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Posts Tagged ‘genetics

epigenetics and imprinting 5: mouse experiments and chromosome 11

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something new, since Carey’s book was published – a healthy mouse, from entirely maternal DNA, with healthy offspring – and in 2018 a healthy bi-paternal mouse was created

 

So we were looking at how we – mammals amongst others – are engaged in a kind of battle for the best way to ensure our genetic survival into the future, beyond our insignificant little selves. This battle begins in the very early phase of life, as zygotes multiply to form a blastocyst. 

Remember from my last post on this topic, the male mammal is interested in the offspring above all else. He’s even happy to sacrifice the mother for the sake of the child – after all there’s plenty more fish in the sea (or mammals in the – you know what I mean). The female, on the other hand, is more interested in self-preservation than in this pregnancy. She wants more than one chance to pass on her genes.

So, by the blastocyst stage, cells have differentiated into those that will form the placenta and those that will form the embryo itself. Experiments on mice have helped to elucidate this male-female genetic struggle. Mouse zygotes were created which contained only paternal DNA and only maternal DNA. These different zygotes were implanted into the uterus of mice. As expected, the zygotes didn’t develop into living mice – it takes DNA from both sexes for that. The zygotes did develop though, but with serious abnormalities, which differed depending on whether they were ‘male’ or ‘female’. In those in which the chromosomes came from the mother, the placental tissues were particularly underdeveloped. For those with the male chromosomes, the embryo was in a bad way, but the placental tissues not so much.

In short, these and other experiments suggested that the male chromosomes favoured placental development while the female chromosomes favoured the embryo. Thus, the male chromosomes are ‘aiming’ to build up the placenta to drain as many nutrients as possible from the mother and feed them into the foetus. The female chromosomes have the opposite aim, resulting in a ‘fine balance’ in the best scenarios.

Further work in this area has identified particular chromosomes responsible for these developments, and some of the epigenetic factors involved. For example, mouse chromosome 11 is important for offspring development. When the offspring inherits a copy of chromosome 11 from each parent, the offspring will be of normal size. If both copies come from the mother it will abnormally small, while if both come from the father it will be abnormally large. These experiments were carried out on inbred mice with identical DNA. Nessa Carey summarises:

If you sequenced both copies of chromosome 11 in any of the three types of offspring, they would be exactly the same. They would contain the same millions of A, C, G and T base-pairs, in the same order. But the two copies of chromosome 11 do clearly behave differently at a functional level, as shown by the different sizes of the different types of mice. Therefore there must be epigenetic differences between the maternal and paternal copies of chromosome 11.

So this means that chromosome 11 is an imprinted chromosome – or at least certain sections of it. This is the same for other chromosomes, some of which aren’t imprinted at all. But how is it done? That’s the complex biochemical stuff, which I’ll try to elucidate in the next post on this topic.

Footnote: the photo above shows a bi-maternal mouse with healthy offspring, and further work in deleting imprinted genetic regions has allowed researchers to create healthy bi-paternal mice too. There’s a fascinating account of it here.

References:

Nessa Carey, The epigenetics revolution, 2011

https://www.the-scientist.com/news-opinion/first-mouse-embryos-made-from-two-fathers-64921

Written by stewart henderson

January 19, 2020 at 12:26 pm

A DNA dialogue 1: the human genome

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what genomics tells us

Canto: I’m often confused when I try to get my head around all the stuff about genes and DNA, and genomes and alleles and chromosomes, and XX and XY, and mitosis and meiosis, and dominant and recessive and so on. I’d like to get clear, if only I could.

Jacinta: That’s a big ask, and of course we’re both in the same boat. So let’s use the magical powers of the internet to find answers. For example, here’s something that confuses me. The Human Genome project, which ended around the year 2000, involved a mapping of the whole human genome, and that includes coding and non-coding genes, and I think it was found to contain 26,000 or so – what? Letters? Genes? Coding genes? Anyway there’s a number of questions there, but they’re not the questions that confuse me. I don’t get that we now, apparently, have worked out the genetic code for all humans, but each of us has different DNA. How, exactly, does our own individual DNA relate to the genome that determines the whole species? Presumably it’s some kind of subset?

Canto: Hmmm. This article from the Smithsonian tells us that the genetic difference between human individuals is very tiny, at around 0.1%. We humans differ from bonobos and chimps, two lineages of apes that separated much more recently, by about 1.2%….

Jacinta: Yes, yes, but how, with this tiny difference between us, are we able to use DNA forensically to identify individuals from a DNA sample?

