Essay on meiosis
... in the four daughter cells has only one chromatid. In the animal life cycle, the daughter cells mature into either sperm or egg cells. Common steps between the two phases of meiosis I and meiosis II includes prophase, prometaphase, metaphase, anaphase, and telophase. In prophase I of meiosis I spindles form as the centrosomes migrate away from one another, the nuclear envelope and nucleolus disappear, and chromatin condense. The homologous chromosomes undergo synapsis and crossing-over may occur between the chromatids. The microtubals connect to the kinetochores in the centromeres of the homologous chromosomes and pull them to the metaphase plate. In metaphase I the homologous pairs align at the metaphase plate. During anaphase I the homologous chromosomes separate and move toward the poles. During telophase, I the nuclear envelop re-forms and nucleoli appear and the haploid number is equal to one. There is a possible interpahse II or cytokinesis that could occur before moving on to meiosis II. The next step is meiosis II that begins with two cells that have one chromosome from each homologous pair and goes into metaphase II where the chromosomes align at the metaphase plate, but are not aligned. During anaphase II the sister chromatids separate, becoming daughter chromosomes that have only one chromatid leading into telophase II where the spindles disappear, nuclei form, and cytokinesis take place to form four haploid cell, and the chromosomes are no longer duplicated and have half the number of original chromosomes. Following meiosis II, the haploid cells, half the number of chromosomes in a parent cell, mature and become gametes in animals. In plants, they become spores, which also divide and grow, through mitosis, into haploid organisms. These haploid cells produce gametes that fuse to become a diploid zygote that develops into a mulitcellular offspring. In males, meiosis is a part of spermatogenesis, which produces sperm formed in the testes. In females, meiosis is a part of oogenesis, which produces eggs formed in the ovaries. In the ovaries of females the primary oocyte divides meiotically into two cells, on is called the secondary oocyte and the other is a polar body. The secondary oocyte undergoes meiosis II but stops at metaphase II, where it can then be met by a sperm and continues with meiosis. Genetic variation is a result of meiosis, which ensures that the offspring...