negative charge is placed on the oxygen anion and produces a pKa of less than or equal to 12. The basic pKa = −3.4, while the acidic pKa = 9.389. In the gas phase, uracil has four sites that are more acidic than water. In DNA Uracil is rarely found in DNA, and this may have been an evolutionary change to increase genetic stability. This is because cytosine can deaminate spontaneously to produce uracil through hydrolytic deamination. Therefore, if there were an organism that used uracil in its 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 trigger unnecessary and inappropriate repair processes. This problem is believed to have been solved in terms of evolution, that is by "tagging" (methylating) uracil. Methylated uracil is identical to thymine. Hence the hypothesis that, over time, thymine became standard in DNA instead of uracil. So cells continue to use uracil in RNA, and not in DNA, because RNA is shorter-lived than DNA, and any potential uracil-related errors do not lead to lasting damage. Apparently, either there was no evolutionary pressure to replace uracil in RNA with the more complex thymine, or uracil has some chemical property that is useful in RNA, which thymine lacks. Uracil-containing DNA still exists, for example in DNA of several phages Endopterygote development Hypermutations during the synthesis of vertebrate antibodies. Synthesis In a scholarly article published in October 2009, NASA scientists reported having reproduced uracil from pyrimidine by exposing it to ultraviolet light under space-like conditions. This suggests that one possible natural original source for uracil in the RNA world could have been panspermia. More recently, in March 2015, NASA scientists reported that, for the first time, additional complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, using starting chemicals, such as pyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), the most carbon-rich chemical found in the Universe, may have been formed in red giants or in interstellar dust and gas clouds, according to the scientists. There are many laboratory syntheses