Splicing

  Next the transcript undergoes splicing to remove intronic sequences. This is a highly regulated and precise process which comprises an additional control point in gene expression, since unspliced transcripts are generally unstable and are rapidly cleared from the cell. Splicing is catalyzed by a large molecular apparatus containing RNA and protein molecules, which recognize short consensus sequences in the intron, and excise the intron through an intermediate form that looks like a lariat (Figure 2). The splicing process must be absolutely precise, because the deletion or addition a single nucleotide at the splice junction would throw out of frame the subsequent 3-base codon translation of the RNA. Splicing is catalyzed by a large molecular apparatus containing RNA and protein molecules, which recognize short consensus sequences in the intron, and excise the intron through an intermediate form that looks like a lariat. The splicing process must be absolutely precise, because the deletion or addition a single nucleotide at the splice junction would throw out of frame the subsequent 3-base codon translation of the RNA. The significance of RNA splicing is not entirely understood, but the process represents an important point of gene control, since in general transcripts cannot leave the nucleus to be translated until their introns are removed.

  The implications of splicing are also important for the manipulation of genetic information. Intron sequences retained in the DNA sequence of a human gene are not present in mature mRNA transcript, from which protein is made. This means that gene libraries made from the RNA sequences present in a specific tissue type (cDNA libraries, discussed below) do not include intron sequences, which in some cases contain sequences, such as enhancers, which are important for the control of gene expression. On the other hand, if the product of a gene, such as insulin, is to be expressed in a bacterium, its introns must first be removed, since bacteria lack introns and the apparatus to splice them out of gene transcripts.