When this happens near land, we see the earth above the two meeting plates rise, forming spectacular mountain ranges. A parallel oceanic trench typically forms just off the shore, as one plate descends deep into the Earthâs core. These convergent boundaries commonly cause volcanoes to form, as old crust melts.
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells â sperm in males, eggs in females. 1  During meiosis one cell divides twice to form four daughter cells. 2  These four daughter cells only have half the number of chromosomes of the parent cell â they are haploid. 3  Meiosis produces our sex cells or gametes (eggs in females and sperm in males).
The cells that form human eggs begin meiosis while the mother is herself still an embryo in the womb. But meiosis stops in the middle of prophase I, before birth, and does not resume until after the egg is released from the ovaries from 13 to 55 years later. Nondisjunction of chromosome 21 may be the result of interrupting the complicated process of meiosis for so long time.