Apoptosis

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

itself. Ultimately, the cell is marked for phagocytosis by the immune system. The organic compounds get recycled.

In apoptosis, there are proteases that have a cysteine moiety on their active site. This makes these proteins called caspases. There is a proteolytic cascade that takes place that cleave proteins within the cell. Lamins, which are part of the nuclear cytoskeleton, get cleaved so that the nuclear envelope breaks down. There are also DNA degrading enzymes called DNases that cut up the DNA in the nucleus. As you can see, there is a neat and orderly breakdown of the cell. The process is an all-or-none process.

There are procaspases in every cell that can get activated to make caspases as part of the caspase cascade. When cells are under stress or damaged, they can commit suicide by triggering this cascade through signals from within the cell. DNA damage itself can trigger apoptosis through the activity of the p53 gene. There are other activators and inhibitors of the caspase cascade.

Apoptosis can get rid of infected or cancerous cells. Their destruction makes the cell unable to do damage to the body, reducing the threat to the organism. DNA damage beyond repair triggers apoptosis from within the cell. Cells that avoid apoptosis when it is necessary can become cancerous cells. Immune cells have specific molecules on their surface that, if not recognized as self-cells, cause the apoptosis of the immune cell.