Of diploidy ( ancient Greek διπλόος diplóos , plural διπλοῖ diploĩ , duality ') is in the genetics speaking, if in a cell nucleus , a twofold set of chromosomes ( 2n present). Its number of chromosomes is twice as large as that of a single set of chromosomes ( 1n ). Cells with such nuclei are diploid . The term is also used for living beings that mainly consist of such cells. If there is only a single set of chromosomes, this is known as haploid . In some living beings there are othersDegrees of ploidy , such as triploid ( 3n ) or tetraploid ( 4n ).
Diploid organisms that reproduce sexually arise from the germ cells of the two parents during fertilization . Two gametes unite and their cell nuclei, each with a single set of chromosomes, fuse to form the common nucleus of the zygote ( karyogamy ). In such living beings, a haploid and a diploid development phase alternate ( nuclear phase change ). The transition from the diploid to the haploid state is meiosis .
In most multicellular animals (including humans), most of the cells in the body are diploid and only the gametes are haploid. They are therefore referred to as Diplonts . In contrast, plants also have a more or less pronounced haploid phase in their life cycle and are therefore referred to as diplohaplonts . The diploid phase is shortest in the haplonts , where it is restricted to the zygote. They include many algae and some unicellular organisms .
Nuclear phase change in plants
- In mosses (Bryophytina), the haploid generation ( gametophyte ) is what one sees as a green moss plant (leafed or as a thallus ). After fertilization , a smaller diploid sporophyte develops , which grows out of the moss plant and forms a spore capsule in which meiosis takes place, so that haploid spores are formed. The spores are scattered in the wind or rain, germinate, and a new haploid gametophyte grows.
- In ferns (Pteridophytina), horsetail (Equisetopsida, belongs to the Pteridophytina) and club moss plants (Lycopodiopsida, also belongs to the Pteridophytina), the large visible plant is the diploid sporophyte, which forms haploid spores after meiosis . A small gametophyte, the prothallium, arises from them . The sporophyte grows again from its fertilized egg cells . But there are also polyploid ferns (sporophytes).
- In the seed plants ( Spermatophytina ) the diploid sporophyte is what we see as trees , bushes or herbaceous plants . The gametophyte is greatly reduced: the pollen grains and the embryo sac in the ovule . Represent the haploid generation In the gymnosperms (gymnosperms) of pollens leads directly to ovule that are not from an ovary is surrounded, so no scar and no stylus possesses. In the case of angiosperms grows after the pollination the pollen tube to the ovum by the pen to the embryo sac where fertilization takes place.
As a rule, multicellular animals result from the fusion of a paternal and a maternal haploid germ cell to form a diploid zygote. The zygote divides many times, creating the animal's body from diploid cells. Finally, through gametogenesis , haploid germ cells are formed again. However, there are numerous exceptions to this general scheme, some of which are described below.
- Some insects are sexed by whether they are haploid or diploid ( haplodiploidy ). Female bees or ants are diploid (workers and queens) while the males ( drones ) are haploid.
- In some actually diploid insects, for example the fruit fly Drosophila melanogaster , most of the cells of the adult animals are polytenized , which means that large parts of the chromosomes have been multiplied several times without the cell having divided.
- Some animals reproduce through virgin generation ( parthenogenesis ).
- The red blood cells ( erythrocytes ) of mammals do not contain a nucleus and therefore no chromosomes.
- Blood platelets ( thrombocytes ) also do not contain a nucleus. They are formed by megakaryocytes , which can contain up to 64 sets of chromosomes.