Core phase change
The nuclear phase change is the periodic alternation between a haploid and a diploid phase in the sexual reproduction of eukaryotes . "Haploid" means that the cell nucleus contains only one set of chromosomes , while in the diploid phase it contains two homologous sets. (Alternatively, one speaks of a single and a double set of chromosomes.)
The core phase change is the basic principle of sexual reproduction, because it enables the recombination of the genome without the chromosome sets doubling with each new generation. The change between the two nuclear phases takes place on the one hand during meiosis (or reduction division) and on the other hand during fertilization . During fertilization, two haploid cell nuclei fuse to form a diploid nucleus of the zygote ( karyogamy ). In meiosis, the degree of ploidy is reduced again by distributing homologous chromosomes to different daughter nuclei.
Three variants of the core phase change
The relative proportion of the haploid and diploid phases varies in different organisms:
- Gametic core phase change (Diplonten)
- In the Diplonts , to which all multicellular animals (including humans ) belong, all body cells are diploid. Only in the sexual organs are haploid gametes (germ cells) formed by meiosis . In most living things, these gametes are divided into two sexes. Often the female gametes are larger and the males smaller. In humans, these gametes are egg cells and sperm . During fertilization, gametes unite with those of the opposite sex. This creates a diploid zygote from which the new organism emerges through numerous mitoses (nuclear divisions without nuclear phase change). In this case, one speaks of a gametic core phase change . The Diplonts also include the unicellular eyelash animals and some green algae and brown algae (the Fucales ).
- Zygotic nuclear phase change (haplonts)
- The opposite case is shown by the haplonts , in which only the zygote is diploid and meiosis takes place again directly after fertilization without intermediate mitoses. Mitoses only take place in the haploid phase. This thus dominates the life cycle, and one speaks of a zygotic core phase change . The haplonts include many mushrooms ( mushrooms and yoke mushrooms ), many thread-like green algae, simply organized red algae and some unicellular organisms , especially flagellates .
- Heterophasic nuclear phase change (diplohaplons)
- Third, there are the diplohaplons , in which mitoses occur in both the diploid and the haploid phase. This heterophasic nuclear phase change occurs in all higher organized plants ( Embryophyta ) and most algae . In the vascular plants ( seed plants and ferns ) the diploid phase dominates, in the deciduous mosses and liverworts the haploid phase dominates . In the hornmoss , both phases are organisms of similar size. The hose fungi are also diplohaplons, although only one mitosis occurs in the diploid phase.
Disturbances in the core phase change
If the meiosis does not take place completely during the gametic or heterophasic nuclear phase change, one or more chromosomes can be contained twice in the germ cell or in the haploid generation. If this germ cell is then fertilized, a trisomy occurs . In the extreme case, the germ cell is completely diploid and fertilization then creates triploidy , i.e. a genome with a triple set of chromosomes. If both germ cells are diploid, tetraploidy with four sets of chromosomes occurs.
Triploidy and tetraploidy are not uncommon in the plant kingdom. They are specifically brought about by breeding methods in some crops , for example to increase yield. Many triploid plants cannot produce seeds, but this is also desirable for crops such as dessert bananas or seedless grapes.
In animals, triploidy mostly leads to sterility and can lead to premature death in some species. Occasionally, animal species such as fish or mussels are deliberately bred triploid. In humans, trisomies often lead to physical and mental disabilities.
Other possible disorders are null isomies and monosomies , in which one chromosome or a pair of chromosomes is missing.
Such chromosome set disorders can also occur in the organism outside of the nuclear phase change, for example in cancer cells . In the liver , a small percentage of cells are naturally tetraploid.
See also
Individual evidence
- ↑ Bernard John: Meiosis . Cambridge University Press 1990, pp. 6f.