Stages of Meiosis

Meiosis Meiosis occurs in both plant and animal cells. It is commonly called reduction division. The process of meiosis reduces the number of chromosomes in gametes to one-half the number contained in the parent cell. Most regular cells typically contain two copies of each chromosome and are described as diploid (2n). Meiosis produces cells with only a single copy of each chromosome and they are described as haploid (n) or monoploid. A plant undergoes alternation of generations in its life cycle as it switches back and forth between diploid and haploid structures. The process of meiosis requires two divisions, called Meiosis I and Meiosis II. Each is subdivided into stages for reference. Stages of Meiosis The order of stages of Meiosis I is Prophase I, Metaphase I, Anaphase I, Telophase I, and Cytokinesis I. The daughter cells are formed and then proceed through a second meiotic division. The names and order of stages of Meiosis II is the same as Meiosis I except that the suffix II is used. The process is similar in plants and animals. Figure 3. Stages of Meiosis I (Animal cells shown) Prophase I During the time before Prophase I, the diploid (2n) cell has already doubled its DNA to become tetraploid (4n). The cell prepares for division by coiling its DNA into chromosomes, pairing homologous chromosomes (matched pairs) to form tetrads, and dissolving the nuclear membrane during Prophase I (as shown in Figure 3). Parts of chromosomes may be exchanged through crossing over. Metaphase I Figure 3 shows the formation of the spindle. It also shows how the tetrads align at the equator of the cell. Each pair of homologous chromosomes attaches to the spindle. Anaphase I Homologous chromosomes separate during Anaphase I, but the sister chromatids stay together as they move toward opposite ends of the cell. Telophase I & Cytokinesis I During Telophase I the chromosomes gather at the poles and uncoil to become the thin, threadlike form called chromatin. The nuclear membrane also reforms. The physical division of the parent cell in Cytokinesis I occurs differently for animals and plants. In animal cells, a cleavage furrow pinches inward near the equator of the cell to form two daughter cells. In plants, a cell plate forms near the center of the cell and spreads outward to separate the two daughter cells. The diploid (2n) daughter cells then begin Prophase II of the second division. 2 Figure 4. Stages of Meiosis II in animal cells Prophase II During Prophase II, the chromosome coil and the nuclear membrane disappears as each daughter cell prepares for division. See Figure 4. Metaphase II The spindle forms and the pairs of sister chromatids align at the equator of the cell. Anaphase II Figure 4 shows sister chromatids separating during Anaphase II. Telophase II & Cytokinesis II The chromatids reach the poles and uncoil into the thin, threadlike chromatin in Telophase II. The nuclear membrane also reforms. In Cytokinesis II each of the two diploid (2n) daughter cells formed in the first division are physically divided into two haploid (n) daughter cells. Summary Let's review the process. The original diploid (2n) cell doubles its DNA to become 4n and divides into two diploid (2n) cells which then each divide into two haploid (n) cells. A male animal produces four haploid sperm from each diploid (2n) primary spermatocyte that enables the male to produce many sperm. This increases the chance of successfully fertilizing a female’s egg. The division of the cell is unequal as a female produces eggs,. Each primary oocyte divides twice to reduce the number of chromosomes from diploid to haploid; however, the unequal division yields only one egg and up to three smaller remnants called polar bodies. The sperm must live only long enough to reach an egg and fertilize it. The egg must have more energy resources than the sperm to support all the vital life processes for the new individual until it can secure nourishment from the mother. The relatively large egg with the additional resources produced by the unequal divisions increases the chances of survival. The process of gamete formation in plants follows a similar pattern as in animals, but the names of the cells are different. 3 Worksheet # 1 – Stages of Meiosis Label each of the stages of Meiosis I & II in the diagram below. 4