Eukaryotic Pre-mRNA Undergoes Extensive Processing

To assist in the modification and processing of mRNAs, eukaryotic cells contain in their nuclei small nuclear RNAs (snRNAs), which are complexed with specific proteins to form small nuclear ribonucleoprotein particles (snRNPs). These RNAs have been named U1, U2, U3,.. . , U13 and range in size from 100 to 220 bases. One, U3, is 

Structure of the 5 methylated cap of eukaryotic mRNA. A 7-methylguanosine (in red) is attached through a triphosphate linkage formed between its 5 -OH and the 5 -OH of the terminal residue in 

It was established in the early 1970s that a great deal of RNA (hnRNA) turns over in the nucleus without ever reaching the cytoplasm. What could this RNA be Was it a specific type of RNA that never made it to the cyto- 

One of the early demonstrations of sequence removal resulted from the finding that mouse /3-globin precursor mRNA did not form a perfect hybrid with DNA complementary to mature mRNA. When the hybrid was observed with an electron microscope, a loop in the DNA of the heteroduplex appeared, which suggested that the precursor contained internal sequences not present in the mature mRNA (fig. 28.18). These noncoding intervening sequences (introns) are interspersed with coding sequences (exons). 

Removal of internal sequences in eukaryotes is not restricted to mRNA processing. It also occurs in the processing of rRNA and some tRNAs. In tRNAs the mechanism appears to be different in that the signal for splicing originates not from the primary sequence but from the secondary or tertiary structure of the pre-tRNA. 

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