Brushing up on some basic biology, it seems the chromosomes only exist during a good part of the act of mitosis (cellular division for non-sex cells). And, they are only paired-up during a brief period during mitosis. Yet, we speak of pairs of chromosomes as if they’re always paired-up, specifically the last pair which determines the sex traits.
So when we say human cells have 23 pairs of chromosomes, are we only speaking of pairs which form during one hour out of about 22 hours (for many cells) in the cell cycle? Also, IIRC, each pair is really two individual chromosomes (i.e.: sister chromatids) attached at the center by a centromere…forming one “X” shape. But, when we speak of the last pair (that which carries the sex traits), it was taught to me that we pair up the “X” and “Y” (males) or “X” and “X” chromosomes…but isn’ each of these last “X” shapes a pair unto itself? And, the “Y” is what? One chromatid?
Ice Wolf, thanks for trying. But, did you read my question carefully? Your link just restates what I have stated…and exactly what I am questioning. We speak of these pairs so loosely, we think they always exist. But, the pairs only form during cell division, it seems. I guess I was under the impression the pairs always exist…not just for the purpose of cell divsion. And, this still doesn’t answer if it is technically correct to speak of an “XY” pair, for example… …and does the “Y” duplicate to form two sister chromatids, as well??? - Jinx
The human cell always contains 46 chromosomes (not including the sex cells, which contain 23). You are correct that they only coil up and form recognizable structures during certain parts of the cell cycle, but they are always present. They just unwind and become too amorphous to recognize individually.
The 46 chromosomes are comprised of 23 pairs, one of which we received from each parent, and one of which will appear in each sex cell. 22 of the pairs are identical – that is, they code for the same genes, although the variety of those genes they code for may be different (for instance, chromosome six may contain the gene for eye color, although one copy may have a green-color gene from your mother and the other copy may have a blue-color gene from your father). This is what we mean by “pair” – that they code for the same genes, not that they are physically in contact or lined up next to each other. The other pair is either identical (XX – a female) or different (XY – a male). The Y chromosome is actually a different chromosome from the X; they are not really a pair at all. The Y is very short and contains a completely different complement of genes.
The normal picture of a chromosome that you see in books and magazines is as an X-shape. This is very confusing, as this is not a pair of chromosomes, but two sister chromatids attached to each other. (A pair is actually two X-shaped chromosomes out of the 46.) The X-shaped chromosomes composed of two sister chromatids only exists during a short portion of the cell cycle, before cell division, so that each daughter cell gets one copy of each of the 46 chromosomes. The rest of the time, the chromosomes can be thought of a single straight bars with the genes at different positions along their length.
Finally, the X and Y chromosomes are just names – they have nothing to do with the shapes of the chromosomes. We could just as well have called them Frank and Alice. In fact, maybe we should have, to prevent confusion. Both the X and Y chromosomes are linear, and form an X-shape after they replicate and before the cell divides.
I know this is all garbled and confusing. If you need any further clarification, just ask.
while you are of course right, when chromosomes were first discovered they would have been quite indistinct under the microscope. I wonder whether the “Y” designation was because the much smaller Y chromosome may have appeared to look like a “Y” e.g. see http://www.contexo.info/DNA_Basics/chromosomes.htm for the sort of thing I mean. Who first designated them “X” and “Y” ?
Well, “chromosome,” of course, means “colored body,” referring to the fact that they appeared as darkly stained objects in the first cellular microscopy observations of the nucleus. The chromosome pairs 1 through 22 are numbered in order of decreasing size. The Y chromosome is the smallest human chromosome, and, as far as I know, is not noticeably y-shaped. I am not aware of where the designations X and Y for the sex chromosomes originated, but perceived shape or mistakenly perceived shape is certainly a possibility.
I have wondered about the names X and Y. Here is my (conjectured) explanation, although I am open to other possibilities. When I was taking biology (the only biology course I ever took was in 10th grade in 1951-52) it was believed that there were 24 pairs of chromosomes. Although it was never so stated, perhaps they were named A, B, …, X and then since the second chromosome was different in males that was called Y. Now we know there are only 23 and they are named 1,…,22, X and Y. Incidentally, in female cells one of the two X chromosomes seems to degenerate into a tiny ball called IIRC a Barr body. This happens at random. I assume it is because double doses of whatever proteins the X codes for would be deleterious. And, sexual markers aside, there is not much on the Y. BTW, in many organisms, females are the ones with two different chromosomes and they are called W and Z in such cases. And there are even some mammals in which the Y has disappeared.
