<P> The word intron is derived from the term intervening sequence, that is, a segment of DNA that "intervenes" between two exons of a gene . The term intron refers to both the DNA sequence within a gene and the corresponding sequence in the unprocessed RNA transcript . As part of the RNA processing pathway, introns are removed by RNA splicing either shortly after or concurrent with transcription . Introns are found in the genes of most organisms and many viruses . They can be located in a wide range of genes, including those that generate proteins, ribosomal RNA (rRNA), and transfer RNA (tRNA). </P> <P> Spliceosomal introns often reside within the sequence of eukaryotic protein - coding genes . Within the intron, a donor site (5' end of the intron), a branch site (near the 3' end of the intron) and an acceptor site (3' end of the intron) are required for splicing . The splice donor site includes an almost invariant sequence GU at the 5' end of the intron, within a larger, less highly conserved region . The splice acceptor site at the 3' end of the intron terminates the intron with an almost invariant AG sequence . Upstream (5' - ward) from the AG there is a region high in pyrimidines (C and U), or polypyrimidine tract . Further upstream from the polypyrimidine tract is the branchpoint, which includes an adenine nucleotide involved in lariat formation . The consensus sequence for an intron (in IUPAC nucleic acid notation) is: G-G - (cut) - G-U-R-A-G-U (donor site)... intron sequence...Y-U-R-A-C (branch sequence 20 - 50 nucleotides upstream of acceptor site)... Y - rich - N-C-A-G - (cut) - G (acceptor site). However, it is noted that the specific sequence of intronic splicing elements and the number of nucleotides between the branchpoint and the nearest 3' acceptor site affect splice site selection . Also, point mutations in the underlying DNA or errors during transcription can activate a cryptic splice site in part of the transcript that usually is not spliced . This results in a mature messenger RNA with a missing section of an exon . In this way, a point mutation, which might otherwise affect only a single amino acid, can manifest as a deletion or truncation in the final protein . </P> <P> Splicing is catalyzed by the spliceosome, a large RNA - protein complex composed of five small nuclear ribonucleoproteins (snRNPs, pronounced' snurps'). Assembly and activity of the spliceosome occurs during transcription of the pre-mRNA . The RNA components of snRNPs interact with the intron and are involved in catalysis . Two types of spliceosomes have been identified (major and minor) which contain different snRNPs . </P> <Ul> <Li> The major spliceosome splices introns containing GU at the 5' splice site and AG at the 3' splice site . It is composed of the U1, U2, U4, U5, and U6 snRNPs and is active in the nucleus . In addition, a number of proteins including U2 small nuclear RNA auxiliary factor 1 (U2AF35), U2AF2 (U2AF65) and SF1 are required for the assembly of the spliceosome . The spliceosome forms different complexes during the splicing process: </Li> </Ul>

Which type of rna catalyzes the removal of introns from pre-mrna