• Question: Why does DNA contain Thymine but RNA contains Uracil?

    Asked by Xx_SaLLY_ScOpE_xX to Sally, Rob, Matt, Marikka, Ceri on 18 Nov 2014.
    • Photo: Ceri Dare

      Ceri Dare answered on 18 Nov 2014:

      Thymine is harder to make, but more stable. DNA needs to keep information safe for a long time, so it makes sense to use thymine. But RNA is made and used quickly, and small mistakes don’t matter as much, so the easier to make uracil does the job.

    • Photo: Marikka Beecroft

      Marikka Beecroft answered on 19 Nov 2014:

      Ceri has given a good answer but there also other reasons as well.

      Uracil can bind to adenine but also other bases including itself, so in a DNA helix it would cause lot’s of different problems and change the DNA structure so Thymine is used instead as it is only binds to one base and won’t ruin the helix in DNA.

      Also the cell uses uracil as a mechanism to repair damaged DNA, if you used Uracil the cell wouldn’t know what to do! You see when DNA is damaged by UV light sometimes a cytosine becomes a Uracil and the cell then knows to switch it back to Cytosine because it know uracil should only be in RNA.

    • Photo: Robert Hampson

      Robert Hampson answered on 19 Nov 2014:

      Early life is proposed to have been RNA based. RNA is not particularly stable and would need to be copied fairly often. Uracil especially is not a very stable base.

      As life evolved towards using DNA to store genetic code, the molecule needed to be much more stable so it didn’t need to be copied so often. Thymine is a simple methylation of Uracil and is considerably more stable whilst still being essentially chemically equivalent to Uracil in a chain of DNA. It therefore makes sense that thymine is now used in DNA whilst Uracil is still used in RNA.

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