Cell Division: Mitosis and Meiosis Explained
Mitosis
Mitosis, throughout the division of the cell, is characterized by the presence of two fundamental components. Normally, mitotic figures are the chromatic axis and the achromatic axis. The primary chromosome consists of the nucleolus, which can be included as part of the mitotic apparatus. The secondary cycle consists of centrioles, asters, and the spindle.
In-Depth Look
Prophase
Prophase accounts for approximately 40% of the total time of mitosis. It involves both the first series of morphological changes and the physicochemical changes in the cell. This stage starts in G2 and involves the first disorganization of the cytoskeleton, which maintains the cell’s shape during interphase. This is not the case in plant cells due to the rigid cell wall.
Metaphase
During metaphase, chromosomal microtubules move toward opposite poles. Each one pulls toward its own pole. The longer the microtubules, the stronger the pull. Thus, if one chromosome is farther from a pole than another chromosome, the closer pole will be pulled more strongly. It is also possible in this phase that chromosomes are farther from both poles. While a chromosome is closer to one pole, it is pushed harder toward the farther pole. Both chromosomes are aligned along the equator of the cell. Metaphase ends when all chromosomes are aligned at the equator.
Anaphase
At the start of anaphase, the centromere of each chromosome divides, and the sister chromatids separate into two independent daughter chromosomes. Molecular movements are used. Protein motors within the kinetochores, similar to those that cause the inclination of flagella and cilia, pull the kinetochores (and the attached daughter chromosomes) along the microtubules. Simultaneously, the microtubules disassemble within the kinetochores in such a way that the microtubules shorten at approximately the same velocity that the kinetochores move toward the poles.
The utility and precision of the mechanisms of classification and separation of chromosomes do not depend on the number of chromosomes in a cell. Mitosis works just as well in cells with few chromosomes, such as those of fruit flies (8 chromosomes), as in those that contain hundreds of chromosomes, such as the tail cells of some newts. The same occurs in haploid, diploid, and polyploid cells.
Telophase
When the chromosomes reach the poles, telophase (the final stage) begins. The spindle disintegrates. The vesicles formed when the old nuclear membrane broke during late prophase fuse around each group of chromosomes, forming two new nuclear membranes.
The chromosomes extend again, and the nucleolus reappears. In almost all cells, cytokinesis occurs during telophase, enclosing each of the nuclei in an independent daughter cell.
Meiosis
The cells specialized for sexual reproduction are called gametes or sex cells. In females, the gametes are called ova or eggs. In males, the gametes are called spermatozoa. The union of an egg and a spermatozoon is called fertilization. The cell formed by the union of two gametes is called a zygote.
Meiosis is cell division in which the number of chromosomes is halved to form gametes. Meiosis involves a cell division that begins with the diploid number of chromosomes. The cell passes through two successive divisions, but the chromosomes only double once. The two divisions result in four daughter cells. Each daughter cell contains only half the number of chromosomes of the parent cell.
The interphase before meiosis is similar to the interphase before mitosis. Proteins and other components are formed, energy is stored, and materials are exchanged with the environment.