3.12.2: Life cycles

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This is a short list of terms associated with life cycles:

3.12.2.01.PNG — Figure 3.12.2.1 : **Plant cell** (compare with two cell schemes from above): 1 vacuole, 2 cell wall, 3 plasmodesm.

mitosis, meiosis (R!), syngamy (Y!)
vegetative reproduction (cloning), sexual reproduction and asexual reproduction
result of syngamy: zygote; participant of syngamy: gamete
smaller gamete: male, bigger gamete: female; movable male gamete: spermatozoon (sperm), motionless female gamete: oocyte (egg cell)
result of meiosis: spores
haplont (plants: gametophyte) and diplont (plants: sporophyte)
sporic life cycle (like in plants), gametic life cycle (like in animals) and sporic (only protists)
sporic: gametophyte dominance (mosses) and sporophyte dominance (ferns and seed plants)

Note that Mendel “saw” genes mixed, segregated and then immediately mixed/recombined again, whereas in the life cycle of unicellular eukaryote, they are segregated, then mixed/recombined and immediately segregated again.

This is because for multicellular organism, diploid condition is better. Since not all genes are strictly dominant, then (1) diploids are broader adapted, due to two variants of gene; (2) if one copy breaks (mutation), the other still works. Diploid condition is also a handy tool to effectively segregate homozygous lethal mutations.

Syngamy is a cheapest and safest way to mix genes within population. After syngamy, the most natural step for unicellular organism is to return DNA amount back to normal, reduce it through the meiosis (of course, genes do not unmix). However, if the organism is multicellular, there is a choice because (a) they already have the developmental program allowing them to exist as stable group of cells and (b) diploid is better. So, while some of multicellular life head to meiosis, zygote of many others proceeds to diplont.

Diplont is a body of diploid cells. It still “keeps in mind” that at some point, meiosis will be required, but this could be postponed for now. Main goal for the diplont is to grow its multicellular body and (if this is reasonable), clone itself with vegetative reproduction.

Then, when time came, meiosis occurs and resulted in 4 cells. They are haploid. Here is the second choice. These new cells could proceed back to syngamy, like in animals and some protists; but in other protists and plants, these cells (now they are called spores) will grow into haplont.

Haplont is a body of haploid cells. Again, it “remembers” that at some point, syngamy will be required, but at the moment, it enjoys multicellular life which could be superficially very similar to diplont.

Finally, some cells of haplont become gametes which go to sexual reproduction, syngamy. Life cycle is now completed.

3.12.2.02.PNG — Figure 3.12.2.2 : Life cycle of multicellular eukaryote.