Intergenic Regions
The gaps or spaces between the genes that lie on chromosomes termed intergenic regions. By definition, intergenic regions are non-coding, (i.e., they are not transcribed into codons) and therefore do not direct the protein synthesis via messenger RNA (mRNA). Although not transcribed, intergenic regions have been shown to play a role in the expression of adjacent genes and there is increasing evidence that they contain important control sequences.
As opposed to often conserved gene sequences, genetic variability increases in intergenic regions. Correspondingly, coding regions of DNA are far less variable than intergenic regions. Such a discrepancy is expected considering that the coding regions are parts of alleles that instruct the synthesis of proteins--and that these proteins in turn regulate cellular structure and function in such a manner that they are more directly influenced by selection pressures.
The physical size of genes, and the related physical size and sequence complexity (i.e., whether there are many repeated sequences or long unique sequences) are specific to the organism under study. Accordingly, intergenic size and complexity can greatly vary. Moreover, unlike genes, that may be similar in a wide variety of species (e.g., mammalian genes), intergenic regions show a far greater diversity or difference. Even among mammalian species very little of intergenic DNA appears to remain conserved (similar).
Although sequences that serve to act as gene enhancers and silencers occur in intergenic regions, they do not directly contribute to the production of protein. Correspondingly, intergenic regions do not directly contribute to the phenotype of the individual and changes in intergenic regions would not be expected to affect the fitness of the organism. For this reason, intergenic regions were previously termed Junk DNA.
Recent research suggests, however, that intergenic regions between genes may play critical roles in the process of imprinting, cancers and some birth defects. Current research projects are attempting to more accurately characterize the intergenic regions near imprinted genes that cause the expression of one parental allele over another (imprinting). In contrast, to equal inheritance of traits, with imprinted genes, only one copy of each allele is active. With imprinted genes, the associated characters and traits rely only on the allele inherited from one parent. Imprinting, perhaps regulated or related to adjacent intergenic areas, is also called genetic silencing. Loss of imprinting (i.e., the sudden expression of both parental alleles is sometime associated with tumor development.
Molecular biologist use sophisticated mathematical techniques and modeling enhanced with modern computer technology (e.g. bioinfomatic analysis) to identify intergenic nucleotide sequences and to investigate their potential role in cell regulation. Sophisticated computer algorithms may lead to a more detailed understanding of the role intergenic regions may play in gene regulation.
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