Canto: Well, perhaps this Smithsonian article provides a clue. It says that the 1.2% difference between us and chimps reflects a particular way of counting. I won’t go into the details here but apparently another way of counting shows a 4-5% difference.

Jacinta: We probably do need to go into the details in the end, but clearly this tiny .1% difference between humans is enough for us to determine the DNA as coming from one individual rather than 7 to 8 billion others. Strangely enough, I can well believe that, given that we can detect gravitational waves and such – obviously using very different technology.

Canto: Yeah the magic of science. So the Human Genome Project was officially completed in April 2003. And here’s an interesting quote from Wikipedia:

The “genome” of any given individual is unique; mapping the “human genome” involved sequencing a small number of individuals and then assembling these together to get a complete sequence for each chromosome. Therefore, the finished human genome is a mosaic, not representing any one individual.

Of course it would have to be a mosaic, but how can it represent the whole human genome when it’s only drawn from a small number? And who were these individuals, how many, and where from?

Jacinta: The Wikipedia article does give more info on this. It tells us that the project isn’t really finished, as we’ve developed techniques and processes for faster and deeper analyses. As to your questions, when the ‘finished’ sequencing was announced, the mosaic was drawn from a small number of anonymous donors, all of European origin.

Canto: But we all originated from Africa anyway, so…

Jacinta: So maybe recent ‘origin’ isn’t so important. Anyway, that first sequencing is now known as the ‘reference genome’, but after that they did sequence the genomes of ‘multiple distinct ethnic groups’, so they’ve been busy. But here are some key findings, to finish off this first post. They found some 22,300 protein-coding genes, as well as a lot of what they used to call junk DNA – now known as non-coding DNA. That number is within the mammalian range for DNA, which no doubt surprised many. Another blow for human specialness? And they also found that there were many more segmental duplications than expected. That’s to say, sections of DNA that are almost identically repeated.We’ll have to explore the significance of this as we go along.

Canto: Yes, that’s enough for starters. Apparently our genome has over 3 billion nucleobase pairs, about which more later no doubt.

References

http://humanorigins.si.edu/evidence/genetics

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

Written by stewart henderson

January 13, 2020 at 11:48 pm

epigenetics and imprinting 3: at the beginning

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stuff that can be done with iPS cells

A zygote is the union of two gametes (haploid cells), the sperm and the egg. It’s the first diploid cell, from which all the other diploid cells – scores of trillions of them – are formed via mitosis.

What’s interesting about this from an epigenetic perspective is that gametes are specialised cells, but zygotes are essentially totipotent – the least specialised cells imaginable – and all this has to do with epigenetics.

I’m not entirely clear about what happens to turn specialist gametes into totipotent zygotes, and that’s what I’m trying to find out. I’m not sure yet whether zygotes immediately start differentiating as they divide and multiply or whether the first divisions – in what is called the zygote phase, which eventually forms the blastocyst – form an identical set of zygotes. 

The two-week period of these first divisions is called the germinal phase. During this phase zygotes divide mitotically while at the same time moving, I’m not sure how, from the fallopian tube to the uterus. Apparently, after the first few divisions, differentiation starts to occur. The cells also divide into two layers, the inner embryo and the outer placenta. The growing group of cells is called a blastocyst. The outer layer burrows into the lining of the uterus and continues to create a web of membranes and blood vessels, a fully developed placenta.

But, as Nessa Carey would say, this is a description not an explanation. How does this initial cell differentiation – into the outer layer (trophectoderm), which becomes the placenta and other extra-embryonic tissues, and the inner cell mass (ICM) – come about? Understanding these mechanisms, and the difference between totipotent cells (zygotes) and pluripotent cells (embryonic stem cells) is clearly essential for comprehending, and so creating, particular forms of life. This PMC article, which examines how the trophectoderm is formed in mice, demonstrates the complexity of all this, and raises questions about when the ‘information’ that gives rise to differentiation becomes established in these initial cells. Note for example this passage from the article, which dates to 2003:

It is now generally accepted that trophectoderm is formed from the outer cell layer of the morula, while the inner cells give rise to the ICM, which subsequently forms the epiblast and primitive endoderm lineages. What remains controversial, however, is whether there is pre-existing information accounting for these cell fate decisions earlier than the 8-cell stage of development, perhaps even as early as the oocyte itself. 

The morula is the early-stage embryo, consisting of 16 totipotent cells. The epiblast is a slightly later differentiation within the ICM. An oocyte is a cytoplasm-rich, immature egg cell.

