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12.1Sexual and Asexual Reproduction

Reproduction is roughly classified into sexual and asexual types. Asexual reproduction is a mechanism by which individuals produce multiple equivalent progeny by division or other means, and is not accompanied by gene-level changes. This mechanism is commonly found in bacteria (Fig. 12-1A) and protista*1, but is also found in animals (jellyfish, hydras, etc.; Fig. 12-1B) and plants (regeneration through vegetative organs (as seen in potatoes) or by cuttage; Fig. 12-1C). Eukaryotic genomes are generally diploid, and have n pairs of homologous chromosomes (n: the ploidy for each cell), generally expressed as 2n. In sexual reproduction, a haploid gamete derived from a paternal source (in humans, a sperm) and one derived from a maternal source (in humans, an ovum) fuse together to form a diploid zygote (in humans, a fertilized egg). In multicellular organisms, this zygote performs repeated cell division to create new individual (Fig. 12-1D). Unicellular organisms such as yeast may also perform sexual reproduction depending on their environment, and in such cases, a zygote creates new progeny. Meiosis is the mechanism by which haploid cells are created from diploid cells.


*1
Protista: The term “protista” is used to collectively describe unicellular eukaryotes, and includes flagellata and ciliates. Their reproduction methods include binary fission, multiple fission and budding.

Sexual and asexual reproduction

Fig. 12-1. Sexual and asexual reproduction

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