MITOSIS, or KARYOKINESIS

    Genuine logistical problems. Why?

 Average human cell is around 30 um in diameter, with a 10 um nucleus.
 Within the nucleus is 10 FEET of DNA, consisting of  46 separate, very thin threads.
 When duplicated, about 20 feet of DNA!

If the cell was 1 foot in diameter, the DNA would be 20 miles long!!!

So, we have a packaging and sorting problem.
   The solution?
 DNA associated with histones as chromatin --   chromosomes can coil up and effectively shorten.
     I.e., DNA divided into units, or chromosomes -- more managable.  

For diagrams of mitotic phases, see Fig. 3.17

Preprophase
 
  Cortical microtubules give way to the PREPROPHASE BAND of microtubules. It’s another microtubule
            ARRAY.
        The PPB encircles the cell and defines the site along the plasma membrane where the new cell wall will fuse
            during cytokinesis. The PPB is ephemeral; it disappears in prophase


Prophase
 Chromatin condenses, so we now see that chromosomes consist of two
    CHROMATIDS.
 The mitotic apparatus, or spindle, begins to form around the nucleus, outside the nuclear envelope.   

At the end of prophase, the nuclear envelope breaks down, exposing chromosomes to microtubules of the mitotic spindle.

Metaphase
 As chromosomes are exposed to spindle microtubules, some attach at a special site on the chromatids called the
       centromere, or KINETOCHORE (3.18).

          (the kinetochore is that part of the centromere where the spindle fibers or microtubles attach)

Up to 200 microtubules in each chromosomal spindle fiber.
  Attachment of microtubules creates spindle fibers, which causes a jockeying for position on the
   METAPHASE PLATE.
 
  Chromosomes oscillate back and forth.
 Spindle is a big biological machine that does work. The work is separation of daughter/sister chromosomes.
        It consists of two HALF spindles, each with a POLE.
   Microtubules emanating from each pole have specific polarity, designated as + and -.
         The polarity is determined by the quaternary protein structure of tubulin and the microtubule.
  Chromatids attached to each other only at the kinetochores, where the spindle fibers are attached.

 Anaphase.
 Cell senses if all chromosomes are attached to the spindle and are aligned on the metaphase plate.
    I.e., another checkpoint.
 If checkpoint is passed, sister chromatids break apart at the centromeres/kinetochores, SIMULTANEOUSLY
 Mass movement of new daughter chromosomes towards the poles then occurs.

 Chromosome arms trail the kinetochores, as if they’re being pulled thru a viscous cytoplasm.
   Like pulling noodles thru chicken soup.
 Chromosomes aggregate at the poles.

Telophase.
 Chromosomes decondense at the poles.
 Nuclear envelope reforms around the chromosomes. 

Summing up: Cell has succeeded in separating daughter chromosomes and creating two daughter nuclei.
Now, it has to put those nuclei in separate cells.
  It has to do CYTOKINESIS.

In late anaphase, early telophase, another microtubule array appears as the spindle disappears.
 It forms between the daughter nuclei.
  It’s called the PHRAGMOPLAST. (3.19)
 It consists of two INTERDIGITATING sets of microtubules.
            The phragmoplast accumulates membranous vesicles, which then FUSE, creating the new plasma
      membrane and cell wall, called collectively the CELL PLATE.
 Fusion continues centrifugally, as new vesicles accumulate on the periphery.

  The extracellular space within the cell plate contains the initial components of the cell wall.
 Original phragmoplast microtubules break down as new ones appear on the periphery of the plate, so plate
    expands outward towards the parental plasma membrane.
 Eventually, plate fuses with parental plasma membrane and divides the cytoplasm into two new cells.
  Plate fuses at the site previously occupied by the preprophase band.

Why do plants do cytokinesis this way?
  Compare: How is it done in animals?
  WHY THE DIFFERENCE?
    Evolutionary perspectives?

======================
MEIOSIS

Let’s take at another kind of division.
 Part and parcel of sexual reproduction.
  MEIOSIS.

Sexual reproduction involves the union of two special cells called
 GAMETES -- Usually called sperm and egg.
   I.e., FERTILIZATION.

What would happen if that’s all there was to sex?
 E.g., let’s take maize, with 20 chromosomes.
  If maize’s sperm and eggs had 20 chromosomes,
   in the next generation how many would it have?
                And in the generation after that?

  So, what is missing in this sexual equation?

In other words, there has to be a mechanism to halve the number of chromosomes in sex cells before
fertilization.
    That process is meiosis.
 >>>Sexual reproduction involves an alternation between fertilization and meiosis.<<<

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?