Molecular biologists have been trying to understand cell differentiation by working backwards, trying to turn specialised cells into pluripotent stem cells, mostly through manipulating their nuclei. You can imagine the benefits, considering the furore created a while back about the use of embryonic stem (ES) cells in medical treatments. To be able to somehow transform a liver or skin cell into this pluripotential multi-dimensional tool would surely be a tremendous breakthrough. Most in the field, however, considered such a transformation to be little more than a pipe-dream.

Carey describes how this breakthrough occurred. Based on previous research, Shinya Yamanaka and his junior associate Kazutoshi Takahashi started with a list of 24 genes already found to be ‘pluripotency genes’, essential to ES cells. If these genes are switched off experimentally, ES cells begin to differentiate. The 24 genes were tested in mouse embryonic fibroblasts, and, to massively over-simplify, they eventually found that only 4 genes, acting together, could transform the fibroblasts into ES-type cells. Further research confirmed this finding, and the method was later found to work with non-embryonic cells. The new cells thus created were given the name ‘induced pluripotent stem cells’, or iPS cells, and the breakthrough has inspired a lot of research since then.

So what exactly does this have to do with epigenetics? The story continues.

Written by stewart henderson

January 6, 2020 at 5:28 pm

epigenetics and imprinting 2: identical genes and non-identical phenotypes

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I’ve now listened to a talk given by Nessa Carey (author of The epigenetic revolution) at the Royal Institution, but I don’t think she even mentioned imprinting, so I may not mention it again in this post, but I’ll get back to it. 

The talk was of course easier to follow than the book, and it didn’t really teach me anything new, but it did hammer home some points that I should’ve mentioned at the outset, and that is that it’s obvious that genetics isn’t the whole story of our inheritance and development because it doesn’t begin to explain how, from one fertilised egg – the union of, or pairing of, two sets of chromosomes – we get, via divisions upon divisions upon divisions, a complex being with brain cells, blood cells, skin cells, liver cells and so forth, all with identical DNA. It also doesn’t explain how a maggot becomes a fly with the same set of genes (or a caterpillar becomes a butterfly, to be a little more uplifting). These transformations, which maintain genetic inheritance while involving massive change, must be instigated and shaped by something over and above genetics but intimately related to it – hence epigenetics. Other examples include whether a crocodile hatchling will turn out male or female – determined epigenetically via the temperature during development, rather than genetically via the Y chromosome in mammals.   

So, to add to the description I gave last time, the histone proteins that the DNA wraps itself round come in batches or clusters of eight. The DNA wraps around one cluster, then another, and so on with millions of these histone clusters (which have much-studied ‘tails’ sticking out of them). And I should also remind myself that our DNA comes in a four-letter code strung together, out of which is constructed 3 billion or so letters.

The detailed description here is important (I hope). One gene will be wrapped around multiple histone clusters. Carey, in her talk, gave the example of a gene that breaks down alcohol faster in response to consumption over time. As Carey says, ‘[the body] has switched on higher expression of the gene that breaks down alcohol’. The response to this higher alcohol consumption is that signals are generated in the liver which induce modifications in the histone tails, which drive up gene expression. If you then reduce your alcohol consumption over time, further modifications will inhibit gene expression. And it won’t necessarily be a matter of off or on, but more like less or more, and the modifications may relate to perhaps an endless variety of other stimuli, so that it can get very complicated. We’re talking about modifications to proteins but there can also be modifications to DNA itself. These modifications are more permanent, generally. This is what creates specialised cells – it’s what prevents brain cells from creating haemoglobin, etc. Those genes are ‘tightened up’ or compacted in neurons by the modifying agents, so that, for example, they’re permanently unable to express the haemoglobin-creating function.

All of this is extremely fascinating and complex, of course, but the most fascinating – the most controversial and headline-creating stuff – has to do with carrying epigenetic changes to the next generation. The inheritance of acquired characteristics, no less. Next time.

References

What is epigenetics? with Nessa Carey – The Royal Institution (video)

The Epigenetics Revolution, by Nessa Carey, 2011

Written by stewart henderson

January 3, 2020 at 3:58 pm

epigenetics and imprinting 1 – it's complex

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A useful summary - not explained in the text

The last book I read was The Epigenetics Revolution by Nessa Carey, though I’m not sure if I’ve really read it. So much of it was about persisting with the next sentence though I hadn’t fully understood the previous one. Biochemistry does that to me – too many proteins, versions of RNA, transposons, transferases, suppressors, catalysers, adjuvants and acronyms. And in the end I’m not at all sure how much progress we’re making in this apparently tantalising field.

