Читать книгу Algorithms in Bioinformatics - Paul A. Gagniuc - Страница 52

The cooperation of eukaryotic cells is best observed in the case of two phenomena, namely chimerism and mosaicism. Mosaicism is represented by two or more cell populations in different tissues originating from one fertilized egg. Namely, one cell population with the original genotype (usually representing the majority) and other cell populations with slight variations of the original genotype. One of the mechanisms that lead to mosaicism is represented by transposomes [161]. With embryonic development, the genotype of an organism can undergo various types of mutations, including transposome-induced mutations above different cell lines. These mutations can occur late in embriogenesis, leading to marginal effects at the organism level, or they can occur early in the embryonic development of an organism with more pronounced/noticeable effects [162]. Transposome-induced mutations represent a normal and nonrandom variability in multicellular organisms, leading to different phenotypic characteristics [161]. The classic example, however, is represented by the experiment performed by Barbara McClintock on corn kernels, where the transposomes inactivate the gene for the pigment protein and the phenotype is easily recognizable (please see the “horizontal gene transfer” subchapter from above). Mosaicism can also be represented by other types of mutations. For example, in humans, Down syndrome is characterized by an additional copy of chromosome 21, which is frequently attached to chromosome 14 (Trisomy 21). The extra copy of chromosome 21 slightly changes the chromosomal territories in the cell nucleus and the way heterochromatin and euchromatin are distributed. This leads to unusual variations in the expression of certain genes, especially those present on the extra chromosome 21 and those present on the neighboring chromosomal territories. Trisomy 21 occurs at the beginning of embryogenesis. However, such a mutation may appear late in embryogenesis, which results in mosaic tissues, part with normal cells and part with cells with an extra copy of chromosome 21 [163, 164]. Such a mosaic can be clinically unnoticeable unless genetic analysis is made on different tissues of the organism. Moreover, it is believed that a combination of germinal and somatic Trisomy 21 mosaicism may be reasonably common in the general population [163]. In development, cells with different genotypes compete in the tissue population. Such a competition can lead to the possibility (especially for mosaicism that appeared late in embryogenesis) in which cells with the original genotype completely marginalize the function of mosaic genotypes or vice versa, depending on which is more fit for a specific function. On the other hand, chimerism is represented by fusion of more than one fertilized zygote, namely cells of different organisms that are orchestrated by common molecular signals to form a single body [165]. Chimerism can be observed in all multicellular species to a greater or lesser extent and may be accompanied by genetic mosaicism in any of the genotypes that form the composite organism.