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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?

Chris
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Dare to ask-I dnt mind punishm
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    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? – Remi.b Jun 11 '18 at 20:53
  • After edit: Is your question why are there 23 small(-ish) chromosomes and not just a single big chromosome? If yes, is your question specific to humans or to any life forms (or any eukaryote at least) that have more that one chromosome? – Remi.b Jun 11 '18 at 21:09
  • Yes. why are there 23 small(-ish) chromosomes and not just a single big chromosome?. And my question is specific to humans – Dare to ask-I dnt mind punishm Jun 11 '18 at 21:21
  • I rewrote your post trying to make it on-topic. Please feel free to roll back if you don't like the edit. I removed the reference to information entropy because really I don't know what you had in mind there and you would weirdly restrict the space of possible reasons by limiting the field of studies that can explain such phenomenon. – Remi.b Jun 11 '18 at 21:30
  • I think the answer is related to Information theory – Dare to ask-I dnt mind punishm Jun 11 '18 at 21:34
  • Can you explain your thoughts for why you think that? Again, please feel free to re-edit your post as you like. You don't have to keep my edit. – Remi.b Jun 11 '18 at 21:36
  • Remi.b I appreciate a lot your help, and I will not re- edit your post, thank you. You are very helpful – Dare to ask-I dnt mind punishm Jun 11 '18 at 21:40
  • I think the answer is related to Information theory because the genome system has : 1- redundancy by doing chomosome pairing. 2- Error Detection and error correction by base pairing (A - T & C - G) – Dare to ask-I dnt mind punishm Jun 11 '18 at 21:43
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    @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. – David Jun 11 '18 at 22:06
  • please add a paragraph that goes something like that "Based on sources (1,2,3) I think that's because of (reason 1,2,3,4). Is that correct? I dont understand why (2,3) are important" or something. In other words, show your work – aaaaa says reinstate Monica Jun 11 '18 at 22:26
  • @David -- When you ask for a revision in the question, Do you mean to add the reference to my conjecture about the relation between the Information theory and the genomic code? – Dare to ask-I dnt mind punishm Jun 11 '18 at 22:31
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    @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 – Dare to ask-I dnt mind punishm Jun 11 '18 at 22:37
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    @ManuelMilla I think you probably know too little about both information theory and genetics to attempt relating them. There is no relevance to information theory in your question and although you listed two possible things you have not made any logical link between them and your question. – Bryan Krause Jun 11 '18 at 22:44
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    @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 – Dare to ask-I dnt mind punishm Jun 11 '18 at 23:10
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    I agree that asking questions is important, but tags are here to help people find questions/answers appropriate to their interests; tagging a post with tags that are not relevant is not helpful. – Bryan Krause Jun 11 '18 at 23:29
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    "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. – David Jun 12 '18 at 12:37
  • If you answer your own question (which is perfectly fine) please use the answer section. And please do it in english, as this is the language of this community. It is also not the first language of most persons here. I edited the question accordingly. – Chris Jun 14 '18 at 09:02
  • Does this answer your question? What limits chromosomal length? – tyersome May 24 '21 at 20:54

2 Answers2

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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.

JayCkat
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  • Related to your answer: https://biology.stackexchange.com/a/35150/6307 – canadianer Jun 12 '18 at 17:49
  • So your answer is "We don't know, but a plausable hypothesis is...". Possibly better as a comment. – David Jun 12 '18 at 19:04
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    @canadianer — a very pertinent reference. Perhaps the question should be closed as a duplicate. – David Jun 12 '18 at 19:06
  • 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. – 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.

user42909
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    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. – David Jun 12 '18 at 12:33
  • 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. – user42909 Jun 12 '18 at 13:05
  • @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: https://en.wikipedia.org/wiki/Polyploid ) Once the duplicate chromosomes exist, they can differentiate, creating more space for genetic diversity. – jamesqf Jun 12 '18 at 18:36
  • @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". – David Jun 12 '18 at 19:03
  • 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. – David Jun 12 '18 at 19:25
  • @David 1. I think you are confusing sex determination with sex reproduction. Sexual Reproduction requires a 2n cell (n being number of chromosomes) becoming a n cell in meiosis. Therefore, the total number of chromosomes must be at least 2, with n=1. 2. Intelligent design is not a West Virginia thing, but an idea at least exposed to students in many states. 3. I don't know how I am preaching – user42909 Jun 12 '18 at 20:05
  • But n can = 1. (My other remark was a reply to @jamesqf.) – David Jun 12 '18 at 20:19
  • @David: My point is that when you ask "why", the only answer is often that it just happened that way. If a few random chances fell out differently, life might be based on single chromosomes (or a single pair). Now you might instead ask whether it's likely that a single long chromosome might break, whether by chance some might break in ways that allowed the organism to survive, and whether there are might be advantages to multiple chromosomes that allowed these broken-chromosome organisms to eventually become the norm. But that's hindsight :-) – jamesqf Jun 13 '18 at 17:16
  • @jamesqf — In my opinion the key point is when you ask why. Often, as with the interminable evolution questions, the answer is never. It is clear that there is no way of knowing why we have five fingers instead of six etc. — we appear to agree on that point. In such cases it is better not to ask, and if the question is asked, not to answer. However for this question there probably is an answer, and there may even be experiments or an evolutionary paper trail that would support or reject the an idea such as maximum viable length. None has been yet been presented here unfortunately. – David Jun 13 '18 at 21:33