Fundamentals of Biochemical Engineering, Part 1 - Bailey, J., & Ollis, D. 1989

Molecular Genetics and Regulatory Systems
Molecular Genetics
Split genes and mRNA modification in eukaryotes

Eukaryotic Messenger RNA is modified by the attachment of a special structural element to its 5' end, known as a cap or cap Structure (a 7'-methylguanosine residue); the cap structure increases the stability of mRNA against the action of Phosphatases and Nucleases. In addition, a poly(A) sequence containing 150 to 200 deoxyadenylate residues is added to the 3' end of most eukaryotic mRNAs. This fact can be exploited in the isolation of eukaryotic mRNAs by sorbing them onto Supports containing covalently linked poly-T oligonucleotide chains.

It can now be considered firmly established that Many eukaryotic genes consist of alternating sequences that encode Cell/13.html">Protein Structure and those that carry no information about protein structure. Sequences of the first type are sometimes called exons, while non-coding sequences are termed introns or intervening sequences. The latter, as their name implies, lie between exons, separating the coding regions with stretches that at first glance contain unnecessary redundant information. Clearly, Eukaryotic Cells must possess mechanisms that recognize exons and introns and ensure that only the exons are directly translated during Gene Expression.

The main Structural elements of a eukaryotic gene and the key stages of its expression are illustrated in Fig. 6.7. The upper part of the figure shows a hypothetical segment of a structural eukaryotic gene containing exons and introns. During METABOLISM/31.html">Transcription, the DNA template strand directs the synthesis of mRNA (the primary transcript) complementary to the entire template sequence—that is, containing both exon and intron sequences.

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FIG. 6.7. During EUKARYOTIC GENE EXPRESSION, the primary mRNA transcript undergoes splicing to yield the mature (active) mRNA involved in Translation. Hypothetical shortened sequences are shown here.

This is followed by a splicing stage that is likely unique to eukaryotes. During splicing, the Regions of the mRNA corresponding to the introns are excised, and the remaining sequences, complementary to the exon sequences, are joined in the same order as they appeared in the primary transcript. The resulting mature mRNA is then translated in the manner described above, yielding the protein corresponding to the specific gene.

Current experimental data indicate that almost all eukaryotic genes contain introns. However, a few exceptions to this rule are known, including the human alpha-interferon genes. Introns can be very large; it is possible that they account for more than half of all DNA nucleotide residues.

At the time of writing of this chapter, the biological function of introns remained unclear. It is possible that introns are involved in the Regulation of Eukaryotic gene expression. From an evolutionary perspective, introns may serve as a tool for bringing about relatively large-scale gene alterations through the inclusion or exclusion of an entire intron or exon sequence. It has also been suggested that exons encode relatively large Structural domains of Proteins and that exon shuffling provides The Cell with a way to synthesize novel proteins and thereby test their functional viability. From a technological standpoint, introns present certain difficulties in the Expression of Eukaryotic genes in prokaryotic host hosts, since the latter lack an RNA splicing machinery. In Section 6.3, dedicated to Recombinant DNA technology, we will see how this difficulty can be overcome.