materials and space are sufficient for this to be carried out effectively. The final stage of interphase is G2 (the second growth) in which organelles are replicated to be distributed between the two daughter cells using the materials collected during the first growth. Energy stores are also increased to enable the motor proteins to carry out the processes on the spindle fibers in the following mitosis phase, which is typically an active process. Finally, DNA is checked for errors before initiating the mitotic division.

Mitosis in eukaryotes - a group of nucleus-containing organisms to which humans belong - involves four main stages. The first is the prophase, during which the chromatin genetic material condenses within the nucleus into chromosomes, which literally means 'colour bodies' since these more compact forms of DNA are clearly visible when using staining techniques such as giemsa staining, or when viewing material on a spectral karyotype. The centrosomes organise themselves to either pole of the cell where they form the tubulin spindle which will later play a pivotal role during the mitotic phase. Finally, the nucleoli break down, which ends the production of ribosomes. By metaphase, the kinetochores have attached to the centromeres on the chromatids and the spindle joins onto the centre of these kinetochores for the purpose of pulling the chromatids towards the equator of the cell. In anaphase A, the centromeres split and the microtubule spindles depolymerise and shorten, causing them to retract and pull each chromosome from the pair of sister chromatids to either pole of the cell. This is followed by anaphase B, involving the movement of the centrosomes closer to the cell membrane to ensure that segregation is easy. The final stage of mitosis is telophase, which finishes off the last of the spindles by full depolymerisation. A nuclear envelope is assembled surrounding the genetic material located on each side of the cell.

The last part of the cell cycle involved in its renewal for further cell activity and growth is cytokinesis, which causes the cell membrane to pinch the cytoplasm into two separate, yet identical, daughter cells, each containing their own full set of genetic information. Since plants have a cell wall, this stage instead involves the formation of a cell plate through the middle of the cytoplasm, which eventually segregates into the product cells.

With a process so complex and laced with so many fine yet vital details, it is unsurprising that there is the potential for errors to occur. Common faults in this system are marked by product cells with the wrong amount of genetic material (aneuploid cells) or with damaged DNA. If sister chromatids fail to separate during anaphase, one daughter will have monosomy as it loses material while the other has too much and experiences trisomy. Binucleated cells also have the potential to occur if during cytokinesis, the cell fails to divide the nuclei evenly between the two product cells. However, the error most commonly tied to cancer occurs as the cell divides too effectively and loses control of its own mechanisms, propelling it into a state of frenzied division and propagation, and planting faulty gene-containing seeds throughout the tissues. This erroneous division is known as a tumour and is often benign, yet can be secretly perfidious if it metastasises and is carried as a vector of malfunction throughout the bloodstream to germinate elsewhere. There are many theories as to how cancer arises, and there is no direct answer. The sparks that ignite this disease vary not only between cancer types, but also from individual to individual, which is what sets it apart from other conditions in its unique difficulty to be studied. A common hypothesis that marries genetic causes with environmental factors is known as the 'two-hit' theory, which suggests that heritable mutations may be passed on in one copy of a tumour suppressor gene. This means that - despite this inheritance - the other copy of the gene is still present to encode the cell-regulating protein and thus the single mutation at first has no effect. This is often coupled with genomic instability, meaning that - in a twisted draw of probability - it is highly likely that environmental factors cause a second hit which eliminates the other copy of the gene and prevents the cell-controlling protein from being synthesised at all, thus resulting in the breakdown of any barriers to incessant replication.

Fortunately, your cells are equipped with reinforced mechanisms to prevent the spread of damaged DNA, in the form of many regular checkpoints throughout the normal cell cycle.

The first of these is known as the restriction point and occurs between the G1 and S phases of the cycle. This confirms that cell conditions are optimal for the cell cycle to proceed, including having the right balance of nutrients and extracellular conditions. Until this point, the cell cycle has relied upon external growth factors to move forwards; following the restriction point, the cell becomes committed to the cycle and no longer requires growth factors, but is also unable to leave the process. If the cell fails at restriction point, the cell cycle is terminated and the cell is instead sent into the G0 phase. This is not a mechanism solely used for errors detected - for example, neurones are fully differentiated and therefore do not need to remain in the cell cycle. Other potential causes for G0 initiation include a lack of nutrients. The G0 phase may be quiescent, allowing it to be reversed, or irreversible such as during senescence and terminal differentiation. Senescence is a way to allow cells to continue their typical function until they die, without further divisions. If a grave mistake in the DNA or irreparable structural damage is present, the cell may undergo a more brutal process of apoptosis, eradicating the cell and physically preventing it from differentiating or carry out erroneous functions which may lead to cancer. Evidently, mutations which cause loopholes in the restriction point may allow damaged cells to slip past these regulations and commence cancerous divisions. 

