Transcription intron removal

Class I introns were originally discovered in ciliated protozoa and subsequently were found in fungi, bacteriophages, and other organisms. The RNA itself in a class I in-tron has catalytic activity and class I introns remove themselves from primary RNA transcripts by a self-splicing reaction. Class I introns are not true enzymes in that they function only once. The nucleotides in the intron that is spliced out are recycled in the cell. 

EXAMPLE 9.17 What modifications other than intron removal and exon splicing occur in the primary mRNA transcript ... 

After transcription, during RNA processing, introns are removed and the exons are ligated together to form the mamre mRNA that appears in the cytoplasm. 

The mechanisms whereby introns are removed from the primary transcript in the nucleus, exons are ligated to form the mRNA molecule, and the mRNA molecule is transported to the cytoplasm are being elucidated. Four different splicing reaction mechanisms have been described. The one most frequently used in eukaryotic cells is described below. Although the sequences of nu-... 

The relationship between hnRNA and the corresponding mature mRNA in eukaryotic cells is now apparent. The hnRNA molecules are the primary transcripts plus their early processed products, which, after the addition of caps and poly(A) tails and removal of the portion corresponding to the introns, are transported to the cytoplasm as mature mRNA molecules. 

To make mRNA, the primary transcript must be spliced to bring the protein-coding sequences (exons) together and to remove the intervening sequences (introns). The splice signals consist of a 5 and a 3 set of sequences that are always found at splice junctions. However, this is generally believed to provide too little information to recognize a splice site specifically and correctly. Some sequences in the intron are also important. 

Immediately after transcription, the hnRNA introns are removed and the exons are linked to form a continuous coding sequence. This process, known as splicing, is supported by complicated RNA-protein complexes in the nucleus, the so-called spliceosomes. The components of these macromolecular machines... 

Our genes are split into coding or exon regions and noncoding or intron regions. The introns are removed from the primary transcript when it is made into a mature or completed RNA such as mRNA, tRNA, and rRNA. 

The primary transcripts of prokaryotic and eukaryotic tRNAs are processed by the removal of sequences from each end (cleavage) and in a few cases by the removal of introns (splicing). Many bases and sugars in tRNAs are also modified mature tRNAs are replete with unusual bases not found in other nucleic acids (see Fig. 26-24). 

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