Question

how does CH3 group in thymine facilitated its presence in DNA (or) why is thymine present in DNA instead of uracil.

Answer 1

Thymine has greater resistance to photochemical mutation, making the genetic message more stable.

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.