Exons, 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. 

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...

The stem-loop structure in the noncoding 3 region of selenoprotein mRNAs has also been termed a SECTS element in mammals although it has a different overall structure. ° In silica analysis of the human genome sequence, using this consensus SECTS element along with the presence of the characteristic UGA codon within an exon, has led to the discovery of several new selenoproteins, including a selenium-dependent methionine sulfoxide reductase. It has been shown that a specific complex exists for selenoprotein synthesis that shuttles between the nucleus and the cytosol. This possibly protects the preformed complex for nonsense-mediated decay to allow for more efficient selenoprotein synthesis. The specific tRNA needed for selenocysteine... 

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 two common isoforms of the insulin receptor (Fig. 11-11) arise because a 36-nucleotide (12-amino acid) exon is spliced out of the mRNA for the shorter protein. Isoenzymes of aldolase34 and of many other proteins are formed in a similar manner. Frame-shifting during protein synthesis (Chapter 29) and also post-translational alterations may give rise to additional modified forms. They are often synthesized in relatively small amounts but may be essential to the life of the cell. In addition, genetic variants of almost any protein will be found in any population. These often differ in sequence by a single amino acid. 

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)... 

Deletions or exon-skipping mutations, resulting in significant loss of El a mRNA and protein (diminished Ela synthesis). These cells will likely also exhibit loss of immunoreactive Eip protein since it has been shown that the presence of both subunits is required for stability of their normal heterotetrameric structure. 

The procollagen peptidase is sometimes lacking. In other cases a person synthesizes an abnormal pro-o2 chain that is resistant to cleavage by the peptidase because of deletion of the normal cleavage site. In others collagen is formed in only small amounts or is degraded rapidly. Some individuals lack lysyl hydroxylase and others have a defect in mRNA splicing which causes loss of an exon from the mRNA and synthesis of shortened pro-o2 chains. ... 

The synthesis of a protein in a human cell can be broadly outlined as follows. A mRNA molecule is transcribed from a single strand of DNA (the sense strand) in the nucleus. The mRNA is processed by splicing out nontranslatable segments of nucleotides (introns) and rejoining the translatable segments (exons). Additional chemical modifications are made to both ends of the mRNA molecule and it is transported into the cytoplasm. The mRNA is translated... 

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