Here is a good picture of a karyotype. This is the type of data that would have been available when the chromosomes were being named. Note the Y chromosome. It’s telocentric, which means the centromere (the bit where the two sides stick together) is close to one end. The two longer ends are long enough to visibly separate, but the shorter ends look clumped together, so it does indeed look like the letter Y.
You remember correctly. The amount of protein product produced is strongly dependent on the number of copies of the genes present. I.e., if you have the wrong number of copies of chromosomes – and therefore the wrong number of genes – the protein abundances will be very screwed up. The only survivable condition I am aware of resulting from having the wrong number of chromosomes (excluding the sex chromosomes) is Trisomy 21, also known as Down’s Syndrome. This results from an extra copy of one of the smallest chromosomes, and the extra copies of whatever genes reside on this chromosome obviously result in some serious problems for the individual’s development. Every other condition that I know of involving the wrong number of chromosomes (excluding sex chromosomes) results in inviable embryos or children that die in infancy.
So obviously, if females had an extra active copy of the large X chromosome, with all of the genes it contains making extra proteins, there would be a problem. Either the males would have serious deficiencies in protein concentrations, or females would have serious excesses. To prevent this, only one copy of the X chromosome is active in every female cell. The shut off X chromosome condenses into a structure called the Barr body. (Incidentally, this is a random process – at a certain point a few cell divisions after fertilization, each cell randomly shuts off one of it’s X chromosomes, and each daughter cell of that cell has the same one shut off. This results in patchy distribution of characters coded on the X chromosome and produces such things as calico cats, where every patch of different color is the descendant lineage of a cell that shut off a particular X chromosome.) This process allows certain conditions involving the wrong number of sex chromosomes to have little to no effect – XO males, XXY males, XXX females, etc. – as all of the X chromosomes except for one copy are turned off and potential problems are prevented. (XYY is also viable because the Y chromosome is so short, and codes so few essential genes, that the effect of having two of them is not great.) Although these conditions may cause some problems, many people are not even aware that they have them.
As regards the sex-determination system, I am not aware of any mammals that have anything other than the XY system, but I may be mistaken. However, birds and some insects have the “WZ” system, where the male has two identical sex chromosomes (WW), and the female has one of each (WZ). Just as in the XY system, the designations W and Z have no significance other than identification, and the only reason the labels X and Y are not used is because the rolls are reversed. Many other strange and creative sex-determining systems exist, including the haploid/diploid system of the Hymenopteran insects (bees and ants), where the male is haploid and the female is diploid. This is particularly interesting, as it means that the queen shares 3/4 of her genes with her daughters, rather than the 1/2 in the XY and WZ systems. This is probably what allowed this group to evolve such wonderful and complex social behavior. See the writings of Richard Dawkins for great explanations of this.
Couple of questions, brianmcc. First, isn’t an XO phenotypically female? I had understood that the Y was necessary for maleness.
Second, what about sex-linked defects? As I understand it, if a woman inherits one copy of the colorblindness gene, for example, she can still see normally, because she’s got the good genes on her other chromosome. But what if all of the cones of her eye or eyes decended from the same progenitor cell, and that cell happened to have the “color vision” chromosome turned off? Surely, which X degenerates can’t be dependent on which one “works right”, since the body would have no mechanism for knowing what’s supposed to be “right”.
Finally, aren’t there some animals where sex isn’t genetic at all? I’m thinking that this might be the case for some reptiles and fishes. Turtles, for instance, are dependent on temperature of incubation, as I recall?
