I am just asking this for curiosity. Normally females have two X chromosomes, but in every cell, only one is active and the other is curled up into something called a “Barr body”. So a woman is a mosaic, with adjacent cells perhaps having made the opposite choice of which X to inactivate.
There must, however, be more to it than that. For you wouldn’t expect any abnormality to result if a woman had only one X chromosome, only she wouldn’t be a mosaic. Every normal male has only one X and no abnormality results. But that is wrong. A single X female is definitely not normal, see Turner syndrome - Wikipedia for details. Can anyone explain how this differs from what happens in normal females?
WAG: whatever regulatory mechanism is responsible for inactivating a single X chromosome in normal females, also acts upon the only X chromosome in Turner Syndrome females? Perhaps it acts on half the cells?
The answer is that the Barr body chromosome isn’t completely deactivated. There are small areas of euchromatin on them - that is, decondensed areas that represent active transcription. Functionally, they fill the same functions as certain regions on the Y chromosome.
The paradox you noted was prompted further investigation of the situation, which led to this discovery.
I suspect that the dosage compensation you’re referring to is what keeps XO from being lethal. Aside from this and other aneuploidies involving sex chromosomes, the only one we hear much about is Down syndrome, which involves an extra copy of chromosome 21. I imagine that the others are usually lethal. I also suspect that it’s no coincidence that Down syndrome involves the smallest chromosome (21 is apparently smaller than 22).
I wondered if it was something like and your explanation is very satisfying. I am amazed how complicated genetics has become. I came of age in the mid 50s (I heard Linus Pauling lecture at the Franklin Institute in Philadelphia on the double helix (how’s that for the unity of science?) and it all seemed so simple then.
I don’t know much, but I’m pretty sure we still don’t know anything now.
There’s a fantastical code in there that turns 800 megabytes of data (in fact much less, considering the junk dna) into the human form–and brain included! When was the last time a Microsoft OS fit into that little?
Not sure what you mean by lethal, but many XO babies die in utero, too damaged to survive until birth. If they are able to make it to birth, they seem to be fairly OK.
Yeah- I had to learn that most XO children die in the womb or right around birth, usually due to hydrocephalus or massive cardiac defects. Some people have thrown around numbers saying that as much as 10% of all spontaneous abortions are due to Turners, but I’m not sure how reliable that is.
That said- the ones that tend to survive and do the best (ie: survive past childhood) tend to be mosaics. Meaning they’re not fully XO, but have a FEW XO cells and a few normal cells as well- they’re the ones who tend to have these deformations and such.
To have a PURE XO Turner’s living past childhood is usually a pretty rare thing, as it DOES cause some pretty severe defects.
There is one other autosomal trisomy that isn’t always lethal - trisomy 18 causes Edwards syndrome, which is much more severe than Downs. Rarely, trisomy 13s can survive as well. But yes, most organisms end up being more tolerant of sex chromosome aneuploidies because of the natural variation between sexes. Fruit flies, for instance, can generally handle lots of weird sex chromosome numbers without serious problems.
Smeghead can elaborate on this, but the so-called “junk dna” turns out to be not junk at all. It is just that we had not been able figure out their value. I think it’s been over 10 years since scientists realize they have a role in ending protein sequences, beginning such sequences, correcting transcription errors, etc.
Indeed. DNA is proving to be far more complicated than people ever imagined. I just sat through a seminar today showing how a region of the yeast genome binds a certain protein which acts as velcro to bring two hugely distant regions of a chromosome together to allow homologous recombination between them. This is in a completely gene-free region. I’ve done some work myself looking at areas that would have been considered “junk” not too long ago. I haven’t figured it out yet - just a few tantalizing snippets. But the point is that coding for proteins is turning out to be just one of many many functions hidden in our genome.
True, but scientists these days - and, to be fair, even back when “junk DNA” was a more common phrase - are hesitant to assume that because we don’t understand it yet, it has no function.
This is absolutely not true, unless you are counting all Turner’s conceptions, including those embryos that are spontaneously aborted, in your overall average. Of those children with Turner’s syndrome who actually make it to birth, most are able to survive childhood because 1) many of them don’t have the riskier cardiac defects associated with Turner’s syndrome and 2) medical science has come up with good ways to help people with cardiac birth defects to survive.
That was so early that I don’t think the one gene = one protein had even been clearly formulated. It was 1955 and Watson and Crick had made their discovery only two years earlier, probably beating Pauling by a few months. He had just published a proposed structure that involved a helix with the bases facing out. But it was complementary pairs with A-T and C-G that was not only consistent with the crystallographic data but also, it did not escape their attention that it could elegantly explain DNA duplication. I think one gene, one protein became dogma in the 60s.
I think it was in 1955 that a mathematician proved that with 3 letter codons made up from 4 letters, the most number of amino acids you could unambiguously code for, assuming there were no start or stop codons, was 20, that being the number of amino acids known to be used in living cells. Of course there are start and stop codons, so that would leave the possibility of 62 amino acids, but there are still only 20.
I don’t follow. The maximum you can encode is 64, and you don’t need to be a mathematician to see that. I don’t know how you’d come up with 20.
Perhaps the point is that to encode 20 you’d need at least three letters per codon, since with two letters you can only encode 16.
How would the existence of start and stop codons increase the number of amino acids that can be encoded?
There are three stop codons. There is a start codon (sort of), but it’s not used exclusively for “start”. In fact it’s the only way to encode the amino acid methionine.
Probably, with the caveat that we may always discover some use for them down the road. That’s my real problem with the term - it implies that we know for sure that it’s useless, when it’s much more accurate to say that we currently don’t know of any purpose for them.
Suppose you wanted to build an unambiguous code that you could look at any random place and find that you only one reading is possible. This puts a severe limit on how many codons make sense. Say you found a sequence of 6 ATGACG, but only GAC coded for an amino acid, while none of ATG, TGA or ACG did. So those three sequnces could not be part of the code. Then the maximum number of amino acids you could code for was proved to be exactly 20. Since that is the number of amino acids, that seemed like too much of a coincidence. But coincidence it was and the reading mechanism must start at a start codon, read without skipping a beat until it finds a stop codon.
In retrospect, the idea of non-consecutive reading seems absurd. It is not amino acids a gene codes for but a sequence of them. If the reading mechanism lost its place, what would you get? But such was the state of knowledge (or ignorance) in those days that somehow the absurdity didn’t seem to occur to anyone, at least immediately. Now a lot more is known, but there is still a lot that isn’t.
Yeah, people were naive. You have to ask how they could not have anticipated all this? Too eager to have everything “explained”–to have a feeling of understanding?
What I strongly suspect is that not only are there fantastically complex codes and processes that control the organizations of tissues as they grow, but that there are equally complex processes that aim to optimize evolution and mutation. Understanding those would be magnificent.