The Cell Cycle - Meiosis

In meiosis, the process is quite similar to mitosis In meiosis, the process is quite similar to mitosis. However, another cell division takes place in which there is no extra DNA replication step. Instead of having a pair of genes (as in a diploid cell), there is only one copy of each gene (a haploid cell). This one copy of genetic information produces gametes of either sperm or eggs. Thus, only one copy of a gene is passed on to each gamete. It is not until the sperm and egg join that there will be two halves of genetic information.

Development Many eukaryotic organisms do not exist as one-celled bodies, and generally, the cells multiply and develop into a more complex organism. This development comes about by way of two processes -- growth and differentiation. During the growth process, cells reproduce and grow more cells through mitosis. This process occurs daily, and even many times a day as cells continually need to be replenished as others die.

As cells form more complex organisms, they must produce different types of cells that result in a functioning integrated organism. A group of cells of the same type form a larger structure called a tissue (e.g. -- skin, muscle). Multiple tissues can form an organ (e.g. - eye, kidney). This variation among cell groups is known as differentiation. Cells that are capable of differentiating into many different cell types are called totipotent.

Meiosis A specialized type of cell division that occurs during the formation of gametes. Although meiosis may seem much more complicated than mitosis, it is really just two cell divisions in sequence. Each of these sequences maintains strong similarities to mitosis.

Meiosis I refers to the first of the two divisions and is often called the reduction division. This is because it is here that the chromosome complement is reduced from diploid (two copies) to haploid (one copy). Interphase in meiosis is identical to interphase in mitosis. At this stage, there is no way to determine what type of division the cell will undergo when it divides. Meiotic division will only occur in cells associated with male or female sex organs.

Prophase I is virtually identical to prophase in mitosis, involving the appearance of the chromosomes, the development of the spindle apparatus, and the breakdown of the nuclear membrane. Metaphase I is where the critical difference occurs between meiosis and mitosis. In mitosis, all of the chromosomes line up on the metaphase plate in no particular order. In Metaphase I, the chromosome pairs are aligned on either side of the metaphase plate. It is during this alignment that the chromatid arms may overlap and temporarily fuse, resulting in what is called crossovers.

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During Anaphase I, the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell. In Telophase I, a cleavage furrow typically forms, followed by cytokinesis, the changes that occur in the cytoplasm of a cell during nuclear division; but the nuclear membrane is usually not reformed, and the chromosomes do not disappear. At the end of Telophase I, each daughter cell has a single set of chromosomes, half the total number in the original cell, that is, while the original cell was diploid; the daughter cells are now haploid.

Meiosis II is quite simply a mitotic division of each of the haploid cells produced in Meiosis I. There is no Interphase between Meiosis I and Meiosis II, and the latter begins with Prophase II. At this stage, a new set of spindle fibers forms and the chromosomes begin to move toward the equator of the cell. During Metaphase II, all of the chromosomes in the two cells align with the metaphase plate.

In Anaphase II, the centromeres split, and the spindle fibers shorten, drawing the chromosomes toward each pole of the cell. In Telophase II, a cleavage furrow develops, followed by cytokinesis and the formation of the nuclear membrane The chromosomes begin to fade and are replaced by the granular chromatin, a characteristic of interphase. When Meiosis II is complete, there will be a total of four daughter cells, each with half the total number of chromosomes as the original cell.

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In the case of male structures, all four cells will eventually develop into sperm cells. In the case of the female life cycles in higher organisms, three of the cells will typically abort, leaving a single cell to develop into an egg cell, which is much larger than a sperm cell.

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Genetic Diversity Independent Assortment The orientation of homologous chromosomes on one side of the metaphase plate or the other in Meiosis I is random. The number of possible orientations is 2n, where n is the haploid number. For humans, the number is 223 = 8.4 million

Random fertilization Any of a male’s 8.4 million sperm can fertilize any of a woman’s 8.4 million eggs. The total number of combinations is over 70 trillion. Crossing over When crossing over is considered, the number of combinations is nearly infinite.

Genetic diversity contributes to evolutionary change Genetic diversity contributes to evolutionary change. If an offspring inherits a combination of genes that gives it a survival advantage, it is better able to survive and pass on its genes. This means the chance that the combination is passed on increases. As a result, there is an accumulation of favorable characteristics.

Nondisjunction Disorders If two homologous chromosomes move toward the same pole, one daughter cell will have an extra chromosome while the other will be missing one. After fertilization, a zygote could have three copies of a chromosome (trisomy) or only one (monosomy), rather than the normal two copies. Each cell of the organism will then have the abnormal chromosome number.

Down syndrome Trisomy of chromosome 21 Symptoms (1) Round, full face; enlarged tongue; short height; large forehead (2) Low mental ability (3) Heart defects (4) Prone to Alzheimer’s and leukemia (5) Mostly sterile Frequency is 1 in 2500 for females under 30 years old. Frequency increases with mothers age to 1in 100 for females over 30.

Turner’s syndrome  Nondisjunction of the sex chromosome Females have only one X chromosome Symptoms (1) Fail to develop sexually; usually sterile (2) Short height; thick, wide neck (3) Normal intelligence (4) Frequency is 1 in 5000

Klinefelter syndrome Nondisjunction during the production of the sperm or egg Individual has XXy sex chromosomes Appears male at birth because of the y chromosome  Testes fail to develop and sterility results. The two X chromosomes trigger the development of breasts and other female characteristics Frequency is between 1 in 1000 and 1 in 2000 births