Mitosis and Meiosis
Mitosis and meiosis are both forms of cell division, however, their processes are not identical. Mitosis happens in somatic cells (cells that are not gametes), and it produces two genetically identical daughter cells. Mitosis happens to replace dead or damaged cells or during growth. Mitosis consists of 4 phases: prophase, metaphase, anaphase, and telophase. DNA replication happens during S phase which is before the mitotic phase. During prophase, the chromatin condense into chromosomes, and the mitotic spindle forms (textbook, 94). During metaphase, the chromosomes line up along the middle of the cell, known as the metaphase plate. During anaphase, the chromatid pairs separate, and each half of the pair is called a chromosome. During telophase, the last stage of mitosis, the identical sets of chromosomes are at opposite poles of the cell, the chromosomes revert back to chromatin, and a nuclear envelope reforms around each group of chromatin. The division of the cytoplasm after nuclear division is called cytokinesis (textbook, 95).
Meiosis, in contrast, occurs in gametes (sex cells), produces four genetically different daughter cells, and involves two divisions of genetic material. Meiosis is broken up into into two parts: meiosis I and meiosis II which both consist of four parts. Meiosis I occurs first after DNA replication and consists of prophase I, metaphase I, anaphase I, and telophase I. Prophase I, much like prophase in mitosis, is when the chromosomes shorten and the mitotic spindle forms. However, unlike prophase in mitosis, the two genetically identical sister chromatids pair off, and crossing over occurs between non sister chromatids. Crossing over is the very important reason that no two people have the same DNA. Metaphase I is very similar to metaphase in mitosis because the homologous pairs of chromosomes line up along the metaphase plate. In anaphase I, the homologous pairs of chromosomes are separated and pulled to opposite sides of the cell leaving the paired chromatids still together (textbook, 97).
Meiosis I results in two genetically different haploid cells after telophase I and cytokinesis which are similar to telophase and cytokinesis of mitosis. Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II. Prophase II is similar to prophase in mitosis, and no crossing over occurs during prophase II. Metaphase II is when the sister chromatids line up along the metaphase plate. During anaphase II, the sister chromatids separate and move to opposite poles of the cell. The final step of meiosis is telophase II and cytokinesis where the nuclear envelopes reform and four, genetically different haploid gametes are formed as a result of the original single diploid cell (textbook, 97).