MEIOSIS, continued...

As we’ll see, meiosis does something else that gets at the heart of why sex is such a big hit, why it’s so popular from a biological standpoint. Why it was so crucial in the success of eukaryotes.
 It produces lots of GENETIC DIVERSITY.
  It shuffles genomes, providing new combinations of genes.
   Why is that so important?

So how does meiosis work?

 It consists of two different kinds of divisions.
    Meiosis I and II.
 The first division actually splits the chromosome pairs.
 The second division is just a kind of mitotic division, but now
  only one set of chromosomes is undergoing mitosis.

In prophase of meiosis I, chromosomes begin to coil up and shorten, as in  mitosis.
But they associate in HOMOLOGOUS pairs.  That is, e.g., both chromosome 2's pair up, both chromosome 17's, etc.
 Since each chromosome has replicated, there are FOUR chromatids in each pair.

By Metaphase I, the pairs line up on the metaphase plate.
 Unlike mitosis, PAIRS line up side by side on the plate. (Fig. 12.2)
 
  However, they do NOT line up as maternal and paternal sets,
    each set on one side of the plate.
  Instead, maternal and paternal chromosomes are SCRAMBLED.
   On either side of the metaphase plate, there is a mix
           of maternal and paternal chromosomes.
                        This is IMPORTANT!  See below....

In Anaphase I, the pairs separate towards opposite poles.
 When daughter nuclei reform, each nucleus now contains
  One set of chromosomes, but from mixed maternal and paternal  origin.
   (Note: this happens each generation!!)

At some point, often very quickly thereafter, another division ensues.
 Chomosomes condense, and eventually line up single file on
   metaphase plate, a la mitosis.
  Sister chromatid kinetochores now face in opposite directions,
    as in mitosis.

 By telophase, nuclei reform, and each one has one set of new chromosomes and becomes a sperm or an egg, depending on the
  organ in which meiosis is occurring.
   E.g. pollen vs ovary.

That’s how we reduce the chromosome number by half.

What about genetic diversity?
 Remember, maternal and paternal sets are shuffled in metaphase.
  Many possible combinations of chromosomes can result in each sperm or egg?

Another souce of diversity: genetic recombination, or
           CROSSING OVER.
  During prophase I, when homologs pair, adjacent chromatids
   exchange segments.  (Figs. 12.2, 12.3, 12.5).
 

ALTERNATION OF GENERATIONS.
            Figs. 12.1, 12.6

Mieosis is also important viz. when it occurs during an organism's life cycle.

Two distinct phases or generations in life cycle of a plant
     One haploid (N), called the gametophyte (gamete bearing plant)
     The other, diploid (2N), called the sporophyte (spore bearing plant)
 
In algae, the group from which land plants emerged, haploid generation is dominant.
Diploid phase very brief and reduced; just fertilized egg or ZYGOTE, which then undergoes meiosis to produce 4 haploid spores.
  I.e., meiosis is said to be ZYGOTIC.

Where do algae live, and does this have anything to do with this????
    In most land plants, diploid generation is EXTENDED, and haploid generation is minimized.
 
Over the course of plant evolution, the diploid generation, or SPOROPHYTE, became dominant and the haploid generation very reduced.
    In mosses, haploid generation, or GAMETOPHYTE, is still dominant, but mosses show limited adaptations to terrestrial conditions. The pin cushiony thing you identify as a moss is haploid. The diploid part of the life cycle, after fertilization, is just a small brown stem like thing growing out of the pincushion.
       In ferns, diploid phase is now dominant (what we commonly see as a fern), while the haploid gametophyte generation consists of a small green 'thallus' on the forest floor. The diploid sporophyte produces haploid SPORES via meiosis, which germinate to produce the green thallus.
          Meiosis is said to be SPORIC in this case.
  Gametophyte in turn produces sperm and eggs via mitosis. When sperm and egg meet via fertilization, they produce a diploid zygote which grows into the fern, or new sporophyte generation.  Got it??
In flowering plants, haploid generation is so reduced it consists of just a few cells, e.g. a pollen grain. Egg is housed in another structure, consisting of just 7 cells. In other words, two different gametophytes: male, or pollen; and female.
      
Just what you would predict given the benefits of diploidy
 for terrestrial plants. WHY? Hint: think aquatic vs. terrestrial environment