Chapter 12.  Meiosis and the Alternation of Generations

• Life perpetuates itself by reproduction, asexual or sexual.
• Asexual reproduction (in the broad sense), some examples:
• Single cells of a multicellular organisms divide to produce identical daughter cells.
• Cells of  a unicellular organism divide to produce identical daughter cells.
• Plants produce daughter plants from runners (e.g. strawberries), root sprouts (e.g. aspens), leaf  "plantlets" (e.g., Kalanchoe), etc.
• Plants produce seeds without benefit of fertilization (e.g. dandelions).
• In all of these examples, offspring are genetically identical, make up a clone.
• Sexual reproduction:
• Requires union of gametes (e.g. eggs and sperms).
• Offspring are genetically variable.
• Asexual vs. sexual reproduction: costs and benefits:
• Asexual reproduction: energetically cheap, only one parent required, offspring almost guaranteed success, more offspring produced.
• Sexual reproduction: energetically expensive, two parents required, since offspring are variable not all will succeed,  fewer offspring produced, but offspring are variable (and this is the only advantage, especially in an unpredictable environment).
• Mitosis and meiosis are mechanisms of nuclear division.
• Mitosis (Fig. 12.3)
• Normal mode of nuclear/cell division during growth and development of vegetative tissues in multicellular plant.
• Number of chromosomes remains the same,
• Daughter cells (actually their nuclei) are identical to each other and to their parent cell..
• Meiosis (Fig. 12.4)
• Occurs only in reproductive tissues (e.g. flowers in angiosperms).
• Number of chromosomes divided in half.
• Daughter cells (actually their nuclei) differ genetically from each other and from their parent cell.  Pay attention to the beginning and end of the process in Fig. 12.4.
• Alternation of generations occurs between diploid and haploid generations.  Diploid plants (sporophytes) produce haploid offspring (gametophytes) and haploid plants (gametophytes) produce diploid offspring (sporophytes).
• Diploid (2n) generation: all cells of plant contain a double set of chromosomes.  The plants are sporophytes and reproduce by spores which result from meiosis.
• Haploid (n) generation: all cells of plant contain a single set of chromosomes.  The plants are gametophytes and reproduce by gametes which result from mitosis.
• The angiosperm life cycle illustrates alternation of generations (Fig. 12.5).
• Superplant life cycle (Fig 12.6) is a way of showing all of the possible steps:
• Diploid plants produce sporangia, within which meiosis produces haploid spores.
• The haploid spores develop into haploid gametophytes which (depending on the plant group) can range in size and complexity from a few cells to a completely independent plant.
• The gametophytes produce gametangia (simple or complex) within which mitosis produces gametes.
• Gametes unite (e.g. when a sperm fertilizes an egg) to produce a zygote.
• Zygote develops into sporophyte.  This completes the life cycle.
• Sporic life cycles: sporophytes and gametophytes are multicellular.   Example: all plants in the narrow sense (excluding algae and fungi).
• Zygotic life cycles: no multicellular diploid generation.  Zygote is the only diploid cell. Example: unicellular algae like Chlamydomonas, Fig. 12.7.
• Gametic life cycle: no multicellular gametophyte generation.   Gametes are the only haploid cells. Example: multicellular seaweeds like Fucus (Fig. 12.8).
• Evolutionary trends in relative development  of diploid and haploid generations.
• Sporophyte generation becomes larger, more complex. compared to gametophyte (Fig. 12.9)
• Possible explanation: recessive and deleterious genes always expressed in haploid gametophytes.  Protection, shortening of gametophyte generation reduces exposure of these genes.

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To skip in this chapter:  The individual stages of mitosis/meiosis (don't worry about what happens in each stage, but you should be clear about the outcomes of mitosis and meiosis with respect to number of chromosomes and genetic variability)