• Question: Good morning! Why does uracil, a demethylated version of thymine, replace thymine in RNA? Thanks!

    Asked by anon-244767 on 30 Apr 2020.
    • Photo: Rachel Tanner

      Rachel Tanner answered on 30 Apr 2020:

      This is a great question! It takes more energy to make thymine because it has an extra methyl group, as you say. DNA needs to take that extra precaution, even though it uses more energy, because it helps to protect the DNA from being broken down by enzymes. RNA doesn’t live as long as DNA, so is at less risk from being broken down. Thymine also makes base-pairing more specific (uracil is a bit less picky). An interesting related question is which came first – the chicken or the egg? (I mean the uracil or the thymine..) – Maybe uracil was the ancestral version, and thymine evolved in DNA as an improvement. Or maybe thymine came first and uracil evolved in RNA because it had an advantage in that context..

    • Photo: Laura Durrant

      Laura Durrant answered on 30 Apr 2020:

      Good afternoon, thank you for your question! Like you’ve correctly stated, uracil is essentially a demethylated version of thymine. The methyl groups on thymine actually protects DNA from enzymes that may try and break it down. Uracil is also a deaminated (removal of an anime group) version of cytosine. Deamination is a common cause of DNA damage – so if uracil was present in DNA in place of thymine, it would be hard for DNA repair mechanisms to find and fix errors caused by deamination. Because RNA is a shorter-lived molecule than DNA, any potential uracil-related errors (like methylation or deamination) do not lead to lasting damage. Because of all of this, it is more energetically efficient to use uracil rather than thymine as a base-pair with adenine in RNA.
      I hope this answers your question! 🙂

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