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Reason why chromosomes come in pairs

  1. May 20, 2006 #1
    Is the reason why chromosomes come in paris, the fact that they look like a cross and each 'slash' (i.e / and \) is one chromosome? So two of them together to make an X shape is counted as a pair? We have 23 X or pairs of chromosomes in every cell?
     
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  3. May 20, 2006 #2

    Hootenanny

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    They are not actually crossed, they are 'bent' V shape coils joined by a structure called the centrimere. A normal human has 23 pairs of chromosomes is every cell in the body, except in haploid cells (gametes) where they have 23 chromosomes.

    ~H
     
  4. May 20, 2006 #3

    Curious3141

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    Just adding on a clarification. The crossed structures the OP is talking about are actually sister chromatids linked by a centromere and they're attached lengthwise (| + |). Chromosomes are asymmetrical structures with a long arm and short arm and in the paired chromatid structure, the chromatids lie parallel to each other (short parallel to short and long parallel to long). They separate along the lengthwise axis.

    Just to clarify, in a normal resting cell, chromosomes aren't found like this. These formations occur only during the preparation for cell division (be it mitosis or meiosis), during a particular phase called metaphase. In a resting (interphase) diploid cell, the chromosomes are not duplicated, and the cell is said to have 2n chromosomes, which is a full diploid complement. In the case of humans, n = 23. Half of those chromosomes came initially from the mother and half came from the father. In a resting diploid cell, the homologous (maternal and paternal) chromosomes are not spatially associated, and all the chromosomes are long, stringy and indistinct.

    In the prelude to a mitotic division of a diploid cell, the chromosomes start condensing into fat cigar shaped structures that we're familiar with. In addition, DNA gets multiplied at this stage to give a total complement of 4n (tetraploidy). This is accomplished by each "cigar" forming a double, which are linked together to get the crossed structure we've been talking about. Each "cigar" in a crossed structure is properly called a sister chromatid. At the end of metaphase, these structures are fully formed and lined up along the mitotic spindle, ready to separate from one another. Note that in a mitotic division, the maternal and paternal DNA does *not* separate. Only the exactly duplicated sister chromatids separate. This results in 2 daughter cells, each with a perfect diploid (2n) complement of identical DNA (the two daughter cells are clones).

    In meiosis things are somewhat more complicated. Meiosis happens only in gametogenic cells (germ cells), which have a 2n complement but must give rise to haploid (n) sperm cells or egg cells. A single sperm cell (or egg cell) contains only a maternal or paternal complement for a particular chromosome (but the choice is random). During the first meiotic division, the 2n germ cell will undergo duplication the usual way to give tetraploidy, *but* here the paternal and maternal chromosomes will become closely associated and crossing over events (chiasmata) often occur (adding to genetic variation). Pictorially, the associated maternal and paternal chromosomes look like 2 crosses with legs intertwined. Each of those cross-structures is one parental chromosome, comprising duplicated sister chromatids.

    During the first meiotic division, the maternal and paternal chromosomes will separate to opposite ends to give a cross structure (2 chromatids to a cross) at each end. At this point maternal and paternal DNA have spatially separated except for the unpredictable cross-over events that happened before. Now the second meiotic division occurs without any further duplication. The sister chromatids at each end undergo a separation in a plane at right angles to the previous one to give haploid (n) cells. In total, a single germ cell gives 4 haploid gametes.

    My explanation is long-winded, but I hope it answers all the OP's questions and more. :biggrin:
     
    Last edited: May 20, 2006
  5. May 20, 2006 #4

    Curious3141

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    To more specifically answer your question (you should first see my long post above), when we say 23 pairs of chromosomes, we mean 23 maternal and 23 paternal homologous chromosomes. As I've explained each paired X shaped structure is just a duplicated single chromosome comprising 2 sister chromatids. This is not a "pair" of homologues, as is commonly understood.
     
  6. May 20, 2006 #5

    Hootenanny

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    A short introduction into cell reproduction by Curious3141 :biggrin: , I'd copyright it :wink: . It is very good though :smile:

    ~H
     
  7. May 20, 2006 #6

    Curious3141

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    Haha, thanks. Maybe I'll upload it to Wiki. :wink: :biggrin:
     
  8. May 21, 2006 #7
    So what I said was basically correct although the chromosomes are not normally crossed but take on many different bends and twists. I don't have good technical understanding of biology so I will say what I have learned, there are 23 pairs of chromosomes in most diploid cells in humans and each pair consisits of two strands which tend to stay together. Although you can clearly see the two distinct chromosomes in each pair. What makes them stay or stick together?
     
