Chimera (genetics)
Chimera is called in medicine and biology an organism which is composed of genetically different cells or tissues, yet represents a single individual. These different cells from different embryos. Whether the different cells of individuals of the same species or of different species are native, is irrelevant to the definition. The chimera must be distinguished from mosaic in which likewise occur genetically different cells, but all from the same fertilized egg come, and from interspecific hybrids, which usually comes from a single fertilized egg cell, but has parents from different species, such B. the mule.
In humans and other mammals blood chimeras are known, which are referred to as free bovine Martins. They can arise with mehreiigen multiples when forming in the placental anastomoses in pregnancies. The blood of the embryos mixed with each other and thus goes back to different blood stem cells, in addition to their own blood stem cells on the multiple siblings. In principle, different blood types are possible in these cases. Also allo- or xenografts make the organ recipient becomes a chimera.
In plants, chimeras created artificially by grafting. Do the cells of a chimera different ploidy levels, such as after a colchicine treatment, they are called Cytochimären. From the study of plant chimeras could be deduced that in these also strongly genetically different cells and tissues can combine into a complex organism. An exchange of genetic information between the genetically different cells of the chimera does not take place according to current doctrine. For genetically modified tobacco plants has been shown 2009 that after grafting transmission of the transgenes to tissue of the other partner in the vicinity of the graft is possible. Whether a transfer in other cases it is possible to initially remains unclear.
Word origin and history
The term chimera is derived from a hybrid creatures of Greek mythology. The Chimera is a fire snorting monster, front lion, goat in the middle, back dragon that was killed by Bellerophon. Literally is " chimera" for figment of imagination, absurdity or fantasy.
Chimeric have long been known, at least in plants. P. 1675 Natus has described a chimera found in Florence from lemon and orange. 1875 in Paris, a chimera from Laburnum Cytisus purpureus anagyroides and was known could show from the 1907 Eduard Strasburger, that it is not a hybrid but a chimera. The term chimera was coined by Hans Winkler, 1908.
Human chimeras
Natural and artificial chimeras
In humans, blood chimeras can occur naturally. Apart from these different cell lines can also form various organs. In one known case came from the womb of a, the rest of the body from the other cell line.
In biomedical research, artificial animal -human embryos are produced. This human genetic material is introduced into oocytes of animals. The resulting embryo can be used for stem cell research. This procedure is ethically controversial. Opponents fear abuse, such as the breeding of hermaphrodites from humans and animals. Proponents argue that human eggs would increasingly scarce and expensive for research. It was difficult to find enough women to donate eggs.
In medicine, a donor organs genetically different from the recipient donor are transplanted in organ transplantation. As in the body of the receiver thereby come together cell lines from different fertilized eggs, the recipient of an organ donation, by definition, is also a chimera. The recipient of a blood transfusion, however, is only a short time becomes a chimera, because the blood cells obtained are reduced again after some time.
British Embryos Act of 2008
The British House of Commons adopted a far -reaching embryos law that allows the growth of human-animal chimeras and human -human chimeras on 22 October 2008. The latter are used among other things for the production of so-called rescue siblings. After the House of Lords agreed to the law, it entered into force in November 2008. By law, the chimeras must be destroyed after two weeks and may not be implanted into a woman's uterus. The then British Prime Minister Gordon Brown is a staunch supporter of the law. His son Fraser suffers from cystic fibrosis, a disease in which there is hope that it can be treated better by findings from research on embryos one day.
Intracellular chimeras
Intracellular chimeras in a cell nucleus, mitochondria, or plastid genomes of different species. This happens about experimentally by protoplast fusion, or hybrid formation at ways in which plastids and mitochondria are inherited by both parents.
Gene chimeras
A chimeric gene composed of portions derived from two or more precursor genes. They are formed by exon shuffling or by retrotransposition. The first such gene discovered was the jingwei gene in Drosophila and Drosophila teissieri Yakuba, consisting of the three exons of the Yellow emperor gene as well as a fourth exon, which is 90 % of the intron -less Adh gene ( alcohol dehydrogenase coded) corresponds.