Hell of a good question, but, unfortunately, I have no idea what the answer is. Perhaps something as simple as diffusion can transport protein products to where they’re needed. Maybe in humans, generation of the Barr bodies happens late enough in fetal development that the regions are sufficiently small so that the entire retina is not the descendant of a single cell. I really have no idea. Hopefully someone who can answer this will come along…
Yes, there are all kinds of crazy schemes out there for determining sex. As you mentioned, some reptiles’ sex is determined by the temperature at which the eggs are incubated. Some fish and amphibians can switch back and forth between sexes depending on their age, environmental conditions, or the sex ratio of the local population. Some species of fish, reptile, and insect are all female and reproduce by parthenogenesis – the growth and development of an unfertilized egg. In fact, some salamanders practice a special form of parthenogenesis where they mate with males of another species; the sperm being used not for its genetic material, but to chemically trigger the egg into developing. Some annelids and mollusks are hermaphroditic, either both sexes at the same time, or each in turn. In some arthropods, sex is determined by infection – all individuals are ZZ, and sex is determined by the presence or absence of a bacterium. Basically, just about any whacked out way to determine sex is practiced somewhere in the animal kingdom.
They’re female, yes, but they suffer from a condition knows as Turner’s syndrome. A quick google search can tell you more. There are a handful of genes on the X chromosome that need two active copies. In normal females, the Barr bodies aren’t 100% deactivated. There are a few regions that are still active. In men, the Y chromosome contains copies of these genes.
I think this would fall in the category of “technically possible but not very common”. The term describing this is mosaic. I remember reading about some human mosaics (though I can’t remember the details offhand), but I don’t think I’ve ever heard of mosaic colorblindness. YMMV, naturally.
Even though Smeghead and brianmcc are doing an admirable job, if I don’t respond in threads like this, I may as well not respond to any threads…
I’d just like to mention that the fruit fly has an X-Y sex determination scheme as well. It is confusing, because it works a little differently. Two X chromosomes = female, but it is all about dosage. Instead of the female downregulating one X chromosome, the male doubles its X chromosome genetic activation. So, in fruit flies, a “Klinefelter” fly (XXY), is female (although the same sex chromosomes in humans is male, with Klinefelter’s syndrome).
There are a number of aneuploidy (disorder in the number of chromosome) syndromes, the most common of which are the sex chromsome ones (Klinefelter’s, Turner’s, XYY syndrome, XXYY, etc.), but Down Syndrome is not the only autosomal one. Interestingly, the human autosomes are misnumbered, and 21 is the smallest of the autosomes. Trisomy 22 is embryonic lethal. But trisomy 18 is not (Edward Syndrome), nor is trisomy 13 (Patau Syndrome). The rare infants born with these suffer multiple malformations and usually don’t live very long, although there are case reports about exceptions. There are even cases of full genome triploid conceptuses surviving until birth.
Next, it is helpful to think of a “C” number and an “N” number when thinking about chromosomes. It is the only way I was able to learn it in first year graduate school, after failing to learn it in high school, undergrad, and medical school.
C is the complement of DNA in a cell. 1C = 23 chromosomes, 2C = 46 chromosomes. Think of this as the number of copies of each chromosome.
N is the haploid number of DNA in a cell. This represents how many different chromosomes are in a cell. 1N = 1 parental copy of each chromosome (1 set of maternal or paternal chromosomes), 2N = 1 copy of paternal, 1 copy of maternal chromosomes. Think of this as “type” of chromosome. We each have two “types” of chromosome 1, etc. Obviously it is not possible to have more than 2N in a cell (as we only have two parents), but it is possible to have more than 2C.
Understanding the distinction between N and C number is crucial. Every cell in the body except germ cells are 2N2C. Germ cells are 1N1C. In mitosis (regular binary division of each cell), the cell is always 2N. But, before it divides, it obviously replicates its DNA so it has two copies of every chromosome, bringing it from 2C to 4C. Then, the cell divides and 2C of DNA goes to each daughter.
In meiosis (sex cell division), it is completely different. You start here with a primary germ cell which is 2N2C like every cell in the body. The cell replicates its DNA, like mitosis, which brings it to 2N4C. But here is the kicker. The first division here reduces the N number (a unique event in the body) after meiotic recombination to reshuffle paternal and maternal genes. The two cells produced from meiosis I are 1N2C now (as each has only one type of each chromosome and two copies of this type). Meiosis II reduces the C number (just like regular mitosis), which makes the final four 1N1C germ cells.
