Previous

Before eukaryotic products of transcription can be moved into the cytoplasm, they must undergo modifications that allow them to become mature messenger RNA. Splicing is the name given to the reaction that removes unnecessary segments of the primary RNA transcript, called introns. The removal of the introns produces mRNA (see the figure below). Messenger RNA contains only exons, those portions of the primary RNA transcript that will be translated into a protein. 

A diagram depicts the product of splicing. At the top of the diagram is a primary m R N A transcript. It's shown as a chain of rectangles that are labeled exon 1, intron 1, exon 2, intron 2, and exon 3 from left to right. An arrow labeled splicing points down to a second chain labeled m R N A. It's represented as a chain of rectangles that are labeled exon 1, exon 2, and exon 3 from left to right.

Figure 6.08: Splicing Product 

Unlike the sequence of an exon, intron sequences are unimportant. Only small portions of an intron sequence are preserved. These portions, located near the end of each intron, serve to identify a sequence as an intron, identifying the sequence for removal. The splicing reaction occurs with the help of the spliceosome, a multi-component protein. 

Alternative splicing occurs when exons are removed in addition to introns. This can occur when an exon is between two introns. The excision of some exons and retention of other exons creates different versions of the mRNA produced. This allows for one segment of primary mRNA transcript to lead to the translation of several different protein products. 

Previous section GTP cap and Poly-A tail

Take a Study Break