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Mulitcellular Organisms

Sexual Life Cycles

Life Cycle of Animals This leads to a paradox of cosmic implications: natural selection is simultaneously pushing for a large stage in the life cycle that can compete for food and for a minute single-cell stage that is essential for sexual reproduction. The result is that all multicellular organisms, from small algae and fungi to elephants and human, have a unicellular stage and a large stage of varying dimensions in their life cycle. Figure 10-05 shows the life cycle of animals for which the unicellular stage is the gamete in the form of egg or sperm. This is called the haploid state where a single cell carries n chromosomes after meiosis (cell division that transmits only n chromosomes) The large stage is the diploid state comprised with all the somatic cells in the rest of the body. Those cells carry 2n chromosomes (one from each parent at fertilization) starting from the zygote. They can differentiate into different cell types (performing different function) during mitosis (cell division in which the daughter cells receive the exact chromosome and genetic makeup of the mother cell).

Figure 10-05 Life Cycle of Animals
[view large image]

Life Cycle of Plants Animal life cycle has the adult (the multicellular form) always in diploid state. However, this is not the case in more primitive species. Figure 10-06 shows the relative importance of the diploid and haploid generations among plants. Mosses are primitive plants, they still require very wet conditions for growth and fertilization. Meiosis produces spores, which germinate into gametophyte (male and female adults in haploid state). The gymnosperms (conifers) and angiosperms (flowering plants) are more advanced; the adult is in diploid state. This evolutionary development seems to indicate that earlier life tried to construct the multicellular body from the simpler cell unit in haploid state. It was switched gradually to the more stable diploid state where two similar copies of the genes are available - a primary and a backup.

Figure 10-06 Life Cycle of Plants [view large image]

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