Gene Splicing

The molecules or molecular complexes that actually splice RNA in the cellular nucleus are called spliceosomes. Spliceosomes are made of small sequences of RNAs bound by additional small proteins. This spliceosome complex recognizes particular nucleotide sequences at the intron-exon boundary. DNA and RNA are both generally read in the 5' to 3' direction. This designation is made on the basis of the phosphodiester bonds which make up the backbone of DNA and RNA strands. Introns are first cut at their 5' end and then at their 3' end. The two adjacent exons are then bonded together without the intron. The spliceosome is an enzymatic complex which performs each of these steps along the pre-RNA to remove introns.

The small RNAs which make up the spliceosome are not mRNAs, rRNAs, or tRNAs; they are small nuclear RNAs (snRNA's). snRNAs are present in very low concentrations in the nucleus. The snRNAs combine with proteins to comprise, small nuclear ribonuclearprotein particles. Several snRNPs aggregate to form a spliceosome. This secondary structure recognizes several key regions in the intron and at the intron-exon border. In essence, snRNPs play a catalytic splicing role. The absence of individual snRNP components can inhibit splicing. snRNPs are only one of many complexes which can regulate gene expression.

In addition to snRNPs, some introns have auto (self) splicing capabilities. These introns are called group II introns. Group II introns are found in some mitochondrial genes, which come from a genome that is separate from the nucleus and is located in small compartments within the cell called mitochondria. Mitochrondria function in provide energy for the cells energy requirements. Although all chromosomal DNA is located in the nucleus, a few genes are located in the cells mitochondria. Group II introns form secondary structures using their internal intron region in a similar way to nuclear introns. However, these mitochondrial introns direct exon-exon rejoining by themselves without snRNPs.