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In many eukaryote species, there are several chromosomes. In humans, for example, there are 23 pairs of chromosomes.

Why are there several chromosomes and not just a join of all chromosomes into a single big chromosome?

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    $\begingroup$ What do you mean by the human chromosome? There are 23 pairs of chromosome in the human genome. What do you mean by chunks? Sounds like instead of "chunks", you meant "chromosome" and instead of "chromosome", you meant "genome". But then the question "why" is more than obvious. There are 23 pairs of chromosomes. There is no arbitrary decision to be made here. We just counted them. Can you please clarify your question? $\endgroup$ – Remi.b Jun 11 '18 at 20:53
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    $\begingroup$ @ManuelMilla — This is better included in a revision of your question. Many people do not read the comments and they are not formally part of the question. $\endgroup$ – David Jun 11 '18 at 22:06
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    $\begingroup$ @aaaaaa - I understand you. I will prepare the possible answer, and when it is ready I will edit the question... I will just add my conjecture at the end of the question $\endgroup$ – Manuel Milla Jun 11 '18 at 22:37
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    $\begingroup$ @Bryan Krause - Imagination is more important than knowledge. Albert Einstein. I have a Degree in Information systems and a specialization in Data mining and knowledge discovery. For me, asking questions is more important than answering questions. I am ignorant in many fields of the knowledge but I use my imagination in order to make questions $\endgroup$ – Manuel Milla Jun 11 '18 at 23:10
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    $\begingroup$ "For me, asking questions is more important than answering questions." Fine, but that's not what SE Biology is about. It's about providing precise answers to answerable biological questions. Questions that only allow opinion, speculation or discussion belong elsewhere. $\endgroup$ – David Jun 12 '18 at 12:37
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The leading contender of "why" in my former lab, is that if a chromosome becomes too long, a cell cannot fully isolate the chromatid to a daughter cell. Ie in a giant chromosome, the chromosome arms trail so far behind centromere, that the arms are of two sister chromosome are still touching each other even through the centromere have reached the opposite poles of a dividing cell. This prevents nuclear reformation. It would also mean, that the size of the cell will determine the chromosome maximum size.

At present this is a hypothesis. We have only just created the method to make ultra large synthetic chromosomes. I expected an answer is 3-5 years (2021-2023). However we say there is nothing special to a chromosome number.

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  • $\begingroup$ Related to your answer: biology.stackexchange.com/a/35150/6307 $\endgroup$ – canadianer Jun 12 '18 at 17:49
  • $\begingroup$ So your answer is "We don't know, but a plausable hypothesis is...". Possibly better as a comment. $\endgroup$ – David Jun 12 '18 at 19:04
  • $\begingroup$ @canadianer — a very pertinent reference. Perhaps the question should be closed as a duplicate. $\endgroup$ – David Jun 12 '18 at 19:06
  • $\begingroup$ Was a bit long to put in to the comments. Right now, this question has no answer, only a hypothesis. However the tools to answer it a systematic manner has been developed. $\endgroup$ – JayCkat Jun 13 '18 at 12:53
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Short Answer: Humans reproduce sexually and evolution, with some random effects, has led to 23 pairs of chromosomes.

Explained:

Reproduce Sexually - Bacteria do have one large piece of DNA. This works for bacteria because they will copy all their DNA, and split the two copies creating two "clones" of each other. Humans, on the other hand, half the amount of genetic material in their sex cells (sperm and egg), which when combined together in a zygote sums to the full or normal amount of DNA. This is why nearly every cell in the human body has 23 pairs, or 46 total chromosomes. To have sexual reproduction we therefore need at least 2 chromosomes in the normal, or diploid, cells.

Randomness - While evolution is often taught as a very directed process, there is actually a great deal of randomness that shift outcomes. The best example is that apes, our closes descendent have 24 chromosomes. In becoming human, two of theses chromosmes somewhat randomly fused together into the human chromosome 2. While there could be some selection advantage, what it would be is unclear. Similarly, many other species have a variety of chromosome number. Biologists have been unable to directly relate a cause for the number of chromosomes in each species (see UCSB and Museum of Innovation), and have therefore determined that the number is mostly based on random effects. A good metaphor is that a chromosome is a bookshelf and genes books, the character of a library is not defined by the number of bookshelves or books, but rather the contents of the books.

Evolution - Chromosomes fill several important roles in a cell. Firstly, during normal cell life they unravel partially and gain/lose different markers (histone marks), which can direct different proteins as to which genes should be transcribed. Secondly, during mitosis the chromosomes must become very compact, and split exactly along the mitotic plane, and during meiosis the chromosomes crossover, exchanging DNA with the other pair. The mechanics of each event is inherently based on the size and number of chromosome. For all of these roles, the number of chromosomes clearly has some impact, although the exact relationship is unclear. So while evolution may select for an optimal number, the strength of the selection is likely very weak and overpowered by the random effects.

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  • $\begingroup$ So you do not actually have an explanation. You just say that meiosis and mitosis are complicated so that it’s best for the DNA not to be too long, the only evidence for which is the circular argument that there are multiple chromosomes. $\endgroup$ – David Jun 12 '18 at 12:33
  • $\begingroup$ Answer has been edited. The key key answer is that the number of chromosomes must be greater than 1, after that it is mostly random as to how many. Although I tried to indicate that since the number of chromosome does change the mechanics of the cell, it would be a property evolution could and therefore likely has selected for. $\endgroup$ – user42909 Jun 12 '18 at 13:05
  • $\begingroup$ @David: You need to remember that this is evolution, not "intelligent" design. There might be no real reason, other than it happened to work well enough for the organisms having multiple chromosomes to survive & reproduce. You might note that in plant breeding, at least, it's quite common to create duplicate chromosomes (polyploidy: en.wikipedia.org/wiki/Polyploid ) Once the duplicate chromosomes exist, they can differentiate, creating more space for genetic diversity. $\endgroup$ – jamesqf Jun 12 '18 at 18:36
  • $\begingroup$ @jamesqf — I found your comment puzzling, so I googled "intelligent design". It appears to be some religious superstition of the native peoples of North America in West Virginia and the like. I live in Northern Europe, in an atmosphere of scientific enlightenment. This taught me to beware of people who substituted scientific dogma for religious dogma and argued that something was so because it was so. It is possible to say "we don't yet know" without implying that it must be the work of some "god". $\endgroup$ – David Jun 12 '18 at 19:03
  • $\begingroup$ So your key argument seems to be that for sexual reproduction there needs to be "at least two chromosomes", in humans X and Y. But this argument does not hold because there are other animals in which sex is determined differently, e.g. in alligators, by temperature. And please do not preach to the poster. Most of your answer is patronizing biological background, rather than considered argument, let alone evidence. $\endgroup$ – David Jun 12 '18 at 19:25

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