Degree of ploidy
In eukaryotes, the degree of ploidy indicates the number of chromosome sets in a cell nucleus . A cell can contain one set of chromosomes, then it is called haploid (1 n ). Many organisms, including humans, mainly have somatic cells with two sets of chromosomes; they are diploid (2 n ). However, there are also living beings that have the ploidy grade 3 n ( triploid ), 4 n ( tetraploid ), 6 n (hexaploid) or even higher degrees (see polyploidy ).
Degree of ploidy in sexual reproduction
In sexual reproduction there is a periodic alternation between a haploid phase and a diploid phase, the nuclear phase change . In the formation of haploid cells from diploid, meiosis or meiosis , the number of chromosomes is halved. During fertilization , the opposite happens, here two haploid cells fuse to form one diploid. This means that offspring have different genes than their two parent organisms, because they each receive a set of chromosomes from each.
Besides meiosis and fertilization, there is also mitosis , a cell nucleus division in which two identical daughter cell nuclei are created. In mitosis, the degree of ploidy does not change. Mitosis is the cell division that enables a multicellular organism to grow.
With asexual reproduction , offspring arise without genetic recombination and not from sex cells . They have identical genes as their parent organism, apart from mutations . Only a few eukaryotes completely forego sexual reproduction, as this enables the creation of possibly advantageous gene combinations and thus offers the species survival advantages.
Most vertebrates have two complete sets of chromosomes in their somatic cells and thus a degree of ploidy of 2 n (diploid). The degree of ploidy of a vertebrate germ cell, i.e. egg cells and sperm , is 1, i.e. haploid (1 n ).
In humans, the simple set of chromosomes consists of 23 different chromosomes , namely a sex chromosome ( gonosome , in humans the X chromosome or the Y chromosome ) and 22 other chromosomes, the autosomes . There are usually two sets of chromosomes in a body cell. However, certain body cells with a high transcription rate can have higher degrees of ploidy, for example liver cells or megakaryocytes .
There is a homologous chromosome for each of the 22 chromosomes and for a woman's sex chromosome (a man usually has an X and Y sex chromosome). Since the chromosome set is present twice, the degree of ploidy is 2, ie the person is diploid .