Mitosis and Meiosis: A Detailed Comparison
Mitosis and Meiosis: A Comparison
In both mitosis and meiosis, chromosomes are duplicated only once, in the preceding interphase. Mitosis involves one division of the nucleus, accompanied by cytokinesis, producing two diploid cells. Meiosis involves two nuclear and cytoplasmic divisions, producing four haploid cells. All events specific to meiosis occur during meiosis I. Homologous chromosomes pair up to form duplicated tetrads, and crossing-over occurs. In metaphase I, the tetrads are aligned on the equatorial plane, so that in anaphase I, the tetrads separate, and each counterpart goes to the cell poles. At the end of meiosis I, there are two cells that have non-separated chromatids. Meiosis II is very similar to mitosis, and sister chromatids separate. But unlike mitosis, in meiosis II, there are four haploid daughter cells.
Both mitosis and meiosis involve chromosome duplication only once, in the preceding interphase. Mitosis includes one nuclear division accompanied by cytokinesis, resulting in two diploid cells. Meiosis includes two nuclear and cytoplasmic divisions, resulting in four haploid cells. All events unique to meiosis happen during meiosis I: homologous chromosomes pair to form duplicated tetrads, and crossing-over occurs. In metaphase I, tetrads align on the equatorial plane. In anaphase I, tetrads separate, with each counterpart moving to opposite cell poles. Meiosis I concludes with two cells containing non-separated chromatids. Meiosis II closely resembles mitosis, with sister chromatids separating. However, unlike mitosis, meiosis II results in four haploid daughter cells.
Division I
Prophase I: Most characteristic events of meiosis happen at this stage. The nuclear envelope is preserved until the end of the phase, when it disintegrates, while the nucleolus disappears and the spindle forms.
- Leptotene: Chromosomes appear as long filaments. Each chromosome consists of two chromatids, but they are not distinctly visible.
- Zygotene: Homologous chromosomes pair point-by-point along their entire length.
- Pachytene: Pairs of homologous chromosomes are tightly linked, forming bivalents.
- Diplotene: Bivalents begin to separate but remain joined at points where crossing-over occurred. These junctions are called chiasmata and reveal the locations of crossing-over.
- Diakinesis: Chromatids are highly condensed, preparing for metaphase.
At the end of prophase, the nuclear envelope has completely disappeared, and the spindle has fully formed.
Metaphase I: Bivalents are arranged on the equator of the cell.
Anaphase I: Chromosomes have only one centromere for the two chromatids. Because of this, metaphase chromosomes with two chromatids are distributed to opposite poles. 2n chromatids are not separated, but rather n double chromosomes.
Telophase I: Gives rise to two daughter cells, whose nuclei have n chromosomes, each with two chromatids.
Interphase: Can be variable in duration, it can even be missing altogether, so that after telophase I, the second division starts without interruption. There is never DNA synthesis; that is, there is no S period in this interphase.