I shouldn’t say that the male fruit fly doubles its X chromosome expression. It does upregulate it, but the nature of sex determination is that it has less total gene expression on a suite of X-linked genes than a female, which causes a cascade of sex-specific gene expression. So XXY flies are female.
Ah yes. I had forgotten that the entire chromosome is not rendered inactive. Too long since I’ve taken genetics. I knew there had to be a simple answer to that question.
That is interesting to know that chromosome 21 is indeed the shortest. I was aware of trisomy 18 and 13 leading to occasional live births, but I believe most infants die within the first two years. I’ve also heard that trisomy 8 is possible, but extremely rare. See “Principles of Genetics” by Robert H. Tamarin for a good coverage of these topics.
Bryanmcc, I once thought I understood this, but I believe I had a simplistic picture in my mind the whole time. Please repeat for me…I believe you said something about how traits need not be paired up (in relation to how we pair up chromosomes)?
This was specifically regarding how the chromosomes uncoil and become non-distinct during interphase. I am most confused about the following:
a) If I could see a two sister chromatids joined by the centromere, would I see two identical DNA helixes joined by the centromere?
b) Also, when Mendel gave us the concept of a dominant allele and a recessive allele joining to give us a trait, so to speak…this is NOT a pairing up of the chromosomes, but rather a simplified model of the two halves of one rung (for sake of argument) on the DNA helix ladder?
Perhaps I am mixing up the two-halves of the DNA ladder and the pairing of chromosomes…? (I know Mendel didn’t have the DNA model to work with…)
Wait, this can’t be correct either, can it? One on hand, you said the chromosomes don’t have to be paired up to determine traits, but when we have XX as a the 1st pair, let’s say, isn’t this:
a) Each “X” shown above just a sister chromatid joined by a centromere?
b) Didn’t each “X” above ultimately come from each parent?
c) If so, then isn’t necessary for each “X” to pair up with its
counterpart “X” to match up the correct alleles to make each of our traits? I assuming each “X” is donated from the parent gamette, and the two corresponding “X” shapes must pair-up to form all the traits.
On the other hand, if each “X” is a sister chromatid, then each half of the “X” contains a complete copy of the DNA coil with the traits determined by each rung of the helix ladder…and if this is so, then why do we even need the donated “X” from the gamete? It seems like the two parts forming a pair of chromosomes doesn’t even interact or bond with each other…
During mitosis, the cell has two copies of all the genetic info both daughter cells will need. The “X” shapes (sister chromatids) simply align alongthe cell equator awaiting for the fission of the cell. They do not pair up with an identical counterpart, do they?
Very confusing! Am I making it harder than need be? Not sure how the individual alleles meet up in all of this… Where am I going wrong? - Jinx : confused:
Jinx, your terminology is a little muddled, and I think you have some confusion about basic concepts, so I’m having some trouble figuring out what exactly it is that you’re asking. But I’ll try to walk you through it.
DNA is a very long, straight, string-like molecule (of course, DNA is made of the famous double helix, but at this scale, it’s basically string-like – imagine a spiral staircase a yard wide, and a hundred miles long). The human cell contains, more or less, 46 seperate, very long DNA molecules – the chromosomes. There are three billion base pairs split among those 46 molecules, so you can see how long each one must be. The vast majority of the time, the chromosomes are unraveled in a big, tangled, spaghetti-like mess. They have to be like this so that the DNA is exposed to allow transcription – the process which leads to protein production – to occur.
Only during a short period immediately prior to and during replication are the chromosomes distinguishable as distinct individual bodies. This is accomplished by winding up the DNA string on structural proteins making bigger loops and spirals, and then twisting those loops and spirals into super-loops and super-spirals, and twisting those into ultra-super-loops and spirals, etc. Now perhaps you can see why the DNA spends most of its time in a loose knot: when it’s all twisted and coiled up tightly, the cellular machinery can’t get in to access the genes and make proteins.
A single copy of a chromosome all coiled up like this will look like a short, squat bar with a kink in it – the centromere. There will be 46 different bars, and 23 pairs of them will have very similar lengths. Those are the pairs of the chromosomes, but note that they are pairs or chromosomes and not identical chromosomes – i.e., you will have two of chromosome 5, one from your mother, and one from your father, and they will have genes that code for the same proteins, although not necessarilly the same exact copies of those genes.
