Uracil in DNA
DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. The information in DNA is stored as a code made up for four nucleotides: adenine(A), guanine(G), cytosine(C), and thymine(T). During protein biosynthesis, DNA is transcribed into RNA, another type of ribonucleid acid.
In RNA, the DNA base thymine is replaced by uracil, a fifth nucleotide which is almost chemically identical to thymine, but lacks its 5′ methyl group.
Cytosine can deaminate spontaneously to produce uracil (U). This process is referred to as hydrolytic deamination. Therefore, if there was an organism that used uracil in DNA, the deamination of cytosine (which undergoes base pairing with guanine), would lead to formation of uracil (which would base pair with adenine) during DNA synthesis. Uracil-DNA glycosylase excises uracil bases from double-stranded DNA. This enzyme would therefore recognize and cut out both types of uracil - the one incorporated naturally and the one formed due to cytosine deamination, which would lead to unnecessary and inappropriate repair processes.
This problem is believed to have been solved in terms of evolution, i.e. by "tagging" (methylating) uracil. Methylated uracil is identical to thymine. Therefore, it is hypothesized that over time thymine became the DNA standard instead of uracil. Therefore, cells now use uracil in RNA and not in DNA because RNA is more short-lived than DNA and any potential uracil-related errors do not lead to any lasting damage. Apparently, there was no evolutionary pressure to replace uracil with the more complex thymine in RNA. Uracil-containing DNA still exists, for example in
- DNA of several phages
- Endopterygote development
- Hypermutations during the synthesis of vertebrate antibodies.