The second checkpoint of the cell cycle is  the G2 point which ensures that there are no further damages to the DNA before mitosis is initiated, and gives cells the opportunity to repair and re-synthesise any faulty sections of DNA. 

Another checkpoint is present during the phase of mitosis itself, known as the spindle checkpoint since it occurs during metaphase, when the microtubule spindle is in use. It makes sure that each chromosome is properly connected tot he spindle by its kinetochore, thus confirming that division during the succeeding anaphase A will segregate chromatids properly and ensure that the daughter cells are truly diploidal and identical. Defects in this checkpoint, as with all other checkpoints, run the risk of cancer arising from aneuploids. Cancer is not the only impact triggered during this stage: Down syndrome is engendered by trisomy in the 21st chromosome if the chromatids are not split evenly across the two daughter cells during embryonic development. 

Every day, your body's complex machinery creates around 330 billion cells each day, many of which are secret precursors to cancer. Thanks to the beautiful and carefully refined mechanisms that act within each cell's cycle, you are constantly halting the formation of tumours before they could even be noticed under a microscope. While these systems are not without their faults and occasional environmental and genetic factors have the potential to tip it into a precarious state of imbalance, it is still worth admiring our capacity to regain order, if not with the help of our ever-advancing oncological technology. It is important that we stop perceiving cancer in a single-faceted manner, as a simple fight between humanity and the human body. We are equipped with nature's all-encompassing user manual deep within our cells, and it is up to us to step up to the task of decoding it. 

https://en.wikipedia.org/wiki/Mitosis#Further_reading 

27.01.2024

Gene Mapping with Yeast

The human genome is often compared to a cell instruction manual – if each page were to represent one of the estimated 25,000 distinct genes, it would claim the title of the longest book ever to be written. These gene units are coded for by a unique fingerprint sequence of up to several million bases, each referred to by one of four basic letters: A, T, C or G. Despite this manuscript of life existing within most somatic cells, it was only in 2003 that the completion of the thirteen year-long Human Genome Project was able to cast light upon a genetic map that would later prove essential in navigating scientific research through the labyrinth of the genome. While the success of this project is widely lauded, the pivotal and unexpected role of S. cerevisiae (yeast) often remains in the shadows. 

The Importance of Yeast 

Despite the title of the Human Genome Project implying that research focused on human cells, yeast rapidly rose to the centre of mapping techniques for its unique properties that distinguished it from other candidates more similar to humans. While human genetic pedigrees – genetic trees used to display the Mendelian patterns of trait heritance - may be useful in analysis, the crevasse of time between generations stunted its potential applications and called for a faster-reproducing organism. The budding time of yeast averages 90 minutes, meaning that the trends in genetic composition and the occurrence of de novo (new) mutations from generation to generation could easily be observed. Furthermore, yeast has the potential to exist in both a diploid and a haploid form in relation to the environmental conditions; this permits researchers to initiate either sexual or asexual reproduction in a certain colony to monitor differences between these modes of replication. 

Tetrad Formation 

Yeast genes are mapped while it is in haploid – halved genetic material - form, requiring the yeast to sporulate under nitrogen-deficient conditions and create a tetrad of meiotically-divided haploids. This is performed via the following general method: 

The yeast sample is first streaked upon a petri dish and incubated, allowing for budding. Each colony appears as a distinct patch of yeast growth; a single colony is then isolated and swirled in a minimal media consisting of salts, minerals, a sugar source, and the absence of nitrogen. Under this lack of nitrogen, the yeast colony undergoes the evolutionary process of sporulation in response to stressful conditions to form an ascospore which would – in the wild – be able to drift to a more nitrogen-rich location. This is achieved as the cells exit the mitotic cell cycle of normal cell division and initiate meiosis within the nuclear envelope. During meiosis, the genetic material divides twice in succession, resulting in four daughter cells, known collectively as a tetrad. The membrane of the mother cell persists around the tetrad, acting as a protective ascus coating around the four inner spores.  

To reach the haploids for study, enzymes are employed for the dissolution of this ascus. The cells may then be observed using a powerful tetrad-dissecting microscope equipped with a fine glass needle designed to isolate the individual haploids from the tetrad.  

