Introns mRNA synthesis

Eukaryotic mRNA synthesis results in a pre-mRNA precursor that contains extensive amounts of excess RNA (introns) that must be precisely removed by RNA splicing to generate functional, translatable mRNA composed of exonic coding and noncoding sequences. 

In prokaryotes, mRNA synthesis can be controlled simply by regulating initiation of transcription. In eukaryotes, mRNA is formed from a primary transcript followed by a series of processing events (e.g., intron excision, polyadenylylation). Eukaryotes regulate not only transcription initiation but the various stages of processing as well. 

Fig. 14.10. Overview of mRNA synthesis. Transcription produces hnRNA from the DNA template. hnRNA processing involves addition of a 5 -cap and a poly(A) tail and splicing to join exons and remove introns. The product, mRNA, migrates to the cytoplasm, where it will...

Removal of introns from hnRNA to leave only the exons or gene regions involved in directing protein synthesis in the finished mRNA is accomplished within the nucleus by processing on spliceosomes (Figure 11-4). 

ID 1MME3. Ribozymes, or RNA enzymes, catalyze a variety of reactions, primarily in RNA metabolism and protein synthesis The complex three-dimensional structures of these RNAs reflect the complexity inherent in catalysis, as described for protein enzymes in Chapter 6. (c) A segment of mRNA known as an intron, from the ciliated protozoan Tetrahymena thermophila (derived from PDB ID 1GRZ). This intron (a ribozyme) catalyzes its own excision from between exons in an mRNA strand (discussed in Chapter 26). 

The triplets of nucleotide units in DNA determine the amino acids in a protein through the intermediary mRNA. One of the DNA strands serves as a template for synthesis of mRNA, which has nucleotide triplets (codons) complementary to those of the DNA. In some bacterial and many eukaryotic genes, coding sequences are interrupted at intervals by regions of noncoding sequences (called introns). 

In at least one eukaryote, Tetmhymem, the pre-rRNA molecule contains an intron. Removal of the intron during processing of the pre-rRNA does not require the assistance of any protein Instead, in the presence of guanosine, GMP, GDP or GTP, the intron excises itself, a phenomenon known as selfsplicing. This was the first demonstration of ribozymes, that is, catalytic RNA molecules that catalyze specific reactions. The list of ribozymes is growing. For example, self-splicing introns have been discovered in some eukaryotic mRNAs and even peptidyl transferase, a key enzyme activity in protein synthesis, is now known to be a ribozyme (see Topic H2). 

INTRON A region of a gene (i.e., ENA) that is transcribed in the synthesis of RNA, but enzymatically removed (by "splicing") from the final mRNA before its translation into an amino acid sequence in protein introns are characteristic of gene structure in eukaryotic, but not prokaryotic, cells. (See also EXON and CODING SEQUENCE)... 

In summary, mRNA processing requires an interconnected set of reactions Cap and polyA synthesis, followed by intron removal. Soluble enzymes catalyze the former two reactions, while the latter set of reactions involves both RNA and protein components. 

The genetic information in a gene is copied (transcribed) into a messenger RNA molecule (mRNA), preserving the sequence by complementary base-pairing. The introns are cut and the mRNA molecule is transported into the cytoplasm where it directs the synthesis of protein at the ribosomes. The sequence of bases is translated into a sequence of amino acid residues by a triplet code wherein three bases specify one amino acid. 

Eukaryotes potentially have many more opportunities for control of gene expression than do bacteria. For example, the cell could take advantage of control at the level of the processing of primary transcripts. It is known that RNA is not transported across the nuclear membrane until all introns are excised. A more subtle form of control could involve alternative modes of splicing a particular transcript. There are now examples known where this occurs to yield different mRNA molecules. Perhaps one of the best-known examples of yet another level of control in eukaryotes is that of translational control of globin synthesis. 

RNA synthesis is a key step in the expression of genetic information. For eukaryotic cells, the initial RNA transcript (the mRNA precursor) is often spliced, removing introns that do not encode protein sequences. Often, the same pre-mRNA is spliced differently in different cell types or at different developmental stages. In the image at the left, proteins... 

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