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They have also been used to study the biochemical events that surround mitosis, since all of the nuclei in a medium-sized plasmodium divide in synchrony. It has been observed that they can find their way through mazes by spreading out and choosing the shortest path, an interesting example of information processing without a nervous system .
Meiosis is an essential part of the life cycle of all sexually reproducing organisms, and in its main features, meiosis in N. crassa seems typical of meiosis generally. As the above events are occurring, the mycelial sheath that had enveloped the ascogonium develops as the wall of the perithecium becomes impregnated with melanin, and blackens.
Photomicrograph of a mature sporangium of an Absidia mold. In some phyla of fungi, the sporangium plays a role in asexual reproduction, and may play an indirect role in sexual reproduction. The sporangium forms on the sporangiophore and contains haploid nuclei and cytoplasm. [3]
Sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. [6] [7] Sexual reproduction also occurs in some unicellular eukaryotes. [2] [8] Sexual reproduction does not occur in prokaryotes, unicellular organisms without cell nuclei, such as bacteria and archaea.
The haploid prothallus does not resemble the sporophyte, and as such ferns and their allies have a heteromorphic alternation of generations. The prothallus is short-lived, but carries out sexual reproduction, producing the diploid zygote that then grows out of the prothallus as the sporophyte.
A megasporocyte inside a megasporangium or ovule undergoes meiosis, producing four megaspores. Only one is a functional megaspore whereas the others stay dysfunctional or degenerate. The megaspore undergoes several mitotic divisions to develop into a female gametophyte (for example the seven-cell/eight-nuclei embryo sac in flowering plants).
The stalk cells undergo apoptosis and die as the prespore cells are lifted high above the substrate. The prespore cells then become spore cells, each one becoming a new myxamoeba upon dispersal. [6] This is an example of how apoptosis is used in the formation of a reproductive organ, the mature fruiting body.
Asexual reproduction often occurs continuously. In heterothallic species, sexual reproduction occurs when opposite mating types (designated + and -) come into close proximity, inducing the formation of specialized hyphae called gametangia. The gametangia grow toward each other, then fuse, forming a diploid zygote at the point of fusion.