So I’m going to pick out imprinting for starters, as a way of familiarising myself a little more with the epigenetic process of leaving tabs or marks on specific genes.

I know nothing about imprinting. Isn’t it what female birds do with their offspring, even when they’re still in the shell? Here’s how Wikipedia introduces it :

Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed in a parent-of-origin-specific manner. Forms of genomic imprinting have been demonstrated in fungi, plants and animals. As of 2014, there are about 150 imprinted genes known in the mouse and about half that in humans. Genomic imprinting is an inheritance process independent of the classical Mendelian inheritance. It is an epigenetic process that involves DNA methylation and histone methylation without altering the genetic sequence. These epigenetic marks are established (“imprinted”) in the germline (sperm or egg cells) of the parents and are maintained through mitotic cell divisions in the somatic cells of an organism.


This suggests that it’s not something life-forms do, it just happens. But there are a number of mysterious terms here that need exploring – ‘a parent-of-origin-specific manner’, ‘DNA methylation’ and ‘histone methylation’.

Briefly, to get all that DNA (between 2 and 3 metres to each nucleus) to fit inside that tiny space you need some expert packaging, and that’s where histones come in. They’re proteins that DNA gets wound around, like cotton reels, and together the histones and the DNA are called chromatin. They’re also divided into sections called nucleosomes.

DNA methylation is when a methyl group, derived from methane (CH3), is added to the DNA, affecting its activity, including repressing gene transcription. Histone methylation is when methyl groups are added to amino acids in histone proteins. Again these can repress or enhance gene transcription, depending on the amino acids and how they’re methylated.

The parent-of-origin thing is most interesting to me, and needs a bit more explaining. When a human sperm cell enters an egg cell, as the first step in fertilisation, it carries its load of 23 chromosomes in what is called a pro-nucleus. In a sense a sperm cell, much smaller than an egg, is nothing but a pro-nucleus surrounded by a membrane, with a tail for motility. Once inside the egg, the tail and the membrane are shed. The egg cell also has its load of 23 chromosomes in its pro-nucleus, but this is considerably larger than the male – and the human egg cell in its entirety has about 100,000 times the volume of a sperm cell. The point is that the differences in the male and female pro-nuclei have a lot to do with epigenetic effects including imprinting, which affect phenotypic traits, including disease prone-ness and structural effects in animals and plants. Tracing these effects in molecular terms to either parent therefore becomes a priority.

So, this is a little starter in what is an overwhelmingly complex topic. I shall return to it.

Written by stewart henderson

December 31, 2019 at 10:11 am

clever Charlie Darwin

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A photo taken by me! King Charles seated in state in the Musuem of Natural History, London. It was a thrill to be granted an audience

A photo taken by me! King Charles seated in state in the Musuem of Natural History, London. It was a thrill to be granted an audience

I recently decided to reread Darwin’s Origin of Species, which was really reading it for the first time as my first reading was pretty cursory, and I could barely follow the wealth of particular knowledge he used for cumulative effect to adduce his theory. This time I’ve been doing a closer reading, and becoming increasingly impressed, and I’ve only read the first chapter, ‘Variation under Domestication’.

Darwin’s argument here of course is that domesticated horses, dogs, birds and plants have been artificially selected over long periods of time, and often unconsciously, to suit human needs and tastes. This might seem screamingly obvious today, and to a degree it was recognised in Darwin’s time, but because of an inability to take the long view, and also because of the then-prevalent paradigm of the fixity of species, breeders and nurserymen tended to under-estimate their own cumulative powers, and to claim, for example, that dogs and pigeons had always come in many varieties. Even Darwin was uncertain, and was willing to concede – writing of course before the advent of Mendelian genetics, never mind the revolution wrought by the identification and analysis of DNA as the molecule of inheritance – that in some cases the breeders might be right:

In the case of most of our anciently domesticated animals and plants, I do not think it is possible to come to any definite conclusion, whether they have descended from one or several species.

He was even prepared to concede that it was ‘highly probable that our domestic dogs have descended from several wild species’, while at the same time arguing that the breeding of dogs, in Egypt, other parts of Africa and Australia (where, in his Beagle travels, he observed dingoes, which he may have seen as semi-domesticated by the Aborigines) extended back far further in time than most people suspected. We now know that Darwin’s concession here was ‘premature’. The latest research strongly suggests that our domesticated dogs trace their ancestry to a group of European wolves dating from 19,000 to 32,000 years ago, and probably now extinct. That’s a time-frame Darwin would’ve baulked at, and it’s both funny and kind of tragic that this is something I’ve ‘discovered’ after 30 seconds of selective internet searching. There’s no doubt, though that Darwin’s bold but always informed speculations were heading in the right direction.

