What is the mechanism for addition of a guanosine to create the 5' methyl cap of a MRNA? O The 5'-OH end of the nascent mRNA performs a nucleophilic attack on the gamma phosphate of GTP O The gamma phosphate is removed from the 3' end of the nascent MRNA and then the beta-phosphate attacks the phosphorus atom of the alpha- phosphate of GTP O The gamma phosphate is removed from the 5' end of the nascent MRNA and then the -OH group from the beta-phosphate attacks the phosphorus atom of the alpha- phosphate of GTP O The 5'-OH end of the beta phosphate present on nascent mRNA performs a nucleophilic attack on the alpha-phosphate of GTP

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Introns are often very large and the cell has devoted mechanisms of eliminating them once they are excised from the pre-mRNA. Following intron excision, what specific ribonucleolytic enzymes or complexes contribute to eliminating the intron RNA immediately after it is excised from the pre-mRNA? Briefly describe the role of each step/enzyme and how it affects its RNA substrate

A common feature of many eukaryotic mRNAs is the presence of a rather long 3′ UTR, which often contains consensus sequences. Creatine kinase B (CK-B) is an important enzyme in cellular metabolism. Certain cells—termed U937D cells—have lots of CK-B mRNA, but no CK-B enzyme is present. In these cells, the 5′ end of the CK-B mRNA is bound to ribosomes, but the mRNA is apparently not translated. Something inhibits the translation of the CK-B mRNA in these cells. Researchers introduced numerous short segments of RNA containing only 3′ UTR sequences into U937D cells. As a result, the U937D cells began to synthesize the CK-B enzyme, but the total amount of CK-B mRNA did not increase. The introduction of short segments of other RNA sequences did not stimulate the synthesis of CK-B; only the 3′ UTR sequences turned on the translation of the enzyme. On the basis of these results, propose a mechanism for the inhibition of CK-B translation in the U937D cells. Explain how the introduction of short…