Question

Can you speculate on why uracil is not one of the naturally-occurring DNA bases?

Which of the following four steps of transcription would you expect to have the most stringent requirements for precision? Initiation; Elongation; Termination; Splicing. Explain why.

Answer 1

Uracil is not one of the naturally-occurring DNA bases because uracil use of uracil doesn't provide enough stability and resistance to Dna molecule as compared to use of thymine in place of Uracil. Dna uses thymine because due to the presence it have more resistance to mutations as it is molecule that is not prepared from any one of the existing bases. Uracil can be produced from cytosine by deamination. Now Dna needs more genetic stability because it is the source of message for the whole life process. So to make that message easily accessible and without any errors, uracil doesn't occur naturally.

Although each and every steps require precision in all ways during transcription but according to the most complex process, various models and accurate precision is required for Splicing.Splicing is the process by the newly transcribed mRNA could be modified and made suitable for translation. Splicing involves firstly 5' cap addition on the nascent mRNA to avoid any degradation. This capping is done in specific manner with certain GTP bound activity. Once the capping is performed, splicing complexes are there. In this the non coding regions( introns) are eliminated from the pre mRNA and exons are ligated with each other. This is done by splicesomal complexes which are present for different class of introns. Introns themselves are divided into 4 classes. Group I and II are self splicing introns and are removed by nucleophilic attack within the pre mRna .Group III introns have special type of splicesomal complexes known as snrps ( small nuclear ribonucleoproteins ) which has 5 components( U1,U2,U4, U5 and U6) and several hundreds of proteins and enzymes . It is a way more complex process which ends with addition of 3' cap on the 3'end of mRna. The process is called polyadenylation. So, the mRna requires many complex steps in splicing and a way more precision than the other steps of transcription.

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