Chromosomes

I'm reading a book about genetics, but I'm really confused about chromosomes. It's mainly vocabulary trouble. It is my understanding that the human genome has twenty-three pairs of chromosomes. I commonly hear people say that humans have twenty-three chromosomes, does that mean chromosome types? Because if there are twenty-three pairs, then that would make forty-six, right? The book I'm reading is called The Language of Genes by Steve Jones. Here is the particular passage that's messing with me:"Every cell contains two copies of each of the chromosomes. The number is halved during a special kind of cell division in the testis or ovary. During the process, the chromosomes lie together in their pairs and exchange parts of their structure. Sperm or egg cells hence contain combinations of chromosomal material which differ from those in the cells of the parents who produced them."Now, when I first started reading this, I initially thought that when it said "every cell contains two copies," it was referring to chromosome pairs (two chromosomes of the same type; one from mom, one from dad). Then it says it halves, so we're left with twenty-three independent chromosomes. Then it says that the chromosomes lie together in their pairs, so then I went back and re-evaluated what it meant when it said "two copies," since based on my initial understanding, after it split in half, there would be no more pairs. So, now, my understanding is that there are forty-six chromosome pairs (which divide into the twenty-three chromosome pairs in the process from the quote). Which would now bring the count of individual chromosomes up to ninety-two.If I'm wrong (which I'm sure I am), please correct me. According to this picture (http://www.ncrtec.org/tl/camp/gene/male.gif), there are at least forty-six chromosomes (two times twenty-three). I'm totally confused by an apparent lack of clarity in what I'm reading (not clarifying whether it's about pairs of chromosomes or individual chromosomes, I'm just confused here).http://biology.udayton.edu/...I230/LECTHELP-2/1DNA-intro.htm - This is another source I was using to clear my confusion. This site further reinforces my idea that there are forty-six.Can someone straighten me out here?[This message has been edited by Tsegamla, 11-18-2003][This message has been edited by Tsegamla, 11-18-2003]

Tsegamla, You are right, they are wrong. In somatic cells (non-sperm or egg) we have 23 chromosome pairs, totalling 46 in all. These cells with paired chromosomes are considered diploid. Meiosis is the type of cell division that produces gametes (sperm or egg) with only 23 chromosomes total. None of them are paired, that is to say one half of the pair ends up in one gamete, the other in another. These cells are termed haploid. When a sperm makes a union with an egg the two individual sets of 23 chromosomes become 23 pairs/46 total again.MarkMark------------------
"The primary purpose of a liberal education is to make one's mind a pleasant place in which to spend one's time" - Thomas Henry Huxley[This message has been edited by mark24, 11-18-2003]

All right. For some reason, I was reading that passage a weird way and I think I get it now. Is the passage saying that as the chromosome pairs are divided, the two chromosomes exchange certain genes back and forth and whatnot right before the division, so the original orientation is lost? Are the X and Y chromosomes actually paired up? If so, why are certain traits always with the X and are never exchanged during this process with the Y? If not, do they just randomly join one of the gametes?EDITED FOR CLARITY[This message has been edited by Tsegamla, 11-18-2003]

Tsegamla,Right, what actually happens during meiosis is that the homologous pairs of chromosomes are brought in close proximity, & entire lengths of chromosomes are swapped over. This is called recombination or crossing over.Consider a chromosome pair, labelled one & two, before recombination.1/ aaaabbbbbgggggtttttt
2/ bbbbaaaaatttttgggggg& after:1/ abbabbbbbgggggttgggg
2/ baabaaaaatttttggttttThis is one of the big advantages of sex, increased variation due to recombination.In retrospect, I may have confused the issue, please consider the letters agtc as genes, not nucleotides, if it makes it easier to understand.Mark

Yes, I only recently found out the the process mixes up parts of the paired chromosomes. You are cleaver it never occured to me to ask what happens to the XY pair.

Tsegamla, No. Or yes, but they don't recombine.Mark

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All right. For some reason, I was reading that passage a weird way and I think I get it now. Is the passage saying that as the chromosome pairs are divided, the two chromosomes exchange certain genes back and forth and whatnot right before the division, so the original orientation is lost?

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Yes.

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Are the X and Y chromosomes actually paired up?

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Yes.

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If so, why are certain traits always with the X and are never exchanged during this process with the Y? If not, do they just randomly join one of the gametes?

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Good question. There are actually just a few genes that cause sexual differentiation to male in the fetus; these are located on the Y chromosome in a region called SRY (Sex Region Y) , which is part of a family of genes related by a type of protein known as an HMG box; genes from this family are referred to as Sox genes. The proteins that they code for bind to a specific region of DNA and can modify chromatin structure by unwinding DNA.SRY lies just outside the pseudoautosomal regions - regions in the chromosome in which gene exchange occurs. As a consequence, it *typically* remains on the chromosome. However, miosis isn't always perfect, and as a consequences all sorts of chromosomal problems can occur, and the sex chromosomes are no exception - thus, you can get XX males and XY females in this way (in addition to several other conditions, such as SRY mutations). Typically, they're sterile, but physically normal. You typically only get physically noticable differences either from irregular hormonal influences from the mother, or from sex-linked mutations further upstream (such as AIS, which is typically caused by a mutation in the child which one or more types of androgen receptors become less receptive to androgens).------------------
"Illuminant light,
illuminate me."

Is "Humans have 23 homologous pairs of chromosomes per body cell" a true statement? It (http://biology.udayton.edu/...I230/LECTHELP-2/1DNA-intro.htm) says:
"Homologous chromosomes are identical in:sizeshapeposition of centromere (the little indentation which is either central off-center)Staining pattern (see above)"Shouldn't there just be twenty-two homologous pairs? The X and Y don't look to be the same size in the drawing or in that other picture I linked above.EDIT: Wouldn't you call it heterologous or something?[This message has been edited by Tsegamla, 11-18-2003]

I think they would say you are nit picking.

You are right, the statement isn't necessarily true. Females have 23 homologous pairs and males have 22 homologous pairs with the other pair considered partly homologous.

Yes, there are reasons for considering XY to not be homologous chromosomes, but I don't see anyone as of yet pointing out the reasons why they should be.Two other properties of homologous chromosomes are as follows:1) One comes from the maternal parent and the other comes from the paternal parent2) They synapse during prophase IThe second is specific to homologous chromosomes and is a key indication that two chromosomes are homologous. For example, many chromosomes can be the same size so that doesn't nail it down. The X and Y chromosomes do synapse to form tetrads (or bivalents).As far as (1), it concerns pairs of chromosomes. If you first match up all of the autosomes you are left with either:a) a pair of X's, one that came from the mother and one that came from the fatherorb)a single X and a single Y, the first of which came from the mother and the second of which came from the fatherSo XX meet all criteria, and XY basically meets 2 of them, with one of those being very indicative.Also, the Y chromosome evolved from an X chromosome, so at some point in time, they were truly homologous.