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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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.
Short Answer: Humans reproduce sexually and evolution, with some random effects, has led to 23 pairs of chromosomes.
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.