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This same pattern, but not the same number of chromosomes, occurs in all organisms that utilize meiosis. Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi. [5] [6] [7] It is an essential process for oogenesis and spermatogenesis.
In humans, on average, about 50 DNA double-strand breaks occur per cell in each cell generation. [21] Meiosis, which facilitates recombinational repair between non-sister chromosomes, can efficiently repair these prevalent damages in the DNA passed on to germ cells, and consequently prevent loss of fertility in humans.
There are two popular and overlapping theories that explain the origins of crossing-over, coming from the different theories on the origin of meiosis.The first theory rests upon the idea that meiosis evolved as another method of DNA repair, and thus crossing-over is a novel way to replace possibly damaged sections of DNA. [9]
In meiosis, non-sister homologous chromosomes pair with each other so that recombination characteristically occurs between non-sister homologues. In both meiotic and mitotic cells, recombination between homologous chromosomes is a common mechanism used in DNA repair .
Animals have life cycles with a single diploid multicellular phase that produces haploid gametes directly by meiosis. Male gametes are called sperm, and female gametes are called eggs or ova. In animals, fertilization of the ovum by a sperm results in the formation of a diploid zygote that develops by repeated mitotic divisions into a diploid ...
In some animals, however, cell division eventually halts. In humans this occurs, on average, after 52 divisions, known as the Hayflick limit. The cell is then referred to as senescent. With each division the cells telomeres, protective sequences of DNA on the end of a chromosome that prevent degradation of the chromosomal DNA, shorten. This ...
Male germ cells produced in the testes of animals are capable of special DNA repair processes that function during meiosis to repair DNA damages and to maintain the integrity of the genomes that are to be passed on to progeny. [19] Such DNA repair processes include homologous recombinational repair as well as non-homologous end joining. [19]
Researching meiosis in mammals plays a crucial role in understanding human infertility. Meiosis research within mammal populations is restricted due to the fundamental nature of meiosis. [2] In order to study mammalian meiosis, a culture technique that would allow for this process to be observed live under a microscope would need to be ...