The use of uracil instead of thymine in transcription may be especially important.

                           The use of uracil instead of thymine in transcription may be very important. 

 The use of uracil instead of thymine in transcription may be very important. 

In eukaryotes, the DNA is in the cell nucleus and is protected from external harmful factors. In prokaryotes, it is found in cell cytoplasm. One of the reasons for the T-U change may be that the DNA remains in the nucleus and protects its original structure. In addition, uracil might be played an important function in protein synthesis by location in a specific position in the start and stop codon’s structure. The special codon for tryptophan is UGG. It has a unique and special codon structure with the first letter U and two letter purines (GG). UAA-UAG-UGA are stop codons and there are no anti-codons The localization of purine and pyrimidine bases in the stop codons is remarkable. There is only uracil as a pyrimidine, and it is always located as the first letter in stop codons and the other two letters are always purine. If stop codons UAA-UAG-UGA had anticodon, they would have to be AUU-AUC-ACU. There is no anti-codon whose first letter is Adenine. Stop codons may not be recognized by ribosomes and tRNA due to unusual molecular codon sequence and protein synthesis might be terminated for this reason.

Positions of Adenine and Uracil in mRNA codons may have special importance in the molecular recognition mechanism.  5-fluoro uracil (5FU) is used in cancer treatment. Uracil and 5-Fu is pyrimidine analogue. This fact may also indicate another importance of uracil in cancer treatment.

Dr.Engin sayin