Now, before cellular replication, each individual chromosome of the 46 must replicate itself to pass a copy to each daughter cell. That is, both mom’s copy and dad’s copy of chromosome 5 must pass a copy of themselves to the daughter cells. So each mile-long spiral staircase splits down the middle of the rungs, and uses the rungs as templates to re-create two copies of the staircase. Now, when these two identical copies of the chromosome coil up, they will produce the x-shaped structure that you are used to seeing in pictures of chromosomes – two sister chromatids attached at the centromere. So now the cell has 92 individual chromosomes in it – two copies of the 46 chromosomes, one of each which will pass to the daughter cells. So, for chromosome 5, you have four copies of it: two from dad forming one X of identical sister chromatids, and two from mom, forming the paired X of sister chromatids, identical to each other, but slightly different from dad’s pair. Once the cell divides, the sister chromatids pull apart, leaving 46 bar-shaped chromosomes in each cell, made up of 23 pairs that are not attached to each other.
Yes. The two sister chromatids attached by the centromere are identical. They are two copies of mom’s (or dad’s) contribution to the pair of chromosomes. They are not the pair itself – there are two sets of two sister chromatids for each chromosome, and during cell division they are pulled apart.
Mendel’s model of genetics was very simplified – as you mentioned, he had no concept of DNA. Most genes don’t have a simple dominant/recessive relationship, but rather a more complex behavior. But we’ll look at one traight which (in our idealization, anyway) can be pictured as strictly dominant/recessive. Say there is one gene for eye color, and that it is on chromosome 5. That means that every cell in your body has two copies of the eye color gene, because they have two copies of chromosome 5. Most of these are turned off, because they are in places in your body where they are not needed. But in your eyes, they are turned on. That means that in the huge ball of spaghetti, there are two lengths where the genes for eye color are going through the motions necessary to create proteins. These two sections are not necessarilly in any way in physical contact, or any other way associated with each other – there just happen to be two copies of the gene making protein. In fact, if some molecular snippers came along and cut one of the copies out of one of the chromosome 5’s, and pasted it into chromosome 6, it wouldn’t matter (at least until cell division occurred) – the two copies of the gene would still do their thing just fine.
The way the dominant/recessive thing works is like this: the two copies of the gene make the same thing – a protein which is used as the eye-color pigment. When these genes are churning out normal protein, the eye becomes pigmented and appears brown. But there may be another version of this gene: one in which the protein doesn’t work in the same manner – it has a kink or a deficiency of some sort that makes it ineffective as a pigment, it is effectively clear. Since you got two copies of chromosome 5, one from each parent, you could have received one copy of the normal gene from one parent, and one defective from the other. The defective gene churns out proteins just fine, but these defective pigments do nothing to color the eye. However, the one functioning copy of the gene makes enough pigment so that the eye appears a normal brown color. If you receive two copies of the defective gene, though, suddenly you have no functional pigment protein molecules, and the eye loses it’s color – it appears the blue of the tissue minus the pigment.
So you see that there is no actual pairing up going on. Each copy of every gene does it’s thing, and the combined effects show themselves in the organism. So, (virtually) every gene you have in your body, you have two copies of, which are not associated with each other at all, other than the fact that they lie on paired chromosomes so that they can be split up in the right way during cell division. Sometimes these two copies will be identical, creating identical proteins, and sometimes these two copies may be slightly different, procucing the “alleles” of Mendelian genetics. Depending on how the chemistry of the individual protein varieties works, the traits may show dominant/recessive, partial dominance, or any other conceivable behavior.
Yes, I think you are. The two halves of the DNA ladder on the same molecule, and they say exactly the same thing (or, rather, one says the inverse of the other). The reason the DNA is two stranded is so that you can rip the molecule in half, and use each half to make a copy of the other. It’s like a cast and mold: with the cast you can make a new mold, and with the mold you can make the same cast. Both contain the same information. So it is with DNA. You can’t code varieties in the same molecule, because both halves have to say the same thing as each other.
Hopefully that clears things up a little bit. If not, just say where you’re confused, and we’ll have another go.
