Uracil DNA glycosylases, specifically remove from DNA the uracil that results

from spontaneous deamination of cytosine. Mutant cells that lack this enzyme have a high rate of G≡C to A꞊T mutations. This glycosylase does not remove uracil residues from RNA or thymine residues from DNA. The capacity to distinguish thymine from uracil, the product of cytosine deamination necessary for the selective repair of the latter may be one reason why DNA evolved to contain thymine instead of uracil. If DNA normally contained uracil, recognition of uracils resulting from cytosine deamination would be more difficult, and unrepaired uracils would lead to permanent sequence changes as they were paired with adenines during replication. Cytosine deamination would gradually lead to a decrease in G≡C base pairs and an increase in A꞊U base pairs in the DNA of all cells. Over the millennia, cytosine deamination could

eliminate G≡C base pairs and the genetic code that depends on them. Establishing thymine as one of the four bases in DNA may well have been one of the crucial

turning points in evolution, making the long-term storage of genetic information possible.

Good Luck!!

Uracil DNA glycosylases, specifically remove from DNA the uracil that results

from spontaneous deamination of cytosine. Mutant cells that lack this enzyme have a high rate of G≡C to A꞊T mutations. This glycosylase does not remove uracil residues from RNA or thymine residues from DNA. The capacity to distinguish thymine from uracil, the product of cytosine deamination necessary for the selective repair of the latter may be one reason why DNA evolved to contain thymine instead of uracil. If DNA normally contained uracil, recognition of uracils resulting from cytosine deamination would be more difficult, and unrepaired uracils would lead to permanent sequence changes as they were paired with adenines during replication. Cytosine deamination would gradually lead to a decrease in G≡C base pairs and an increase in A꞊U base pairs in the DNA of all cells. Over the millennia, cytosine deamination could

eliminate G≡C base pairs and the genetic code that depends on them. Establishing thymine as one of the four bases in DNA may well have been one of the crucial

turning points in evolution, making the long-term storage of genetic information possible.

Good Luck!!

Uracil and Thymine are very close relatives with respect to their molecular structure differing in just a methyl group. In an evolutionary perspective it is now well accepted that RNA molecules evolved in to DNA. when the life began it was necessary for it to be unstable and process the tendency to be highly dynamic with respect to molecular aggregations, base pairing etc. this helped to have a biological diversity in a relatively short amount of time and the best variants where selected. if this did not happen the life may have started with very limited diversity and finally became extinct just because the diversification was too small to sustain in a stringent natural selective environment.

however, as it evolved those primitive organisms where far less energy efficient and unstable. it became a robust need to transform these organisms to be more stable and energy efficient. this was achieved by simple substitution of DNA as genetic material and making RNA as just an intermediate. to attain this goal uracil was problematic as it can pair with itself and many other bases.if incorporated to DNA, this would again make DNA error prone. the reason for this behavior of uracil was its freedom with respect to steric factors and hydrophilicity.

adding a methyl group in 5th position of uracil resolved this problem. this enabled an organism to save its genetic information in complimentary copies adding redundancy and more stability to genetic information.

still DNA can mutate to form variants but in a more controlled manner to sustain evolution.

Hope this answers your question in a simple manner.

Dear Siva Kumar,

You put a question, in solution of which we are interested very much.

In DNA molecules, the occurrence of thymine is associated with the occurrence of desoxy-D-ribose. In RNA, the occurrence of uracil is associated with the occurrence of D-ribose. In addition, it is known that the DNA strands form dimers and the RNA strands, oppositely, form no dimers. I think that these differences between DNA and RNA are explained by one cause. This cause may be of structural or thermodynamic nature. I think that this cause is of structural nature, and we intend to study this assumption on the basis of the techniques described in the work: E.A. Kadyshevich, A.V. Dzyabchenko, and V.E. Ostrovskii, Life Origination Hydrate hypothesis (LOH-hypothesis): computer simulation of rearrangement of different DNA components within CH4-hydrate structure II, EPSC Abstracts Vol. 8, EPSC2013-285, 2013 (is available at the ResearchGate as well as other our works about the Hydrate Hypothesis of Living Matter Origintion (LOH-hypothesis) and Mitosis and Replication Hydrate Hypothesis (MRH-hypothesis)). Apparently, the occurrence of one of these peculiarities determines the two other ones as the geometric consequence. We showed that DNAs (and RNAs) originated within the methane-hydrate matrix and that water structuring-destructuring determines the DNA replication process. In addition, we showed that the thymine-adenine distance within the methane-hydrate structure II is equal to the thymine-adenine distance determined by X-ray techniques in the crystalline DNA double helixes; this result allows the conclusion that DNA double helixes form at the step of DNA formation within the gas-hydrate matrix. We believe that further studies of the DNA/hydrate-matrix systems will allow clarification of the question under consideration.

The deamination of CYTOSINE lead to the formation of URACIL. If URACIL is present in DNA, then it will be difficult to know that it is the original URACIL or it has been formed by the deamination of CYTOSINE. In this way, the mutations occurring by the deamination of CYTOSINE could not be detected.

Thymine has greater resistance to photochemical mutation, making the genetic message more stable. A rough explanation of why thymine is more protected then uracil, can be found in the article.

Arthur M, L., Why does DNA contain thymine and RNA uracil? Journal of Theoretical Biology, 1969. 22(3): p. 537-540.

"Excitation energy in DNA is mobile, and is eventually transferred to thymine residues, which are the sites of radiation damage."

"Uracil but not thymine forms a stable photohydration product. The dimerization of thymine can be partially photoreversed by irradiation at relatively longer wavelengths, while this process is less effective for uracil dimers because of the competing photohydration reaction"

"Photochemical mutation is, or at least was at one time, a serious problem, since there exists a series of enzymes to repair radiation damage. Therefore resistance to radiation damage was an important selective advantage."

In DNA, deamination of CYTOSINE (conversion of CYTOSINE to URACIL) take place spontaneously.

There is a proofread system which turns the deaminated URACIL back to CYTOSINE. Now if DNA had URACIL instead of THYMINE, the proofread system would have done this:

     Since the proofread system would have got confused in detecting which is the original URACIL (as we have assume DNA has URACIL instead of THYMINE) or which is the deaminated URACIL, so it would have converted all the URACIL to CYTOSINE leading to availability of only three bases that is ADENINE, GUANINE and CYTOSINE. This contradict the fact that DNA has 4 bases. SO this can't happen.

If instead of URACIL, THYMINE is present in the DNA, the above problem does not comes. So this is the reason why DNA contain THYMINE instead of URACIL.

Moreover, URACIL has more base pair affinity to ADENINE, but it can also pair up with other bases including itself. This can lead to mutations in our genetic material.

For pictorial representation, this figure looks well-explanatory:

https://www.facebook.com/beingscientist.17/photos/a.1733471750218250.1073741828.1733445293554229/1744093829156042/?type=3&theater

I think the answer of "M. Pradhap" is more convincing because the subject is associated with the resistance of DNA to radiation damage.