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Mitosis and meiosis are types of cell division. Mitosis produces genetically identical cells for asexual reproduction, growth, and repair, while meiosis produces cells for sexual reproduction. Both have stages, with mitosis having four and meiosis having eight. Interphase occurs before the stages of division. During prophase, chromosomes condense, centrioles form, and the nuclear envelope breaks down. During metaphase, chromosomes line up along the center of the cell. Anaphase and telophase follow. Meiosis I differs from mitosis during prophase I when bivalents form. Chromosomes are pulled to opposite ends of the cell during anaphase I, and a nuclear membrane forms around them in telophase I. Meiosis II follows the same process as mitosis. The end result of meiosis is four new cells with half the genetic information of the parent cell.
Metaphase is one of the phases of mitosis and meiosis, which are the two types of cell division. During mitosis, cells genetically identical to the parent, or clones, are produced. It is used for asexual reproduction, growth of multicellular organisms, and to repair and replace damaged tissue. Meiosis is the cell division used to produce cells for sexual reproduction. Mitosis occurs in all cells, whereas meiosis occurs only in the sex organs of an organism, such as the testicles and ovaries of mammals or the ovaries and anthers of flowering plants.
Both mitosis and meiosis are continuous processes, but are each described as a series of stages. During mitosis, there are four phases: prophase, metaphase, anaphase and telophase, which occur in this order. Meiosis has two divisions, meiosis I and meiosis II, each made up of the same four stages as mitosis. There is one more stage for both processes called an interphase. Interphase occurs before the stages of division and is when cells grow and prepare to divide by replicating their DNA.
All cells have a cell cycle that begins when they were produced through cell division and ends when they divide to produce identical cells. Mitosis is the period of cell division and the rest of the cell cycle is interphase. Interphase is commonly called the resting phase, but it is a time of great cellular activity. During this stage, the cell grows and produces organelles and proteins. The DNA in the nucleus is replicated in preparation for mitosis and continues to grow and produce duplicate organelles.
During prophase, the chromosomes in the nucleus shorten and thicken, condense, and become visible. Each chromosome appears to have two chromatids joined by a centromere. Centrioles form and move to opposite ends of cells where microtubules develop to form a star-shaped structure called an aster. Some of the microtubules, or spindle fibers, cross the cell from end to end to form the spindle. Finally, the nucleolus and nuclear envelope break down so that the chromosomes are free to float in the cytoplasm.
The next stage of division after prophase is metaphase. During this stage, chromosomes line up along the center of the cell. Each of the chromosomes is attached to a spindle fiber at their centromere. The chromatids are then pushed apart slightly due to contraction of the microtubules. Anaphase and then telophase follow the metaphase.
During anaphase, the spindle fibers are fully contracted so that the separate chromatids of each chromosome are pulled to either side of the cell. Once the chromatids reach the poles of the cell, a new nuclear membrane forms around them, indicating the onset of telophase. The spindle fibers break, the chromosomes uncoil and lengthen, the nucleolus reforms, and finally, the cell splits in two, terminating the mitotic division.
Meiosis is similar to mitosis, but two divisions occur. It involves the division of chromosomes followed by two divisions of the nucleus and the cell. Meiosis I differs from mitosis during prophase, but meiosis II is a typical mitotic division, as described above. The end result of meiosis is four new cells that have half the genetic information of the parent cell.
The key difference in meiosis I occurs during prophase I when pairs of chromosomes join together to form a bivalent instead of each chromosome forming a chromatid. During metaphase I, the bivalents randomly line up along the center of the cell to be separated. This random orientation leads to greater genetic variety. Each chromosome in the pair has genes that determine the same characteristics, but they are not always the same gene. The random distribution and subsequent independent assortment of chromosomes creates new genetic combinations in cells.
Chromosomes are pulled to opposite ends of the cell during anaphase I, and a nuclear membrane forms around them in telophase I. The resulting two cells now have half the genetic material of the parent cell. Meiosis II follows the same process as mitosis, in which chromosomes form a pair of chromatids joined by a centromere. They line up along the center of the cell and are pulled by their centromeres to opposite ends of the cell. Once the poles are reached, cell division completes resulting in four new cells, each with half the genetic material of the original cell.