Particularly informed –  and bold – were his speculations about pigeons. This is hardly surprising as he spent several years studying and breeding them himself. Interestingly, he started doing so because he’d become convinced that all the fancy pigeons then on show were most likely derived from one common species, the rock pigeon or rock dove (Columba livia), a view already held by some naturalists but few breeders.  He devotes several pages in Chapter 1 to arguing his case, for example pointing out that the ‘several distinct species’ argued for by breeders can be crossed with complete success, that’s to say with no signs of sterility or more than usually defective offspring.

So, as with dogs, I decided to look up what the latest research was on the ancestry of English carriers, short-faced tumblers, runts, fantails, common tumblers, barbs, pouters, trumpeters and laughers, to name some of the pigeons Darwin mentions in the chapter, and was excited to find that a piece of research published as recently as 2013 has confirmed Darwin’s hypothesis. Cheaper and faster genome sequencing technologies have enabled researchers to sequence the genomes of many wild and domesticated birds, and they’ve found that all of the latter are clearly closer to C livia than to any other wild species. It only took just over 150 years for Darwin to be proven correct.

Close reading like this really does reap some fun rewards, and I’ll finish with two more examples. Darwin wrote of how in the world of breeding, quite a drastic change can be brought about in one breeding step, as in the case of the fuller’s teasel with its hooks. He goes on:

So it has probably been with the turnspit dog; and this is known to have been the case with the ancon sheep.

Not knowing wtf he was talking about, I irritatedly decided to look up these unknown creatures. The turnspit dog is a now-extinct breed, bred specifically from around the 16th century to provide the dogpower to turn meat on a spit, the only conceivable way of cooking large joints of meat in your average fancy household for a couple of centuries. The dog, or dogs, because the system worked better if you had two of them engaged in shift work, turned a wheel by running inside it, rat-like, until the meat was cooked. They were known to be long-bodied and short-legged, but details of how they were bred aren’t known, as they were apparently beneath scholarly consideration. They certainly weren’t seen as cuddly pets – if you treat creatures as slaves it heightens your contempt for then (cf Aristotle) – and they were even taken to church as foot-warmers. They’d disappeared entirely by the end of the 19th century.

It's a dog's life?

It’s a dog’s life?

The ancon sheep was a short-legged type, apparently bred from a single individual in the USA in the late nineteenth century, its short legs having the singular advantage, to some, of curtailing its hopes of freedom by jumping the fence. The term ‘ancon’ has since been used by breeding researchers to describe strains of creatures arising from an individual with the same phenotype.

Achondroplastic_sheep

Written by stewart henderson

June 4, 2016 at 11:00 am

What is a trisomy?

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img014

Canto: So I happened to watch an excellent video from the Royal Institute recently, a talk by the beautifully named and beautifully voiced Irish geneticist, Aoife Mclysaght…

Jacinta: How do you pronounce that?

Canto: It’s pronounced Aoife Mclysaght…

Jacinta: Oh right.

Canto: So the theme was that everything in biology makes sense only in the light of evolution, and she was illustrating this through her area of interest and research, gene duplication. And along the way she talked about trisomies, particularly trisomy 21, usually referred to as Down Syndome.

Jacinta: A trisomy involves having an extra copy of a chromosome, in this case chromosome 21.

Canto: Very good, and the extra copy is a perfectly good copy, but having that extra copy causes major problems, obviously.

Jacinta: The term ‘trisomy’ refers of course to three – having three rather than two sets of a particular chromosome. Humans normally have two sets of 23 chromosomes. I have a relative who has a rare and unnamed form of trisomy, or at least a rare form of chromosomal disorder, which, when looking into it, I decided must be a trisomy. But since then I’ve discovered that Williams syndrome – which I learned about from another person I know with that condition – isn’t a trisomy, but the result of genes missing from chromosome 7. So now I’ve gone from thinking that trisomies accounted for all or most sorts of genetic intellectual disabilities to… I don’t know.

