occurs so that unique chromosomes can separate. Meiosis I is when the cell turns from being haploid to being diploid. In prophase I, DNA is exchanged and recombinant chromosomes are made. There are five separate phases to prophase I, including leptotene, zygotene, pachytene, diplotene, and diakinesis. During pachytene is when the actual crossing over takes place through the formation of chiasmata between the chromosomes. In metaphase I, the pairs of chromosomes are arranged in rows along the metaphase plate. The arrangement of chromosomes is random so that their can be genetic variation. There are more than 8 million different combinations that can occur because of the random assortment of the 23 pairs of chromosomes. In anaphase I, the chromosomes separate and in telophase I, the chromosomes become diffuse again. Cytokinesis happens with these cells as well, creating two new cells. Meiosis I is considered a reduction division so that the haploid cell is created. This is followed by meiosis II. Meiosis II separates the chromosome into two chromatids. The process is different in males and females. In males, four spermatozoa are created, while, in females, three polar bodies are formed along with one egg cell so that just one egg cell is made in the process. Meiosis II is similar to mitosis.
APOPTOSIS Apoptosis is also referred to as “programmed cell death”. If cells are not necessary, there is a process that takes place in which the cell commits suicide. Apoptosis is extremely common with billions of cells in the healthy human adult dying every hour, particularly in the bone marrow and the intestinal tract. It occurs in embryos and in fetuses in order to sculpt the features of the embryo. In adults, this apoptosis balances cell division so the size of the organism’s organs stays the same over time. Cells die in necrosis by swelling and bursting, spilling contents throughout the extracellular space. Cells die in apoptosis do this differently. They die neatly, without spilling their contents throughout the environment. This can be called shrinkage and condensation rather than swelling and bursting. The cytoskeleton is allowed to collapse and the nuclear DNA is broken up after the envelope disassembles 197
