MITOSIS
"Prophase
Prophase: The two round objects above the nucleus are the centrosomes. Note the condensed chromatin.Main article: Prophase
Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome. Since the genetic material has already been duplicated earlier in S phase, the chromosomes have two sister chromatids, bound together at the centromere by the protein cohesin. Chromosomes are visible at high magnification through a light microscope.
Just outside the nucleus are two centrosomes. Each centrosome, which was replicated earlier independent of mitosis, acts as a coordinating center for the cell's microtubules. The two centrosomes sprout microtubules (which may be thought of as cellular ropes or poles) by polymerizing free-floating tubulin protein. By repulsive interaction of these microtubules with each other, the centrosomes push themselves to opposite ends of the cell (although new research has shown that there might be a mechanism inside the centromeres that also grab the microtubules and pull the chromatids apart). The network of microtubules is the beginning of the mitotic spindle.
Some centrosomes contain a pair of centrioles that may help organize microtubule assembly, but they are not essential to formation of the mitotic spindle. Plant cells that lack centrioles have no trouble undergoing mitosis.
Metaphase
Metaphase: The chromosomes have aligned at the metaphase plate.Main article: Metaphase
As microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes convene themselves on the metaphase plate or equatorial plane, an imaginary line that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug of war between equally strong people. In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline.
Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules, it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned. The signal creates the mitotic spindle checkpoint.
Anaphase
Early anaphase: Kinetochore microtubules shorten.Main article: Anaphase
When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase. Two events occur in order:The proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids turned sister chromosomes are pulled apart by shortening kinetochore microtubules and toward the respective centrosomes to which they are attached. The nonkinetochore microtubules elongate, pushing the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell.
These two stages are sometimes called early and late anaphase. At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations."
MEIOSIS
"Prophase I
In the prophase stage, the cell's genetic material, which is normally in a loosely arranged pile known as chromatin, condenses into visible threadlike structures. Along the thread, centromeres are visible as small beads of tightly coiled chromatin. Recall that centromeres are connection sites between sister chromatids, which are not yet distinguishable. As the chromatin becomes progressively ordered and visible, homologous chromosomes find each other and bind together. In this process, called synapsis, a protein structure called the synaptonemal complex attaches the homologous chromosomes tightly together all along their lengths.
The zygotene stage sees the completion of synapsis. The combined homologous chromosomes are said to be bivalent, a reference to the two homologous chromosomes. They may also be referred to as a tetrad, a reference to the four sister chromatids. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear.
The pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
During the diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. They are held together by virtue of recombination nodules, betraying the sites of previous crossing over, the chiasmata.
Chromosomes recondense during the diakinesis stage. Sites of crossing over entangle together, effectively overlapping, making chismata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles.
During these stages, centrioles are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are two kinetochores on each tetrad, one for each centrosome. Prophase I is the longest phase in meiosis.
Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole. These are called nonkinetochore microtubules.
Metaphase I
As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align equidistant above and below an imaginary equatorial plane, due to continuous counterbalancing forces exerted by the two kinetochores of the bivalent. Because of independent assortment, the orientation of the bivalent along the plane is random. Maternal or paternal homologues may point to either pole.
Anaphase I
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. Nonkinetochore microtubules lengthen, pushing the centrioles further apart. The cell elongates in preparation for division down the middle."
http://en.wikipedia.org/wiki/Meiosis#Meiosis_I
http://en.wikipedia.org/wiki/Mitosis#Metaphase
