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Question: Why do different organisms have different numbers of chromosomes?

Asked by jumute to Amy, Drew, Julia, Kimberley, Sara on 17 Jun 2011.

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  • Photo: Amy MacQueenAmy MacQueen answered on 16 Jun 2011:

    This is a toughie jumute!

    People used to think the more complex an organism the more DNA it would have…and therefore the bigger number of chromosomes. But this is simply not the case – scientists had to have a re-think when they discovered things like onions having 5x as much DNA as a human and goldfish have ~100 chromosomes!! (onions are great at what they do but we like to think we are more complicated than them!).

    Its a given that a lot of this “extra” DNA in an onion is probably duplicated pieces that essentially contain the same information and therefore the actual amount of DNA is not so important as the content. Its the control of our genes – when they are expressed, at what times and in what circumstances that is important. Its kind of like we’re all made of lego – us, goldfish, onions – but what we DO with the lego makes the difference…we can make houses, cars, spaceships etc…obviously some organisms have extra lego pieces (like wheels) that others don’t have but its what order we use the pieces that makes a difference!

    Chromosomes are essentially just the way the DNA is organised and differences occur;
    1. variation between the two sexes
    2. variation between the germ-line and soma (between gametes and the rest of the body)
    3. variation between members of a population, due to balanced genetic polymorphism
    4. geographical variation between races
    5. mosaics or otherwise abnormal individuals.

    Some plants especially have cases of tetraploidy etc which means they have 4 sets of chromosomes. Some diseases like Downs Syndrome involve chromosome abnormalities.

    So really I don’t think anyone’s very clear! if you want to ask more please do!! Sorry I can’t be more definitive! :)

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  • Photo: Drew RaeDrew Rae answered on 16 Jun 2011:

    Hi Jumute,
    One thing to remember is that evolution is never about perfect solutions. If something is “good enough” and doesn’t impose a disadvantage, then there is very little pressure to optimise.

    DNA is a form of digital encoding – just like data is stored on a computer, only instead of 1s and 0s, it is A, C, G or T Nucleotides. Computer scientists would say, “I’ve got only so much space, and 4 symbols to work with, what’s the optimal encoding to store as much information as possible”.

    Evolution doesn’t have a brain. If a mutation happens to create a whole new chromosome but gives an advantage to the organism that has it, evolution doesn’t say “but I could do that better simply by tweaking the code on an existing chromosome”.

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  • Photo: Sara Imari WalkerSara Imari Walker answered on 17 Jun 2011:

    Hello jumute! You’ve really hit on a big question in biology here. We really don’t know a lot about how genomic information manages all of the amazing activities of a cell, or why genome size doesn’t seem to necessarily scale with an organisms complexity (Amy gave the great example of humans versus people). Both of these things probably relate to your question. There are pluses and minuses to having a large number of chromosomes. A larger number means you can code for more stuff, and have more room for duplicates (backups) in case something goes wrong. However more DNA means more stuff to copy, which takes a lot of energy and so can be a drawback – especially if you are copying a bunch of junk DNA. Plus you have to pack it all somewhere and be able to access the information you need when you need it. So certain organisms it seems take advantage of different aspects of this give and take.

    Evolutionarily, its hard to imagine how an organism could evolve with fewer chromosomes than its ancestor. But, it happens – the jack jumper ant for example only has one pair! (males are haploid so they actually only have a single chromosome). This probably arose by chromosome deletion events in reproduction. Most, and I really mean most, chromosomal deletion events will be extremely fatal. But perhaps some individual might survive. Chromosomal numbers can also change by fusion (two coming together) or duplication events (double copying of one). These newly formed chromosomes could evolve independently of their progenitor chromosomes leading to speciation of new organisms with different numbers of chromosomes. This was a cool question! Keep it up!!

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