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Selfing DNA prevents genomes from mixing

11.10.2002


Genomes of multicellular organisms are one of the greatest mysteries of biology. The more is discovered about them, the more questions are to be answered. One of such questions is connected with the size of a genome. As is known since the middle of the 20th century, the level of organization of an organism does not depend on the genome size, i.e., on the amount of DNA in the nucleus of a cell. Sometimes, a primitive organism contains much more DNA than a mammal. For example, the genome of certain amoebas is 200 times as large as that of humans. The nature of this phenomenon has been understood very recently. The most part of DNA does not contain any of protein-coding genes. Because of its unclear function, it is called the selfing or junk DNA, which is somewhat abusive. Its share in genomes of some species riches 95% (in human genome, its content is 75%). The selfing DNA can hardly serve as a material for evolution: it is so unstable that has no time to develop into a functional structure. However, as long as each species has its specific junk DNA, it must serve for something.



Different scientists tried to find an explanation for the biological purpose of the selfing DNA. About fifteen hypotheses were offered, and most of them turned to be invalid. In Russia, this problem has been studied for many years by doctor of biological sciences Aleksei Akifyev and his colleagues. The scientists believe that an actual function of the selfing DNA stands behind a phenomenon known for already 100 years and called the chromatin diminution. This is a key term in this context, let us remember it. The chromatin diminution is the elimination of an inactive chromatin from a genome. Some multicellular animals, such as ascarids and small crustaceans Cyclopoida, lose an important part of their chromosomal DNA at the early stages of embryogenesis. The diminution normally takes place in cells that are to build the body and never occurs in developmental precursors of germ cells. The latter still have all their selfing DNA. Apparently, these are the cells, in which the selfing DNA is functionally significant. The scientists have revealed that the selfing DNA prevents the confusion of closely related species.

Aleksei Akifyev and his colleagues have studied the genetic isolation mechanism using near species of crustaceans Cyclopoida as an example. The chromatin diminution is characteristic of one species and never occurs with the other. The researchers suggest that the DNA elimination is performed by certain enzymes that cut the genome at the right time and in the right place and are contained in the cytoplasm of an ovum. If a sperm cell of one species enters an ovum of another species, then the embryo dies either because of a failure to perform a necessary diminution or because of an unreasonable elimination of vital chromosomes by aforementioned enzymes.


The chromatin diminution never takes place in cells of humans and most animals, but they isolate the selfing DNA in another way ? by placing it in a position unreachable for most enzymes and thus making it functionally cut off. Each species has a unique structure and packing of the selfing DNA. In other words, indirect ways of a physical separation of some part of genome are known for many organisms.

Thus, a certain part of a genome with a particular size and structure serves for the genetic isolation of species. Therefore the selfing or junk DNA is not a piece of garbage, but a vital component. If one mechanism eliminates it, another mechanism should restore it while a species still exists. The scientists expect to understand more, if they will manage to reveal the molecular organization of the selfing DNA. The most suitable material for this study is DNA lost from cells as a result of the diminution.

Natalia Reznik | alfa

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