Overview of the Respiratory System
Regulation of the Human Genome (III)
Another type of genetic regulation is exerted at the level of RNA processing through a mechanism called alternative splicing. In alternative splicing, certain exons (coding segments of DNA) of a gene may be alternately included in the final mRNA product. In this way, inclusion of particular exons only in certain tissues or only at certain times during development permits cells to modify the performance characteristics of a particular protein, thereby altering the function or effect of a protein in a cell or tissue. As shown in the graphic, this precursor RNA is alternatively spliced into 3 different isoforms. Isoform 1 contains all 4 exons; isoform 2 contains only exons 1, 2, and 4; and, isoform 3 contains only exons 1 and 4.
Various factors, including DNA sequences, also contribute to the rate of production and stability of mRNAs. By controlling the rate at which new mRNAs are transcribed from a particular gene and the decay kinetics of a mRNA (the rate at which a mRNA is degraded within a cell), cells exert control over the amount of protein that is made from an mRNA, thereby influencing the level of particular proteins within cells and tissues. Different mRNAs have different decay kinetics, meaning that some are degraded very quickly and others are degraded much more slowly. For example, the mRNAs of many cellular growth factors and transcription factors are degraded very quickly, resulting in tight control of the temporal aspects of their cellular signaling properties. On the other hand, mRNAs that encode enzymes can be much longer lived.
Keywords: alternative splicing | chromosome | DNA | enhancers | exons | gene regulation | human genome | imprinting | introns | methylation | promoters | silencers | genome
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