Why Uracil?

Mario Bergami mc8366 at mclink.it
Fri Sep 27 20:00:25 EST 1996

"Roger J. Young" <RYoung at lib.drury.edu> wrote:

>With apologies if this is the incorrect newsgroup; but the question came up
>yesterday as to why RNA uses Uracil rather than Thymine. One theory
>suggested was that the presence of the extra "O" in ribose compared to
>deoxyribose was compensated for by the loss of the "CH3" group in U
>compared to T, based on some wild assumptions about steric hindrance etc.
>Anybody actually know the real reason?

>Many thanks

>Roger Young

You may find the answer to your question in:
"Molecular Biology of the Gene"
by Watson et al.
4th edition
p. 1142
"Why does DNA contain Thymine instead of Uracil?
Supposing that the first functional ribonucleotide reductase simply
reduced each of the four RNA precursors to make DNA precursors, we
would expect the original DNA genomes to have contained uracil. Why,
then, does DNA today have thymine in place of uracil? Spontaneous
deamination of cytosine to uracil is known to occur at a low but
significant rate. The potentially harmful effect of these C to U
mutations are prevented by the enzyme uracil-DNA glycosylase, which
excises uracil (but not thymine) from DNA. This suggests that uracil
was replaced by thymine (5-methyluracil) after the initial conversion
from RNA to DNA genomes, so that the increasingly efficient repair
enzymes could recognize the hydrolysis products of cytosine in DNA."

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