This document was written when I was in high school. I have little or no authority on this topic. Make of this what you will.

The Use of Thymine Instead of Uracil in DNA

  1. Uracil is energy inexpensive compared to thymine
    1. Thymine is created from uracil in a methylation process
      1. Methylation process is the addition of a methyl group (CHx) to a molecule
      2. Methylation carried out by DNA methyltransferase
        • Most methylated nucleotides are in the coding region of DNA; little in the promoter region
  2. Thymine & other nitrogenous bases undergo methylation; uracil has a problem with this
    1. Methyl groups are hydrophobic; most other parts of DNA and RNA are hydrophillic
      1. So, this process protects from DNA solubility in water
      2. Being hydrophobic the methyl groups are forced to a certain position in the DNA strand and give a more stable structure
    2. If uracil is methylated it can pair with almost any other base, including itself
      1. If uracil was used complementary bases could be changed during DNA replication and therefore can code for different amino acids
      2. Thymine doesn’t do this
    3. Thymine is methylated before incorporation into DNA
      1. The other bases are methylated at another time by different enzymes
        • DAM and DCM methylate adenine and cytosine respectively
    4. Becoming methylated makes the DNA “invisible” to nucleases, enzymes that break DNA and RNA down
      1. Viruses, bacteria and other invaders can’t destroy the code using the nucleases
  3. Cytosine can be easily deaminated to uracil
    1. Demaination is the removal of an amine group (NH2) from a molecule
      1. Cytosine’s amine group can react with water (H2O) and form ammonia (NH3+) and leave behind an oxygen which becomes double bonded to the carbon. Formula:
        • H2O + 2H2N → 2NH3 + O2
    1. If uracil was used in DNA instead of cytosine, the deaminated cytosine groups, which appear as uracil could not be replaced with a regular cytosine groups when DNA polymerase proofreads during DNA replication
      1. Therefore different protines would be produced, perhaps killing the organism
        • Note: In RNA cytosine shouldn’t have time to deaminate (I think)


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Re: Why does uracil replace thymine in RNA?