Gene Splicing

Various splicing signal sequences are universal and are found within every intron site spliced, while some signal sequences are unique to individual genes. DNA is made up of bases called nucleotides, which represent the DNA alphabet. There are four bases, Adenine (A), Guanine (G), Thymine (T), and Cytosine (C). Most introns in higher life forms begin with the nucleotide sequence G-T and end with the sequence A-G. The sequences define the "left" (5') and "right" (3') borders of the intron and are described as conforming to the GT-AG rule. Mutations in any of these four positions produce introns that cannot be removed by normal splicing mechanisms. Within the intron is another highly conserved sequence that has some variability in the genes of a species; this region (called the branch site) is the area that connects to the 5' end of the intron as it is cut and then curls around to form a lariat shape. This lariat is a loop in the intron which is formed as it is removed from the maturing RNA.