  9. May 21, 2006 #8

    Hootenanny

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    As Curious said, the two homologous chromosomes are only visible, only take up the distincive shape and are only joined during cell division. Otherwise, the chromosomes are unwound as long strands contained by the nuclear envolope. The two chromosomes are joined by a centromere, which is basically a series of non-coding DNA bases on each chromosome responsible for binding the chromosomes together.

    Just to add clarification you said;
    However, diploid cells by definition contain 23 pairs of homologous chromosomes (2n), therefore diploid cells always containt 23 pairs of homologous chromosomes.

    ~H
     
  10. May 21, 2006 #9
    But when they are not ready for cell division and are unwounded, can the pairs still be differentiated as opposed to 64 individual chromosomes? They would just look like pairs of straight 'sticks'? Or am I not getting the point and they are invisible according to current equipment to detect when they are not ready for cell division.
     
    Last edited: May 21, 2006
  11. May 21, 2006 #10

    Curious3141

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    You meant 46 right?

    Interphase chromosomes are tough to detect, no doubt. There are specialised techniques used to study them, like fluorescent in-situ hybridisation (FISH) and chromosomal painting which uses sequence specific fluorophore (fluorescing molecule) tagged nucleic acid probes to attach to the complementary sequences on chromosomes. You can make probes that attach specifically to one gene. Since you know that on homologous chromosomes, the same gene is present (only in possibly different allelic form), you will get the same region "lighting up" during FISH. In a diploid cell, you will see two distinct spots under confocal microscopy, whereas, in haploid cells, you will see only one spot.

    We now have the technology to do banding on prometaphase (early metaphase) chromosomes, which are much thinner and longer than the traditional Giemsa banded metaphase chromosomes you see in karyotype diagrams. The longer chromosomes allow for more bands to be recognised and hence more resolution to be studied.
     
  12. May 22, 2006 #11
    Yes. I was in a hurry.


    I am probably seeing things in a too simplistic manner where in actual fact they are extremely complicated. But it seems true from your information that the chromosomes generally always come in pairs and that is why we call them 23 pairs of chromosomes rather than 46 chromosomes.
     
  13. May 22, 2006 #12

    Curious3141

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    In diploid cells (that's basically all the cells in your body except some of the ones in your testes (male) or ovaries (female)), there are 23 pairs of chromosomes. One member of the pair came from your mom, the other from your dad. They are not located closely together at any time, even during "usual" cell division, which is mitosis. They only come together in special cells in the gonads (testes or ovaries) when undergoing the special process of meiosis.

    In diploid cells, of the 23 pairs, 22 pairs are perfect homologues, meaning they are similar in structure. The maternal and paternal chromosomes have the same "map" and the genes occur in the same order. The individual genes may occur in different forms (alleles) but the organisation on the chromosome is the same between them. The exception is the last pair which constitute the sex chromosomes. In the case of the male, he has an X and a Y. The X came from his mother, and the Y came from his father. They are not homologous, the Y is much smaller than the X, and codes for less stuff. The mapping is completely different between the X and the Y.

    In a female, each diploid cell contains two X chromosomes. One came from her mother and the other from her father. They are homologous, but in every diploid cell, one entire X chromosome is inactivated in a process called Lyonisation. So, for all intents and purposes, in a single cell, only one of the X chromosomes is active at any one time. The inactivation is random, in some cells, the maternal X is switched off, in others, its the paternal X.

    This is a much more complex topic than I can convey to you by means of this forum. I'd suggest you read a basic high school biology text, and once you understand that, progress to some undergrad level stuff to get the basics right. :smile:
     
  14. Apr 28, 2009 #13
    Re: chromosomes?

    Finding this was a life saver! Do you have any idea how many books and websites can't manage to be this clear and straight forward? One website referred to sister chromatids as a "chromosome"... it did point out the chromatids, but then, in a large bracket, called the whole thing one chromosome. That was confusing. Is the coiled dna a chromosome AND the pairs after duplication a chromosome?

    From your explanation, I take it that the whole pair is not "a chromosome", but a pair of chromosomes that are referred to at that stage as sister chromatids. That makes more sense, since a single dna molecule coiled up makes a chromosome... Anyway, there are other instances of confusing explanation on various websites and even some books. You have to sort through them all to get to the right stuff, it seems. Curiously enough, all this searching causes one to accidentally memorize stuff one is made to memorize if one takes a class (the names of the phases and such).
     
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