Using Tetrads to Measure Gene Linkage 

During the meiotic process, the genes do not segregate into identical cells as they would during mitosis. Instead, genetic variety of offspring is caused by recombination: this is the crossing over of DNA between different chromosomes to exchange genetic material at a certain point. Two genes are usually selected for observation to determine their genetic distance, and thus position within the yeast genome. The closer together the genes are, the more likely they are to remain on the same chromosome and in the same daughter cell following recombination.  

Without recombination, all haploids have what is known as the ‘parental ditype’ genotype; this is identical to that of the mother cell. A potential genotype of the mother cell could be AB/ab, in which A and a are two alleles of the same gene (as are B and b) and AB and ab represent the combinations of these alleles present on each chromosome belonging to a pair. If recombination does not occur between the loci of the two genes on the chromosomes, all haploid daughter cells have either an AB or ab genotype, which matches that of the mother chromosomes.  

However, in the event of recombination, one of two different offspring types may arise. The first is the non-parental ditype, in which none of the daughter cells have chromosomes that match the mother cell, as recombination has switched the arrangement of the two genes. This would be represented by a mix of Ab or aB haploids. The second is the tetratype, with four different possible genotypes – two of which are recombinant and two are parental. Therefore, the daughter cells would exhibit a mix of AB, ab, Ab and aB genotypes. Both these types of tetrads show that the chromosomes have crossed over and swapped material at some point between the genes A and B.  

Observing these haploids is critical in the measurement of genetic distance between yeast genes, since the relative numbers of each type of tetrad (parental ditype, non-parental ditype and tetratype) can be directly input into this formula, from which genetic distance measured in centimorgans (cM) can be derived:  

Genetic Distance = 100 x (T + 6NPD)/(2E) 

where T corresponds to the number of tetratypes, NPD to the number of non-parental ditypes and E to the total number of haploid cells in the sample. As recombination events increase in frequency, the numerator of the fraction rises since the number of tetratypes and non-parental ditypes increases in relation to the total number of cells. This causes the overall fraction to increase, displaying a proportional increase in genetic distance.  

Applications 

Although yeast seems an unlikely subject to map genetic distances and determine the degree to which genes are linked, it is to this unique organism that the Human Genome Project owes its success. While morphologically, humans and yeast are highly distinguishable, 23% of genes are homologous between these two species and observations of genetic distances in yeast are frequently mirrored in the human genome. These genetic distances can – like distances on a geographical map – be used to physically place the genes relative to each other to construct a highly accurate sequencing of bases.  

The most notable application of yeast technology resides in the study of genetic markers: these more visible and easily identifiable ‘flags’ are linked to and signal the presence of other, more significant alleles and mutations close by on the same chromosome. Among the marker loci identified using yeast are even genes which point towards antibiotic resistance in bacteria – a corner of research with the future potential to revolutionise healthcare and accelerate pharmaceutical evolution.  

References 

MITx 7.03.1 Genetics: The Fundamentals  

accessed: 19th November 2023 

https://www.uvm.edu/~dstratto/bcor101/mapping3.htm  

Accessed: 27th December 2023 

K-State Parasitology Laboratory: Mendelian Genetics Problems 

Accessed: 

https://www.k-state.edu/parasitology/biology198/answers2.html 27th December 2023 

A. Neiman: Ascospore Formation in the Yeast Saccharomyces cerevisiae 

Accessed: 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1306807/#:~:text=The%20presence%20of%20a%20poor,%2C%20and%20sporulate%20(40). 27th December 2023 

A Glowing Review of 'Below the Edge of Darkness'

         Many months ago I found myself captivated by the idea of bioluminescence and spent many days researching it until every Wikipedia link had turned purple and I concluded my findings in a short article. After this short-lived love affair with light I returned to my non-luminescent world and this interest gradually became diluted by other discoveries and ideas I encountered in the following weeks. This was all to change after an animated afternoon discussion with a friend that led to me brushing the dust off my old notes about the deep ocean. At the reveal of my hidden interest, her eyes lit up as if they, too, were bioluminescent, and she insisted that I must at once read Widder's 'Below the Edge of Darkness'.

