Introns removal from transcripts

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-... 

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 term splicing refers to the process by which introns are removed and the mRNA put back together to form a continuous coding sequence in the 5 -3 direction. Remembering how accurate this process must be is important. If only a single nucleotide of an intron were left in the processed mRNA, the protein made from that mRNA would be non-functional, because the ribosome would read the wrong codons. The cellular machinery that splices pre-mRNAs uses information at the splice junctions to determine where to cut and where to rejoin the mRNA. Removal of introns from transcripts containing more than one intron usually occurs in a preferred but not exclusive order. Several pathways are used. 

A. Introns are removed from RNA transcripts by splicing mechanisms. 

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. 

Eukaryotic pre-mRNA transcripts contain regions known as exons and introns. Exons appear in the matnre mRNA introns are removed from the transcript and are not found in the matnre mRNA (see Eig. 14.10). Introns, therefore, do not contribute to the amino acid seqnence of the protein. Some genes contain 50 or more introns. These introns are carefnlly removed from the pre-mRNA transcript and the exons spliced together, so that the appropriate protein is prodnced from the gene. 

In eukaryotes the introns are removed from the primary transcript by processing, to produce the mRNA that is translated. Prokaryotes lack the machinery to perform this processing consequently, the translation product of the primary transcript would not be functional because it would encode amino acid sequences that are specified by the in-tron sequences. The problem can be circumvented by using cDNA prepared from the... 

---