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Thread: evolutionary advantage of uracil?

  1. #1 evolutionary advantage of uracil? 
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    Is there an evolutionary purpose/advantage of having uracil in RNA
    rather than thymine (as seen in DNA)?

    Why does only thymine get replaced in RNA?


    -cheers


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  3. #2  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    It can form wobbly bonds, which allows for more tRNAs, I guess this can improve fitness, not really sure hoq. Thymine is more stable and adds to the stability of DNA.


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  4. #3  
    Forum Professor Zwirko's Avatar
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    An examination of the structure of thymine shows that you could also call it 5-methyluracil - it's just a methylated uracil. So cells go to the trouble of methylating uracil before incorporating it in to DNA.

    There are two reasons:

    1) Uracil is able to form weak base pairs with the other bases of DNA, while 5-methyluracil (thymine) only forms base pairs with adenine.

    2) Cytosine can, at a low rate, spontaneously lose an amino group and form a uracil, thus creating a C > U base-conversion mutation. If uracil was the base of choice within DNA, the cell would be unable to tell whether a uracil present in DNA was supposed to be there or if it was the result of a C > U conversion. When the DNA replicates this C > U change results in a G > A mutation. The cell actively removes uracil from DNA through the action of the Uracil-DNA glycosylase enzyme. (Note that C > U conversions in RNA don't really matter, due to their high copy number).


    So for evolution to work, DNA needs to be stable and faithfully replicated to a high degree of accuracy. Uracil in DNA is no good for life as we know it,
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  5. #4  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    Well that was a substantially better answer than my own
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  6. #5  
    Forum Professor Zwirko's Avatar
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    Thanks But I should add a disclaimer:

    I may not have actually answered the question directly when I think about it some more. It seems I answered why uracil is not found in DNA, rather than what is the evolutionary advantage of it being in RNA.

    I'm not too sure why uracil has been kept in RNA, except to point out that uracil works fine, so why fix it when it ain't broke? Perhaps the transcription/translation machinery already evolved to use uracil? To be honest I don't know. I guess I'll put that in my "to do folder", unless someone unless can enlighten me?
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  7. #6  
    Forum Cosmic Wizard i_feel_tiredsleepy's Avatar
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    This feels like one of those things that is going to bother me for days until I find a suitable answer.

    I'm also interested in evolutionary benefits of inosine and pseudouridine in tRNA.

    I'm thinking this all has to do with some sort of fitness benefit of wobble base pairing and allowing the cell to function with fewer tRNA.
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  8. #7  
    Forum Professor Zwirko's Avatar
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    Fact of the day: a typical sized human might experience 10 quadrillion+ spontaneous cytosine > uracil deaminations per day.
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  9. #8  
    Forum Freshman dickies994's Avatar
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    It is hard to say what would be an advantage to using uracil just looking at the two compounds quick uracil has a lower molar mass than thymine so possibly it is easier to use uracil when creating RNA. Or using uracil works just fine and there has been no evolutionary reason to change the use of it phylogenetic inertia?
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  10. #9  
    Forum Cosmic Wizard spuriousmonkey's Avatar
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    Evolution doesn't care about the best solution.

    Just the one that does the job and lies within the realm of historic possibilities
    "Kill them all and let God sort them out."

    - Arnaud Amalric

    http://spuriousforums.com/index.php
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  11. #10  
    Forum Professor Zwirko's Avatar
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    I was doing a little background reading and discovered that the small RNAs found in the nucleolus (snRNA and snoRNA) are sometimes referred to collectively as U-RNA - because they are rich in uracil. There's also a whole bunch of non-standard base-pairing going on in RNA; I don't know what role uracil plays in this though. RNAs also have all sorts of weird and wonderful cellular functions besides their role as messengers between DNA and protein. These facts and the logic of the RNA before DNA hypothesis would suggest that the original question is wrong, or at least approaching the puzzle from the wrong angle. Thus, the question should really be why is uracil not in DNA (which, hopefully has been answered).
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