Never once have I read a non-fiction book disguised beautifully as a work of fiction. Widder is as much the protagonist of her piece as she is the writer. Typically as a reader of factual texts, I am left to feel as though I am sat hidden at the back of a standard, uninspiring lecture given by a professor with textbook-accurate fact recall. Widder instead seemed to reach a hand through the page and talk to me - as an older relative would an eager child - with the kind of wisdom only a life could give, not a textbook or a website. Just as it is perceived a sin to try to cleave art from artist, it is surprisingly hard to separate science from scientist. By the end of the book, I felt as though I knew Widder personally, and could see her human experiences shining through her discoveries. 

A particularly touching scene I found was the story of her close encounter with blindness. For a book clearly about light and the visible world, to open by plunging the reader into a shared period of both physical and emotional darkness was immensely impactful in developing an appreciation for the light we are so privileged to experience surrounding us. By prompting me to see even the everyday colour as beautiful and vibrant, she could then go on to truly dazzle with her descriptions of the unusual and breathtaking underwater scenes. And it was these depictions that were nothing like I had read before, even in fantasy novels. Widder takes us on a journey, bundling the readers into her claustrophobia-inducing submersibles and descending into the pelagic abyss. She skilfully manipulates our emotions to the point that - from the comfort of my bedroom - I felt my panic rise with hers as the precarity of these expeditions was unveiled and began to pray with her for a glimpse of the magical underwater world. At many points my heart momentarily halted as the world fell into an inky shadow. Then all of a sudden, the lights would flicker on and this array of colours would glow through the black and white pages like stars illuminating the night sky. I don't think I'll ever be able to look at the ocean in the same way again.

In terms of the science scaffolding these paintings, Widder has the perfect approach. Dealing with a subject you have decades of expertise in when communicating with the general public is rather like resurfacing after a deep-sea dive. You have to come up for air eventually and give an accurate and detailed narrative that represents your topic in an informative manner. But move too rapidly and abruptly and you risk decompression sickness. Not once did I find myself at a loss for detail while reading this. And not once did I find myself lost within the detail. It is clear that she not only has the wide knowledge about every facet of this topic, but also the intellect to express it in the best possible manner for her audience. And believe me, the audience was engaged. In fact, I had to check in the mirror after reading the final page that I hadn't begun to glow myself from the excitement for bioluminescence that Widder managed to rekindle within me!

The Legal Development of Genetic Testing

Recently, the development of genetic testing has given us greater control over our genes, from pre-implantation optimisation of in-vitro fertilisation to chorionic villus sampling. Some adult genetic tests even provide a window through which people can observe the future coiled within their own DNA, predicting changes that may lie dormant for as long as several decades. With this range of opportunity, however, comes a range of factors to consider including finance, ethics and legislation, and a deficit of information for those most concerned by it. 

        Perhaps the most widely-debated issue that genetic testing has created is the option of terminating gestation on the basis of results indicating a life-limiting disorder. Although this discussion comes with its own problems and ethical questions, a more overlooked topic surrounds adult genetic testing for later-onset conditions such as Huntington's disease. This is an autosomal dominant, neurodegenerative disorder resulting in chorea and mental decline between the ages of 30 and 50. The hidden implications of these genetic tests is explored by Bishop and Waldholz (1991, p322) in their book, Genome. These issues include the possibility of social and economic discrimination, such as potential biases against those with pre-diagnosed conditions in the workplace or by insurance companies maximising profit by exploiting those who will need the support most in the future. 

        Aside from the problems outlined by Bishop and Waldholz, other concerns also become apparent: what education and mental health guidance is on offer to those facing the results of these genetic tests? What is being done to ensure that people are able to make supported decisions regarding their results, and are fully informed during each section of the process? Since Genome was written over three decades ago, it acts as an interesting guideline to compare to today's legislatures and technology surrounding genetic testing. With this, we can view what has developed over recent years, and also what still remains to improve upon.

        In October 2018, the UK governmental Code on Genetic Testing and Insurance was put in place to tackle financial disadvantages experienced by many after receiving the results of genetic testing. The principle of the code is that insurance companies no longer have the right to require an applicant to undergo diagnostic or predictive genetic testing before receiving insurance, unless for a test relevant to the terms of the insurance itself. Furthermore, it is now against the law for insurance agencies to demand the results of a test of a blood-related family member, during the insurance cover, or when the result was obtained as part of a clinical trial. This is in the hopes that more people with a genetic history or predisposition to diseases such as Huntington's will feel more in control of their results and are able to make a better-informed decision to obtain a genetic test without the concern of data sharing and of bias based on their genetics. 

        The ability to recognise potential ethical and legal dangers in the field of testing has thus proven critical in its development: as much as genetic technology advances, so must our legislations.