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The karyotype is performed on a cell whose cycle has been stopped in metaphase or pro metaphase, using colchicine or by other means. In the textbooks I read that during the S phase each of the 46 chromosomes is duplicated, giving, again, 46 chromosomes, but, and here I am deducing, 92 chromatids. When we perform the karyotype, we only see 23 pairs of chromosomes (46, that is), and, it looks like to me, the same number of chromatids (46 as well). What am I missing? Why can't we see 92 chromatids in the karyotype?

Thanks a lot for your attention.

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    $\begingroup$ Possible duplicate of Karyotypes Appearance $\endgroup$ – De Novo Jul 13 '18 at 21:56
  • $\begingroup$ See this question. Karyotypes do not show the entire structure. It's a good question, though, and shows you've been putting some thought into this. $\endgroup$ – De Novo Jul 13 '18 at 21:58
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Chromosomes and chromatides are referring to the same physical objects, although a chromatid is mostly referring to the DNA strand, and chromosome to the "colored object" (chromo=color, some=body) that people first saw under a microscope. So, yeah, don't get confused by that, it is just that biologists like to coin terms for specific contexts.

You can see a detailed explanation here.

Cheers,

Pedro

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  • $\begingroup$ I'm familiar with the difference between chromosomes and chromatids, what I don't get is why we see 46 objects in metaphase, and not 92 (and by objects I mean independent molecules and also molecules linked at the center, so that two sister chromatids count as two objects, even though they are linked, by this definition)? If the chromosomes duplicate in the S phase, my reasoning goes, even though the copies may be 'linked' to the original in some way, shouldn't we still be able to distinguish them clearly (and thus distinguish 92 molecules, however they may be connected to each other)? $\endgroup$ – piadinas Jul 13 '18 at 6:06
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As you noted, karyotypes are prepared from cells halted in metaphase or prometaphase. In either of these phases, the chromosomes will have two chromatids bound together at the centromere with the chromatid arms lying parallel to each other.

The difficulty in seeing the chromatids in most karyotypes is due to the way they are prepared. Most karyotypes are prepared using Giemsa staining which causes contrasting bands to appear on the arms. These bands have patterns characteristic for each chromosome type and thus allow the chromosome images to be identified and sorted into pairs. Between the stain, lighting and exposure used to show the banding, the images don't show the separate forms of the chromatids well.

With alternative staining and lighting, the chromatids can stand out (all images from commons.wikimedia.org):

enter image description here

Still, in typical karotype photographs of sufficient resolution you can at times make out the separate forms of chromotid arms, or perhaps the "pinched waist" shape where the centromere draws the chromotid arms together:

enter image description here enter image description here enter image description here

So, yes, a normal human karyotype has 92 chromatids present; they just tend not to be apparent due to the preparation done to show banding.

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