Reproduction is the creation of new, independent offspring of a living being . As a rule (except for some unicellular organisms ) it is associated with an increase in the number of specimens. A distinction is made between sexual reproduction , in which two sexes usually mate, and asexual reproduction , in which no mating takes place.
The notion that living beings reproduce, which is taken for granted today, did not emerge until the end of the 18th century: Until then, the creation, the "generation" of a living being, was viewed as an act of creation . A distinction was made between “semen production”, as it occurs in humans and higher animals, from spontaneous production through which lower animals such as snakes or flies seemed to emerge from putrid and muddy matter. In either case, the Creator's intervention was considered necessary.
In the 17th century the idea arose that humans and higher animals were not created anew, but were already preformed (preformed) and only had to "unroll" ( preformation theory ). She gained great persuasiveness through the microscopic examinations by Antoni van Leeuwenhoek and others who found "seed animals" ( sperm ) in the semen of humans and animals and thought they could see tiny "people" (homunculi) in them. At the same time, Francesco Redi's experiments on the emergence of flies from rotting meat made it clear that such lower animals do not arise spontaneously, but from tiny eggs. Now one imagined that all living beings were already nested in one another and were created at the same time during creation . The discovery Charles Bonnet in 1740 that female aphids can reproduce without males ( parthenogenesis ), was considered a brilliant proof, although it was contrary to the assumption that future generations in the "spermatozoa" are nested.
That living properties of both can unite parents in itself, was the case of the mule's been around since the ancient times known and Joseph Gottlieb Kölreuter described in 1761 that this even at intersections of different tobacco TYPES occurs. It was also known that deformities can be passed on to offspring from both the father and the mother. However, these were neglected phenomena that could not be explained within the framework of prevailing beliefs. Likewise, the detailed investigations by Caspar Friedrich Wolff (1759) on the gradual development of chicks in the egg from the initially quite shapeless yolk mass ( epigenesis ) did not convince contemporaries.
Johann Friedrich Blumenbach suggested a new line of thought by postulating an educational instinct as "one of the first causes of generation, nutrition and reproduction" in 1781 . This vitalistic concept offered an alternative to the preformistic ideas and also included the concept of procreation (reproduction). The question that remained open at first was whether humans and mammals, like other animals, develop from eggs or, as Albrecht von Haller postulated, from coagulating menstrual fluid, until Karl Ernst von Baer discovered the human egg in the ovarian follicle in 1827 .
The process of fertilization , ie the union of egg and sperm, described Oscar Hertwig 1876-1878 with sea urchins : He used osmium tetroxide for fixing microscopic preparations and borax - Carmine for specific staining of the nuclei . When he compared successive stages, he discovered that the sperm enters the egg cell with its head and releases its nucleus, whereupon both nuclei move towards each other and unite. Nuclear division ( mitosis ), in which the chromosomes , which can also be stained with carmine, appear and are evenly distributed between the two daughter cells, was already described by Friedrich Anton Schneider in 1873 .
However, no meaning or function could be ascribed to these processes. Although had Gregor Mendel , the results of his experiments crossing, where he later named after him inheritance rules had worked out, published in 1866, but he found it in his lifetime not understand. It was not until 1900 that Hugo de Vries , Carl Correns and Erich Tschermak became aware of this foreman after they had received corresponding results themselves - simultaneously but independently of one another. In the meantime, the behavior of the chromosomes in meiosis was better known, and in 1902 Walter Sutton pointed out that the occurrence of chromosomes of the same shape in pairs could have something to do with the characteristics that were also found in pairs in the work of Mendel and his rediscoverers, which was ultimately Theodor Boveri 1904 explicitly formulated as the chromosome theory of inheritance .
Boveri, Correns and others assumed, however, that the cell nucleus or the chromosomes only had a rather subordinate function in inheritance and that the cytoplasm played the main role. On the other hand, Thomas Hunt Morgan and Hermann Joseph Muller, based on their investigations into mutually inherited traits ( gene coupling ) , develop gene maps on which a certain arrangement of genes was recorded on a chromosome, and in the 1920s they formulated the theory that genes are basically on the chromosomes localized and the cytoplasm only plays a secondary role. The gene maps were based on the fact that linkage groups can be separated, and it was assumed that this happens more often the further apart the genes in question are on the chromosome. The process of crossing-over on which the coupling break is based was clarified in 1930/31 by Barbara McClintock and Harriet B. Creighton .
However, it was by no means clear which components of the chromosomes are the material carriers of the genetic information. It was known ( Richard Altmann 1889) that the chromosomes contain basic proteins and “nucleic acid”. The latter hardly seemed to come into question as a genetic material because it only consists of sugar, phosphate and five different nucleobases , while an increasing number of components ( amino acids ) were discovered for proteins . It was not until 1953 that the double helix structure model by James Watson and Francis Crick made it clear that deoxyribonucleic acid (DNA) actually has a very complex structure.
Sexual or sexual reproduction is characterized by the fact that cell nuclei alternately fuse with one another ( karyogamy ), whereby the number of chromosomes doubles ( diploidy ), and in a special form of nucleus division, meiosis , the number of chromosomes is halved again ( haploidy ). This nuclear phase change leads to the fact that the chromosomes and thus the genes on them are recombined ( recombination ), because homologous chromosomes are randomly distributed to the daughter nuclei during meiosis. In humans, as in all multicellular animals and in most fungi , this happens from generation to generation, while in more highly organized plants (land plants or embryophytes) diploid and haploid generations alternate.
Most eukaryotes (living things with cell nuclei) reproduce sexually at least occasionally. It is the only form of reproduction in humans and more highly organized animals, while asexual reproduction also occurs in other eukaryotes. In the most common case (as in humans) there are two sexes that form different sex cells ( gametes ), which unite during fertilization . Living things often have genital organs of both sexes, which is known as hermaphroditism . This is to be distinguished from intersexuality , the expression of characteristics of both sexes, which is mostly associated with infertility .
Where there are no gender differences, such as mushrooms and many algae , one speaks of mating types. There can also be more than two of these. A derived special form in which offspring arise from unfertilized egg cells is unisexual or unisexual reproduction. It is known as parthenogenesis in animals and apomixis in plants and can alternate with bisexual reproduction, such as in aphids , or it can be the only form of reproduction, as in most dandelion species.
In asexual or asexual reproduction there is no karyogamy or meiosis; the degree of ploidy remains unchanged. It occurs very frequently in plants, algae, fungi and protozoa and is often the primary form of reproduction in these organisms, for example through spores . In the animal kingdom it occurs only in relatively simply organized representatives, such as worms , polyps and tunicates .
- François Jacob : The logic of the living . Fischer, Frankfurt / Main 1972, pp. 27f and 32f.
- François Jacob : The logic of the living . Fischer, Frankfurt / Main 1972, pp. 61-75.
- Ilse Jahn , Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, pp. 219f.
- Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, p. 232f.
- Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, p. 244f.
- Ilse Jahn (ed.): History of Biology, 3rd edition, Nikol, Hamburg 1998, pp. 336–338.
- Ilse Jahn (ed.): History of Biology, 3rd edition, Nikol, Hamburg 1998, p. 347.
- Ilse Jahn, Rolf Löther, Konrad Senglaub (eds.): History of Biology. Theories, methods, institutions, short biographies. 2nd, revised edition. VEB Fischer, Jena 1985, p. 463f.
- Lexicon of Biology : Intersexuality . Spectrum, Heidelberg 1999.
- Lexicon of Biology : Asexual Reproduction . Spectrum, Heidelberg 1999.