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My textbook says that prochromosome is a false chromosome present in the nucleoid of prokaryotes.

I looked up Wikipedia and all over internet and this word is kinda sus. So I'm asking about it here. It doesn't seem legit. Wikipedia has just mentioned that the DNA like thing is present in nucleoid but never calls it prochromosome.

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It would help a bit to get the exact quote from the text. That definition doesn't seem to agree with what I find when I research the term. For prokaryotes, I find the term here but it is poorly defined and does not agree with what you write. It is described as a "naked" chromosome that is only DNA, but that also doesn't make sense as bacterial chromosomes are bound by proteins. There is no page on wikipedia, as you note, supporting your "sus" designation.

There are current definitions available from online resources. It basically corresponds to condensed pieces of chromosomes (heterochromatin) that are apparent under a microscope under certain conditions. But it is not very well defined biologically, it is a name that humans give to something they see that they don't totally understand.

The term itself appears to be archaic. An early usage in reference to plant genetics is here. I think that it was probably a label that someone put on observations made microscopically before people knew what they were seeing. It looks to have become mostly outdated as cytology matured as a scientific field, here is an example quote from that paper:

Rosenberg (i8) was the first to show that the number of prochromosomes is approximately the same as the number of chromosomes. Prochromosomes have been considered as structures identical with chromosomes (Malte, I3; Stout, 28; Eichhorn, 2), as centers about which chromosomes form during the prophases (Laibach, 9; Overton, 17; Rosenberg, 20) and as portions of chromosomes which persist through the telophases (Heitz, 5; Gregoire, 3; Doutreligne, i). It has also been considered that prochromosomes bear no direct relation to chromosomes (Lundegardh, 12; Tischler, 30) or that they are not directly concerned in the formation of prophase chromosomes (Kuhn, 8). De Smet (24) stated that prochromosomes may not be present in meristematic nuclei.

This does not really correspond to any structure I can think of in the nucleus or nucleoid. It also doesn't correspond to the current heterochromatin definition.

Looking through search results, I am inclined to believe that the term as you have found it is an archaism that has somehow remained in some textbooks through bad editing. Very little current scholarly work is using the term. The scholarly work that does use it recently is using it in the more obscure cytological observational sense (heterochromatin), not in the prokaryotic sense that you have pointed to.

So it makes sense that you were confused given the inconsistent usage and the lack of current rigorous work on it. I would recommend sticking to recent scholarly work or definitions that come from traditional publishers (e.g. here). I would more or less entirely ignore question and answer sites that don't directly cite the scholarly literature.

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  • $\begingroup$ Interphase chromatin can be divided into "chromosome territories" by chromosome painting probes. It does seem like people a century ago without these probes may have been able to track these from telophase to prophase to a certain extent (see Figure 4). $\endgroup$ Jun 7, 2021 at 3:37
  • $\begingroup$ @MikeSerfas I'm a little confused; the later reference has no reference to prochromosomes. Are you just speculating that well-known nuclear compartmentalizations are what such terms referred to? This seems uncontroversial, I guess, for the heterochromatic/eukaryotic definition of the term. But it doesn't seem to bear on the asker's question? $\endgroup$ Jun 10, 2021 at 0:24
  • $\begingroup$ See Figure 2 / page 80, for example $\endgroup$ Jun 10, 2021 at 3:34
  • $\begingroup$ @MikeSerfas The PLOS paper (the later reference) has only 1 figure? The other paper is relevant to eukaryotes but not prokaryotes. $\endgroup$ Jun 11, 2021 at 0:48
  • $\begingroup$ Sorry - I had read "latter" for "later" in your comment. The eukaryotic definition is by far the most prevalent. Its extension to prokaryotes, however, is logical -- if a chromosome is defined only as a condensed, distinct structure, and "prochromosome" refers to the diffuse chromatin, then a bacterium should not be said to have a "chromosome" throughout its cell cycle, but rather a "prochromosome". But I think that most people, rather sensibly, have not attempted to keep track of this distinction. It relies on a uniform morphological distinction made in very different situations. $\endgroup$ Jun 13, 2021 at 13:19

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