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EUKARYOTIC MRNA IS PROCESSED BEFORE MAKING PROTEIN
Bacterial mRNA may be translated without any processing. Indeed, bacteria often start translating their mRNA while it is still being transcribed (known as coupled transcription/ translation). However, eukaryotic RNA is processed in a variety of ways before it can leave the nucleus and be translated into protein. First, eukaryotic mRNA must have a cap added to the 5′ end of the message (Fig. 2.14). The cap is a GTP added in reverse orientation and which is methylated on position 7 of the guanine base. Methyl groups may also be added to the first one or two nucleotides of the mRNA.
The second modification of eukaryotic mRNA is adding a long stretch of adenines to the 3′ end—the poly(A) tail. Three sequences at the end of a new mRNA mediate the addition of the tail: the recognition sequence for the polyadenylation complex (AAUAAA); the cut site for cleavage binding factor; and the recognition sequence for polyadenylation binding protein (a length of GU repeats). First, the polyadenylation complex binds to the AAUAAA, and an endonuclease in the complex cuts the mRNA after a CA dinucleotide downstream from the AAUAAA recognition sequence. Next poly(A) polymerase adds 100 to 200 adenine nucleotides. Finally, the poly(A) binding protein binds to both the poly(A) tail and the cap structure. This circularizes the mRNA.
A third modification made to eukaryotic mRNA is the removal of introns. Eukaryotic DNA contains many stretches of intervening sequence (introns) between regions that will
ultimately code for a protein (exons). First the entire region is transcribed into an RNA molecule called the primary transcript. After capping and tailing, this is processed to remove the introns. The exons are spliced together to form the mRNA. Proteins called splicing factors recognize the exon/intron borders, cut the DNA, and join the neighboring exons.
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