What is meiosis and why is it important.

Having the correct number of chromosomes in a cell is vital, because chromosomes contain the body's DNA, and DNA is the chemical in which genes are encoded. When a cell is missing a particular chromosome, or has an extra copy, it doesn't have the proper mix of genes. Many cellular errors result, with the details depending upon which chromosome is absent (or present in surplus) and when. An embryo or fetus having the wrong number of chromosomes is the biggest known cause of spontaneous miscarriage in people. Errors in chromosome number are extremely common in cancers, indeed some researchers hypthesize that chromosome number errors may cause cancer.

A normal human cell contains two nearly identical copies of 23 different chromosomes, a total of 46 chromosomes altogether. The karyotype to the right shows the pairs of each of the 22 numbered chromosome pairs plus the X and Y chromosomes. The two members of each pair are called homologues, meaning nearly identical genetically.

adapted from Raven and Johnson 1991

 

Most cell division winds up with the daughter cell possessing the same number of chromosome as its parent, for example, the cell division involved in the growth of an arm or a liver or in the production of brain cells. This type of cell division is called mitosis.

To produce sperm or eggs, however, a different process is followed, called meiosis. In meiosis, the number of chromosomes after cell division is halved. Thus in humans each sperm or egg gets one copy only of each of the 23 different types of chromosomes. Then when sperm and egg combine during fertilization, the resulting embryo once again has the appropriate number of chromosomes, one copy of each type from both parents for a total of 46.

Meiosis. The drawing below shows what happens to chromosomes during meiosis. People inherit a copy of each chromosome from their parent and thus have what are called "homologous pairs" of chromosomes in each cell. Prior to meiosis, each chromosome has made a copy of itself via DNA replication. Thus at the outset of meiosis, each cell begins with two copies of each pair.

In the first phase of meiosis, the chromosomes normally align with their homologous pair within the nucleus in a characteristic pattern, perpendicular to the "microtubule spindle apparatus."

 

The homologous pairs then separate from one another and migrate along the spindle apparatus toward opposite ends of the cell. A cleft then forms, separating the two poles and cell division completes. In this first phase of meiosis, the daughter cells each wind up with two copies of each chromosome. This is because prior to the onset of meiosis, each chromosome was duplicated in a process called DNA replication.

A second phase of meiosis (not shown) repeats this pattern, except it proceeds without the initial DNA replication. The result of that second phase is for the daughter cells to have only one copy of each chromosome.

In women, the process of meiosis takes place in two stages. The cells that will become a woman's eggs actually begin meiosis during fetal life, but the process is arrested during what scientist's call the "first prophase," the left-most cell in the drawing above. Then after puberty, during each menstrual cycle a small number of oocytes resume meiosis. The final stage of meiosis occurs after fertilization by a sperm.

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