Meiosis

Meiosis, process of cell division in which the cell’s genetic information, contained in chromosomes, is mixed and divided into sex cells with half the normal number of chromosomes. The sex cells can later combine to form offspring with the full number of chromosomes. The random sorting of chromosomes during meiosis assures that each new sex cell, and therefore each new offspring, has a unique genetic inheritance.

Meiosis differs from normal cell division, or mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell.

To illustrate the steps of meiosis, consider a corn plant cell with 10 pairs of chromosomes. The normal number of chromosomes, or diploid number, for corn is 20. In order for the diploid corn cell to reproduce, it must undergo meiosis to produce cells with half the normal number of chromosomes, called the haploid number. Each haploid corn cell contains only 10 chromosomes.

Prior to meiosis, the corn cell undergoes interphase, in which it synthesizes materials needed for cell growth and prepares for cell division. During this stage the cell’s genetic information, in the form of deoxyribonucleic acid (DNA), is replicated. Each of the two consecutive cell divisions consists of four stages: prophase, metaphase, anaphase, and telophase.

In prophase I each long DNA strand wraps around proteins that in turn coil and condense to form a chromosome. Since the DNA was copied during interphase, each chromosome condenses to form two identical chromatids, joined at a centromere. A corn cell has 20 chromosomes at this stage, each with two identical chromatids, making a total of 40 chromatids.

Chromosomes exist in pairs; one is inherited from the mother (maternal) and one from the father (paternal). When the chromosomes duplicate, two maternal and two paternal chromatids are produced. These two pairs of chromatids gather together in groups of four called tetrads. Each corn cell contains 10 tetrads. While grouped together in tetrads, sections of the chromatids from the maternal pair may randomly exchange, or cross over, with sections of the paternal chromatid pair. Called genetic recombination, this process is the first of two ways that meiosis mixes genetic information during sexual reproduction.

Also in prophase I, two structures called centrioles, both located on one side of the nucleus, separate and move toward opposite sides of the cell. As the centrioles move apart, they radiate thin hollow structures called spindle fibers. The membrane around the nucleus of the cell breaks down, marking the beginning of the next stage.

During metaphase I, the spindle fibers attach to the chromatids near the centrioles. The spindle fibers move the tetrads so that they line up in a plane halfway between two centrioles.

Anaphase I begins when the spindle fibers pull the tetrads apart, pulling the maternal and paternal chromosomes toward opposite sides of the cell. The first meiotic division concludes with telophase I, when the two new groups of chromosomes reach opposite sides of the cell. A nuclear membrane may form around the two new groups of chromosomes and a division of cell cytoplasm forms two new daughter cells.

Each daughter corn cell receives 10 chromosomes made up of a random mixture of maternal and paternal chromosomes. This second mixing of genetic information is called independent assortment. Genetic recombination and independent assortment make it possible for parents to have many offspring who are all different from each other.

In the second meiotic division the cell moves directly into prophase II, skipping the interphase replication of DNA. Each corn cell begins the second division with 10 chromosomes. Once again the centrioles radiate spindle fibers as they move to opposite sides of the cell. During metaphase II, the chromosomes line up along the plane in the center of the cell, and in anaphase II the pairs of chromatids are pulled apart, each moving toward opposite ends of the cell.

Telophase II completes meiosis. The spindle fibers disappear and a new nuclear membrane forms around each new group of chromosomes to form four haploid cells. The original diploid corn cell with 20 chromosomes has undergone meiosis to form four haploid daughter cells, each containing 10 chromatids. It is now possible for two haploid sex cells to join during fertilization to form one egg cell with the normal diploid number of chromatids. After fusion and DNA replication, two haploid corn cells will yield one diploid egg cell with 10 pairs of chromosomes.

In humans meiosis occurs only in the reproductive organs, the testes in males and the ovaries in females. In males, each of the meiotic divisions result in four equally sized haploid cells that mature into functional sperm cells. In females, the meiotic divisions are uneven, resulting in three tiny cells called polar bodies and one large egg that can be fertilized.