Mitosis

Purpose of mitosis: Mitosis is the process of cell division, and occurs only in "somatic" or body cells. When haploid sperm meets haploid egg, a chain of events that begins with a single diploid cell and ends with an adult organism made of billions of cells is set in motion. The single cell divides into 2, and each of those 2 divide again, and this process continues geometrically along the following progression: 1, 2, 4, 8, 16, 32, 64, 128, and so on into the billions. Therefore, the first purpose of mitosis is growth. The second function of mitosis is repair. Cells are constantly wearing out and getting damaged and unless an organism replaces them at least as fast as they are lost, a gradual deterioration will occur.

Interphase: Each chromosome undergoes replication, making an identical copy of itself. At this point, the chromosomes are still long and thin, and are not visible inside the nucleus. Cells spend most of their life in this non-reproductive phase.

Prophase: The chromosomes coil and shorten, and become visible. It becomes apparent that the chromosomes have duplicated. Pairs of identical chromosomes remain attached to each other at the centromere and each chromosome is called a chromatid.

Metaphase: Chromosomes line up along the center of the cell. A pair of structures called centrioles form at the poles of the cell, and produce spindle fibers which attach to the centromeres of each chromosome pair.

Anaphase: The paired chromosomes split at the centromere and the two halves migrate along the spindle fibers to opposite sides of the cell. At the same time, the center of the cell begins to pinch.

Telophase: Cell division occurs, and each is identical to the original. Cells return to Interphase and prepare for another round of division.


Meiosis

Purpose of meiosis: Meiosis is a special version of cell division that occurs only in the testes and ovaries; the organs that produce the sperm and eggs. Why is this different? Normal body cells have a complete set of chromosomes. If normal body cells from mom and dad fused to form a baby, the fertilized egg would have twice as many chromosomes as it should. Meiosis is sometimes called "reduction division" because it reduces the number of chromosomes to half the normal number so that when fusion of sperm and egg occurs, baby will have the correct number. Therefore the purpose of meiosis is to produce gametes; the sperm and eggs.

Interphase I: Identical to Interphase in mitosis.

Prophase I: Identical to Prophase in mitosis.

Metaphase I: Instead of all chromosomes pairing up along the midline of the cell as in mitosis, homologous chromosome pairs line up next to each other. This is called synapsis. Homologous chromosomes contain the matching alleles donated from mother and father. This is also when meiotic recombination, also know as "crossing over" (see below) occurs. This process allows for a genetic shuffling of the characteristics of the two parents, creating an almost infinite variety of possible combinations. See the close-up diagram below.

Anaphase I: Instead of chromatids splitting at the centromere, homologous chromosome pairs (now shuffled by crossing over) move along the spindle fibers to opposite poles.

Telophase I: The cell pinches and divides.

Prophase II: It is visibly obvious that replication has not occurred.

Metaphase II: The paired chromosomes line up.

Anaphase II: The chromatids split at the centromere and migrate along the spindle fibers to opposite poles.

Telophase II: The cells pinch in the center and divide again. The final outcome is four cells, each with half of the genetic material found in the original. In the case of males, each cell becomes a sperm. In the case of females, one cell becomes an egg and the other three become polar bodies which are not used.


Meiotic Recombination or Crossing over: