6: Multicellularity, the Cell Cycle and the Life Cycle
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- 6.1: Mitosis and the Cell Cycle
- Mitosis is a process of equal cell division, where each of the new cells receives the same number of chromosomes as the original cell. Mitosis does not change the cells’ genotype. The goal of mitosis is to distribute pre-combined genetic material equally. Actually, mitosis is a kind of karyokinesis, or splitting of the nucleus, as opposed to cytokinesis, which is the splitting of the whole cell. Karyokinesis and cytokinesis are parts of the cell cycle.
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- 6.2: Syngamy and Meiosis
- A sexual process is important to the survival of a species. First, it makes the population more diverse, which allows more flexibility to adapt via natural selection. Natural selection means that all organisms are different, but if environmental conditions change, only most adapted will survive. If the population is uniform, it has less chance of survival. Second, it prevents lethal mutations from being transferred to the offspring, because those with the mutations will die and not pass on genes
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- 6.3: Life Cycle of the Unicellular Eukaryote
- The life cycle of a unicellular organism begins with syngamy: one cell unites with cell having different genotype. The life cycle has all three possible ways of reproduction: sexual (ploidy doubles: syngamy), asexual (ploidy reduces: meiosis of zygote) and vegetative (ploidy does not change: mitotic divisions)
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- 6.4: Life Cycle of the Multicellular Eukaryote
- Cells do not always part after mitosis, but sometimes stay together to form multicellular organisms. Cells in the multicellular body are not connected forever. Sometimes, one or few cells escape and start a new body. This body will be exact copy (clone) of the previous one (vegetative reproduction). It is also possible that when these “escaped cells” go the different route: they become “sex delegates”, gametes.
Thumbnail: Principal scheme of meiosis. Only one of two telophase I variants is shown.