Cell division is the process by which a living cell proliferates from one cell to two cells. The cells before division are called mother cells, and the new cells formed after division are called daughter cells. Generally, the cell division includes two steps: nuclear division and cytokinesis. During nuclear division, the mother cells pass the genetic material to the daughter cells. In single-celled organisms, cell division is the reproduction of individuals. In multicellular organisms, cell division is the basis of individual growth, development and reproduction.
Types of Cell Division
|Eukaryotes||Mitosis||A vegetative division, whereby each daughter cell is genetically identical to the parent cell.|
|Meiosis||A reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes.|
|Prokaryotes||Binary fission||A vegetative cell division, where their genetic material is segregated equally into two daughter cells.|
Eukaryotic Cell Division: Mitosis
|Interphase||The interphase is divided into three stages: G1, S, and G2. DNA replication is mainly carried out in the S phase, the synthesis of chromosomal proteins and DNA helicases is mainly carried out in the G1 phase, the synthesis of enzymes involved in cell division and spindle proteins is mainly carried out in the G2 phase.|
|Prophase||The prophase of the cell cycle refers to the period from the beginning of the mitosis to the disintegration of the nuclear membrane. In the prophase, the nucleus volume increases. The chromatin gradually shortens and becomes thicker, forming a chromosome. Since chromatin has been replicated in the interphase, each chromosome consists of two chromatids, which have a common centromere junction. The nucleolus gradually disappeared in the prophase, and the chromosomes were scattered in the cytoplasm. In the prophase, animal cells have two centrosomes near the nuclear membrane. Spindle wires are emitted from the centrosomes. The two centrosomes with spindle gradually separate and move toward the opposite poles.|
|Prometaphase||The prometaphase refers to the rupture of the nuclear membrane until the chromosomes are arranged on the equatorial plane. The main process of prometaphase is the final formation of the spindle and the movement of the chromosome to the equatorial plane. After the nuclear membrane ruptures, the chromosomes are dispersed in the cytoplasm. The centromeres of the two chromatids of each chromosome are connected to the two poles by a spindle wire. Due to the interaction between the microtubules, the chromosomes move toward the equatorial plane. Finally, the various forces reach equilibrium, and the chromosomes are arranged on the equatorial plane.|
|Metaphase||The metaphase refers to the period from the arrangement of chromosomes on the equatorial plate to the time when their chromatids begin to split to the two poles. These chromosomes are arranged radially on the equatorial plate, when they are not stationary, but are in a state of constant oscillation. The chromosomes become thicker and thicker, showing the number and morphology unique to the species in the metaphase.|
|Anaphase||The anaphase is the period in which the two sister chromatids of each chromosome are separated and moved to the poles. Separation of the chromatids often begins at the point of centromere, and then the arms of the two daughter chromosomes are gradually separated. When they are completely separated, they move to the opposite poles.|
|Telophase||The telophase refers to the period from the time the daughter chromosomes reach the two poles until the formation of two daughter cells. The main process in this period is the formation of daughter nucleus and the division of cells. The formation of the daughter nucleus is generally a process that is reversed from the prophase. The daughter chromosomes that reach the two poles first unwind. All the daughter chromosomes form a large chromatin block, and the nuclear membrane components are gathered around them. Then the nuclear membrane of the daughter nucleus is fused, and the nucleus appears.|
|Cytokinesis||The division of the cell is called cytokinesis. Cytokinesis is not a phase of mitosis but rather a separate process, necessary for completing cell division. The end of cytokinesis marks the end of the mitosis.|
Figure 1. Diagram of the mitotic phases.
Eukaryotic Cell Division: Meiosis
|Meiosis I||Prophase I||Slim and linear chromosomes appear in the nucleus and the volume of nucleus increases. Each chromosome contains two sister chromatids. Pairing of homologous chromosomes within a cell is called a synapsis. Since there are 4 chromatids in a pair of homologous chromosomes, they are called tetrad chromosomes. The chromosomes are shortened and thickened continuously. At the same time, DNA fragment exchange occurs between non-sister chromatids in the tetrads, which leads to the exchange of parental genes and genetic recombination. Then the chromatids begin to separate. The chromosome becomes compact and approaches the periphery of the nucleus. Then, the nuclear membrane, the nucleolus disappear, and finally the spindle is formed.|
|Metaphase I||Each pair of homologous chromosomes is moved to the equatorial plate in the center of the cell, and the centromeres are arranged in pairs on both sides of the equatorial plate, and a spindle is formed in the cytoplasm.|
|Anaphase I||By the pulling of the spindle, the paired homologous chromosomes are separated and moved to the two poles respectively.|
|Telophase I||The non-homologous chromosomes that reach the poles are gathered again to reproduce the nuclear membrane and nucleolus, and then the cells divide into two daughter cells. The number of chromosomes in these two daughter cells is only half of the original. The regenerated cells then undergo a second division.|
|Meiosis II||Prophase II||Similar to the prophase I, the chromosomes are scattered in the cells. Then the nuclear membrane and nucleolus disappeared, the spindle was formed again.|
|Metaphase II||The centromere of the chromosomes is arranged on the central equatorial plate of the cell. Note that there are no homologous chromosomes.|
|Anaphase II||The centromere of each chromosome is separated, and the two sister chromatids are separated to form two chromosomes. Under the traction of the spindle, the two chromosomes move to the poles of the cell.|
|Telophase II||The reappearing nuclear membrane, nucleolus, and chromosomes reach the poles and enter the two daughter cells respectively. The number of chromosomes in the two daughter cells is reduced by half compared to the primary spermatocytes or oocyte.|
Figure 2. Diagram of the meiotic phases.
Prokaryotic Cell Division
Prokaryotes replicate through a type of cell division known as binary fission. Prokaryotes are simple organism, with only one membrane and no division internally. Thus, when a prokaryote divides, it simply replicates the DNA and splits in half. The process is a little more complicated than this, as DNA must first be unwound by special proteins. Although the DNA in prokaryotes usually exists in a ring, it can get quite tangled when it is being used by the cell. To copy the DNA efficiently, it must be stretched out. This also allows the two new rings of DNA created to be separated after they are produced. The two strands of DNA separate into two different sides of the prokaryote cell. The cell then gets longer, and divides in the middle.