Canto: To a position of deeper ignorance. So people with trisomies have 47 chromosomes, with Down syndrome being the most common. Others include Edward syndrome (trisomy 18) and Patau syndrome (trisomy 13). Interestingly, though, there’s another rarer form of Down syndrome that’s due to translocation – that’s when a part of a chromosome – in this case chromosome 21 – migrates to another chromosome, usually chromosome 14, during cell division

Jacinta: That complicates matters… So do we know what causes these trisomies, and these translocations? They seem to be very specific, occurring for only particular chromosomes, or bits of them.

Canto: Well you’re right in that trisomies 21, 18 and 13 are relatively common – I mean rare but more common than a trisomy 9 or 15 or 19, just to pick out any numbers less than 23. We do know that trisomies become more common with older egg cells. As you know, your egg cells are as old as you are, and they become a little decrepit with age like yourself.

Jacinta: We’re both slouching to oblivion.

Canto: It’s also the case that most trisomies don’t survive to term, in fact they mostly miscarry so early that the mother doesn’t even know she’s been pregnant. So presumably those trisomies I just picked at random, if they occur at all, have more fatal consequences. It seems in any case that a trisomy occurs when cells divide but one chromosome somehow sticks to its homologue and is carried with it into the new cell. So maybe some chromosomes are more ‘sticky’ than others?

Jacinta: I think we need to do a deeper dive, as one pundit likes to say, into meiosis and aneuploidy.

Canto: Aneuploidy?

Jacinta: That’s just when you have an abnormal number of chromosomes per cell: it could be less or more. Actually trisomy 16 is the most common form, but fatal in its full-blown version. It can exist in mosaic form – when not all the cells have it.

Canto: So can you explain meiosis for us?

Jacinta: A long story but fascinating of course. It’s the basis of sexual reproduction for all eukaryotes. So before eukaryotic germ cells or gametes divide they need to replicate their chromosomes so that the resulting pair of cells has an equal share. This period of replication is known as the S phase.

Canto: Wait a minute, does this mean that in the S phase humans have 92 chromosomes per cell instead of 46?

Jacinta: Don’t bog me down with clever questions. Taking another step back, we have this whole process called the cell cycle, which we divide into phases. We can start anywhere, since it’s a cycle, if you know what I mean, but if you need a beginning it’s the prophase. Anyway, the S phase comes after the G1 phase and before the G2 phase. S, by the way, stands for synthesis, and G here stands for gap. Together these three phases make up the interphase, at the end of which we have the prophase of a new cell cycle, though actually meiosis isn’t a cycle the way mitosis (non-sexual reproduction or cell division) is. To be accurate, the next phase is called prophase 1, which is followed by metaphase 1, anaphase 1 and telophase 1 before we have prophase 2….

Canto: Stop cycling I’m getting dizzy.

Jacinta: Well yes believe me it’s complicated, and I haven’t begun yet. But you did ask for it.

Canto: Can you give the simplified version?

Jacinta: Not really.

Canto: Okay, we’ll leave that for another day. Focusing in on the part of meiosis when these trisomies and other anomalies occur, it seems that the problem isn’t so much stickiness as non-stickiness. Think of gametes. In mammals such as humans there are two types, egg and sperm cells. They’re differentiated by their sex chromosomes, chromosome 23…

Jacinta: And also by the fact that the egg cell is like the sun and the sperm cell is like the earth.

Canto: Well, sort of, in terms of volume. Now, after meiosis – which occurs in phases, meiosis 1 and meiosis 2, creating two daughter cells then four grand-daughter cells, so to speak – each of these grand-daughter gametes should be haploid. That’s to say, they should contain only one of each of the 23 chromosomes. But nothing’s perfect and sometimes there are errors, and we’re not clear about why, though the chances of error rise with the age of the female as mentioned before. Mostly the problem is that the chromosomes didn’t properly separate, a state called chromosome nondisjunction. Something to do with the spindle apparatus not functioning properly due to a lack of cohesion of the chromosome. This occurs rather more frequently in female meiosis, or oogenesis, than in male meiosis, or spermatogenesis, they’re not sure exactly why.

chromosome_nondisjunction_meiosis

Jacinta: Well I must say that’s all very enlightening, and salutary, as it’s made me aware of how little I know about genetics in general. Now I know a teensy bit more. As to trisomies and other such chromosomal problems, what they know just makes me keen to know more about how we might detect them and possibly in the deep future rectify them at source. But the science is clearly a long way from that…

Canto: Well you never know. Genetics is a fast-moving field.

Jacinta: we must explore it more. It’s serious fun.

Written by stewart henderson

January 31, 2016 at 10:06 pm