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Genetics, replication of heterozygous diploids – conceptual

  1. Jul 31, 2017 #1
    So you have two alleles for every one gene, and the chromosome is composed of 2 homologous chromatids that have one of the 2 alleles for the specified gene on each chromatid.

    when you have to replicate that chromosome the way I understand DNA replication is is that you have a serious of functioning proteins that make a one-to-one photocopy of that entire chromosome. Despite the minor mutations poss. involved with DNA replication in general. But if I have a heterozygous diploid whose chromosome splits off, both chromatids should just replicate themselves. So the diploid cell will duplicate into 2 individual homozygous chromosomes. (I'm not trying to talk about meiosis right now, just talking about cellular division)

    I'm sorry that these are massive run-on sentences
  2. jcsd
  3. Jul 31, 2017 #2


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    You have a misconception here:
    The chromatid is the result of the duplication of a single chromosome before the two sister chromosomes are separated. They are held together for a while at the centromere.
    When this happens in a diploid cell, you will have 4 chromatids for each chromosome pair (such as the chromosome 2 pair of chromosomes). For non-paired chromosomes (like an X and Y) there will be only two chromatids for each of the unpaired chromosomes (like the X and Y).

    In a diploid organism, there will be two separate chromosomes that are not derived from each other. Normally they will come from different parents and be unrelated.
    This is why you can have a heterozygous organism, the chromosomes can be unrelated with different evolutionary histories.

    I don't understand what you mean by this (the splitting off part):
  4. Jul 31, 2017 #3
    Sorry I should've clarified,

    After telophase individual chromatids from chromosomes were broken apart and delivered to opp. sides leading to newly forming cells. But the confusing part for this diploid to be a replica of the previous cell that it comes from, it should need both chromatids to do this. Otherwise I imagine they singular chromatid of the 2 that was put into that individual cell, replicating itself, therefore, ending up with a chromosome that contains only one allele of the previous 2.

    unless something before happens I guess?
  5. Jul 31, 2017 #4


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    During mitosis, it is extremely unlikely that two chromatids for a single chromosome would go in the same direction and end up in the same daughter cell.

    This is because all the chromosomes line up on the plane between the two poles (metaphase). Each chromosome's pair of chromatids are attached to microtubules in the mitotic spindle from different poles at their joined centromere. Things giggle around until some sensing mechanism (recently discovered, which I don't know much about) decides the tension is equal and then all the chromatid pair's centromeres split apart and the newly separated chromosomes go in different directions.

    If this does not work right then you get problems of too many or too few chromosomes in a cell (anuploidy). This can lead to a variety of problems. There is a strong selection against this, so evolution has resulted in mechanisms to prevent it (usually).

    When I was studying this, I would draw cartoons over and over until I understood where everything came from and was going to and could always draw it right. For me anyway, drawing is a good method for learning certain aspects of biology. Something like this might be a good drawing to work on.

    This is of course different from meiosis where (in the first division) duplicated pairs of chromosomes (four chromatids) are lined up and separated (after crossing over occurs).
  6. Jul 31, 2017 #5
    So I didn't really mean the nonstandard # of chromatids are separated into Difference cells coming from the original cell. I meant in the situation where all things are normal, you have one chromatid for each individual daughter cell. Chromosomes are composed of 2 chromatids, a heterozygous diploid (Aa) will have 2 diff. alleles for a gene. Each one is located on its own chromatid. So when the 2 daughter cells have one of the chromatids each, how are they going to replace the missing allele they no longer have a copy for? A daughter cell with the "a" allele chromatid can't replicate the "a" allele chromatid to get the "A" allele chromatid, so how does it compensate to become heterozygous just like its parent heterozygous diploid cell?
  7. Jul 31, 2017 #6


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    Duplicated chromosomes are composed of two chromatids before they are separated into the two daughter cells that result from cell division.
    when they separate they are just two separate chromosomes, kind of composed of one chromatid each, except they would not be called chromatids then.
    Before DNA replication there is only one chromosome, composed of one chromatid, which would not be called a chromatid then either.
    Lots of terminology issues.
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