Mitosis
Mitosis is the process of cell division; as a result of it, two identical cells emerge. The cells of all types, except for sex cells, multiply via mitosis. Mitosis occurs in several phases (Dawson, 2002; The University of Arizona Biology Project, 2004):
- Interphase is the phase of preparation; during it, a cell grows and produces proteins and organelles; chromosomes are duplicated.
- Prophase is the stage at which the nucleoli in the nucleus vanish, and chromatin in the nucleus condenses into chromosomes; also, centrioles move to the opposite ends of the cell.
- During prometaphase, the nuclear membrane disappears, and the chromosomes also start their motion.
- At metaphase, chromosomes are lined up along the middle of the nucleus of the cell, and their centromeres are also aligned about each other. This occurs to ensure that each daughter cell will obtain one copy of every chromosome in the end.
- During anaphase, the paired chromosomes are divided and transported along the microtubes to the opposite parts of the cell.
- At telophase, daughter nuclei (which are identical) are formed in the opposite ends of the cell; the spindle fibers disappear.
- Finally, cytokinesis starts, i.e. the cell begins to divide into two identical daughter cells.
Meiosis
Meiosis is the division of cells that results in the production of four sex cells each of which contains half the number of chromosomes of the parent cell (Harigaya & Yamamoto, 2007). It occurs in several phases (Bailey, n.d.):
- During interphase, the cell grows and prepares for division; additional DNA is synthesized, chromosomes are duplicated, but remain in the form of chromatin.
- Then, prophase I, the longest phase, begins. Homologous chromosomes pair, forming tetrads, and exchange DNA (chromosomal crossover). Thus, new combinations of DNA are created. Centrioles move to opposite parts of the cell; so do chromosomes after the nuclear envelope and nucleoli disappear.
- During metaphase I, tetrads line up at the metaphase plate.
- At anaphase I, chromosomes migrate to the opposite ends of the cell; sister chromatids, however, stay together (which does not happen in mitosis).
- During telophase I, homologous chromosomes finish moving to the opposite parts of the cell, so that each part has a haploid quantity of chromosomes (i.e., contains only one full set of chromosomes). Cytokinesis occurs here, resulting in two daughter cells, each of which contains half the quantity of chromosomes of the parent cell. After that, the cell prepares for the next part of meiosis, but the genetic material never replicates further.
- Next, during prophase II, nuclei break up, and chromosomes start moving to the metaphase II plate (without further replication).
- At metaphase II, chromosomes align along the metaphase II plate located in the middle of the cell.
- At anaphase II, sister chromatids are separated and transported to the opposite ends of the cell (since this occurs, they are regarded as full chromosomes); the cell is also elongated, and the poles move in the opposite directions. At the end of this phase, there is a full set of chromosomes in each pole of the cell.
- During telophase II, separate nuclei are formed in each of the poles; the cytoplasm is divided, and two separate cells are produced.
As a result, four sex cells, each with half the number of chromosomes of the initial cell, are produced.
Genetic Variation and Mendel’s Law of Independent Assortment
According to Mendel’s law of independent assortment, alleles for different characteristics are passed independently from each other (unless they are located in the same chromosome). This occurs because chromosomes containing various alleles “separate and independently sort” (Callihan, 2005, p. 88) during the chromosomal crossover at prophase I of meiosis; as a result, sex cells contain different sets of chromosomes, and, because of this, genetic variation occurs.
On the other hand, during mitosis, no chromosomal crossover takes place; each daughter cell contains the same DNA as the mother cell. Therefore, no genetic variation happens as a result of mitosis.
References
Bailey, R. (n.d.). Stages of meiosis.
Callihan, L. A. (2005). CLEP biology. Piscataway, NJ: Research & Education Association.
Dawson, M. (2002). Stages of mitosis. Web.
Harigaya, Y., & Yamamoto, M. (2007). Molecular mechanisms underlying the mitosis-meiosis decision. Chromosome Research, 15(5), 523-537.
The University of Arizona Biology Project. (2004). The cell cycle & mitosis tutorial: Mitosis.