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    Hey!
    I'm a little confused and was wondering if somebody could help... i understand that a homologous chromosome are 'a set of one maternal and one paternal chromosome that pair up'. But i'm confused as to when they are formed (they are definitely in meiosis) but are they in mitosis also? Are they in a normal diploid cell in interphase?
    Thanks!
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    (Original post by ProbablyJade)
    Hey!
    I'm a little confused and was wondering if somebody could help... i understand that a homologous chromosome are 'a set of one maternal and one paternal chromosome that pair up'. But i'm confused as to when they are formed (they are definitely in meiosis) but are they in mitosis also? Are they in a normal diploid cell in interphase?
    Thanks!
    from my understanding, we have homologous chromosomes generally as each characteristic is coded for by 2 copies of the same gene (one maternal and one paternal) therefore each nucleus contains matching sets of chromosomes called homologous chromosomes - diploid cell (23 from mom, 23 from dad)

    so yes to answer your question they do exist in a diploid cell and I believe before interphase we would have these homologous chromosome but it is during interphase our DNA replicates so that we have 2 sister chromatids for each chromosome. The chromosome from dad now made up of chromatids pairs up with the homologous chromosome from mom to form a bivalent

    therefore I believe that they would exist in mitosis but they just aren't mentioned in the process as they don't pair up to form bivalents in mitosis but do in meiosis

    I hope that helps, don't worry too much its a very confusing process Im struggling to get my head around it too :afraid::afraid:
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    (Original post by Gingeyy)
    from my understanding, we have homologous chromosomes generally as each characteristic is coded for by 2 copies of the same gene (one maternal and one paternal) therefore each nucleus contains matching sets of chromosomes called homologous chromosomes - diploid cell (23 from mom, 23 from dad)

    so yes to answer your question they do exist in a diploid cell and I believe before interphase we would have these homologous chromosome but it is during interphase our DNA replicates so that we have 2 sister chromatids for each chromosome. The chromosome from dad now made up of chromatids pairs up with the homologous chromosome from mom to form a bivalent

    therefore I believe that they would exist in mitosis but they just aren't mentioned in the process as they don't pair up to form bivalents in mitosis but do in meiosis

    I hope that helps, don't worry too much its a very confusing process Im struggling to get my head around it too :afraid::afraid:
    Sorry for the late reply. I think i understand what you are saying. On wikipedia it says 'humans have two homologous chromosome sets in each cell', which i assume means the 22 pairs (discluding the last pair because not homologous.... i think). The thing that was confusing me was i thought they had to be attached to be homologous chromosomes, but from my reading of your post and some more on good old wikipedia, that's not the case? So homologous chromosomes do not have to be attached by a centromere? Which is why homologous chromosomes do not join in mitosis, instead sister chromatids join to form a chromosome, with is not homologous.
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    All pairs of chromosomes are homologous, except X and Y chromosomes, so homologous pairs are not FORMED in meiosis, as the chromosomes are not genetically identical. The homologous pairs have the same genes but can have different alleles of the same gene, which is what i swapped during crossing over.
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    It is very confusing and hardly any texts explain this clearly.

    Prior to interphase, the DNA that makes up each chromosome is present in the nucleus, but it is not organised into a chromosome. Instead, it is busy being transcribed and so it takes the form of an extended double helix of DNA plus associated histones, transcription complexes and so on. The best you can usually manage when you do try to see it in a microscope is to see a diffuse staining of chromatin (which is the DNA plus associated proteins) spread all over the nucleus.

    All of the chromosomes, including the maternal and paternal homologues are present at all times. If you think about it, this is why heterozygous individuals carrying a gene for a disease (say) can survive. The normal gene on the chromosome inherited from the normal parent goes into production to compensate for the deficiencies of the disease copy that was inherited from the other parent.

    Anyway, in mitosis, there is no need for homologous chromosomes to pair up. But in meiosis, they do, because they will need to exchange sections of DNA by recombination.

    I think that answers the question, but in case you need more explanation of mitosis, here it is:

    At interphase, as the cell prepares to divide, the whole genome of DNA replicates itself. You now have a nucleus which contains twice the usual amount of DNA, but you still don't see the chromosomes yet, as the DNA is opened up and extended while it is being replicated.

    When replication is finished, there is little or no DNA transcription going on, so the DNA condenses into tighter structures which we see as the chromosomes. This is prophase. Remember that each chromosome now contains twice the amount of DNA that was originally there because it has just replicated itself. What we see is this "doubled" chromosome, made up of the original DNA double helix plus its new copy, joined together with a centromere. To denote this special state, each copy is called a chromatid.

    At anaphase, the proteins that bind the sister chromatids together are cleaved, and the chromatids split apart. One goes to one daughter cell, the other to the other daughter cell. Once separated, each is just a double helix of DNA, just like they were in the original cell. So they assume their "true" identity and are called chromosomes again!

    Hope that helps.
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    (Original post by OxFossil)
    It is very confusing and hardly any texts explain this clearly.

    All of the chromosomes, including the maternal and paternal homologues are present at all times. If you think about it, this is why heterozygous individuals carrying a gene for a disease (say) can survive. The normal gene on the chromosome inherited from the normal parent goes into production to compensate for the deficiencies of the disease copy that was inherited from the other parent.

    Anyway, in mitosis, there is no need for homologous chromosomes to pair up. But in meiosis, they do, because they will need to exchange sections of DNA by recombination.


    Hope that helps.
    @OxFossil That was really helpful, i'm pretty sure i understand what you are saying, thank you!
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    (Original post by ProbablyJade)
    @OxFossil That was really helpful, i'm pretty sure i understand what you are saying, thank you!
    You are welcome. It was a long-winded reply, let me know if there is anything you couldn't follow.
 
 
 
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