VI. Cytokinesis
The process of cytoplasmic division known as, cytokinesis, follows mitosis. Cytokenesis is sometimes considered its own phase; other times it's simply lumped in with telophase. The term cytokinesis comes from the Greek words cyto which means " cell " and kinesis which means " movement," hence cytokinesis literally means cell movement. In cytokinesis, the cytoplasm is divided between the two new cells.
Figure 7. A pictorial representation of mitosis, each cell depicts a different phase.
- Prophase—the nucleolus disappears, represented by the green dashed line around the nucleus. The chromatin also condenses into chromosomes, which is represented by the X shapes in the center in the color orange. Along the outer edge of the cell you can see the separation of entrosome and formation of the miotic spindle, shown in blue.
- Prometaphase—the nuclear envelope disassembles represented by the dashed light green circle in the center of the cell. The chromosome forms two kinetochores at the centromere (in the center of the orange Xs) and the microtubules (blue fiber-like structures) attach to the chromosomes (shown as the orange Xs).
- Metaphase—the chromosomes represented by the orange X shapes, align in the metaphase plate or along the center of the cell. The blue spindles from each side are attached to the kinetochore, also known as the center of the chromosome.
- Anaphase—the sister chromatids (each half of the orange X) separate towards opposite poles pulled by the blue spindle fibers.
- Telophase—new nuclear envelope forms, shown as the dashed green circles, one for each new cell. The chromosomes unfold back into chromatin represented by the orange dots and mass in the two new nuclei. The nucleoli (yellow shading in the center of the two new nuclei) reappear and the cell continues to elongate.
- Cytokinesis—follows mitosis and is sometimes considered its own phase. The cytoplasm is divided between the two new cells. You can see the cell membrane (light blue line surrounding the cell) cleaving or pinching in two to split into two new identical cells
Each cell contains a huge amount of DNA. For example, if the DNA in a single human cell were stretched out, it would reach two meters (more than six feet) in length. Each species of organism has its DNA separated into a specific number of chromosomes. In bacteria and archaea, there is typically only a single circular chromosome, (sometimes with a few smaller circular plasmids containing only a few genes each). But eukaryotic organisms have multiple linear chromosomes. Each chromosome chain can contain from 100,000 to 10,000,000,000 nucleotides, separated into several hundred to several thousand genes. The number of chromosomes varies by species. For example, humans have 46, chimpanzees have 48, cabbage plants have 18, dogs have 78, one algal species has 148, and so on.
In sexually reproducing organisms, these chromosomes are in pairs - one member of each pair is derived from each parent. For example, in humans, the 46 chromosomes are divided into 23 pairs. Every cell must have a complete set of the organism's genetic material to be able to divide and reproduce itself. Most cells are body cells, also called somatic cells. In humans, somatic cells contain 46 chromosomes; that is, each chromosome is paired, so there are two complete copies of the genome. A few of the cells are gametes; eggs (ova) in females and sperm in males. Each gamete contains only 23 chromosomes (one set of genes) instead of 46 (two sets). When an egg and sperm combine during reproduction, the somatic cell chromosome number, such as the 46 chromosomes in humans, will be restored.
The two daughter cells produced by mitosis have a complete set of chromosomes, identical to those of parent cell that initially divided. As a part of this process, organelles and cytoplasm belonging to the mother cell are also divided up, providing the newly established cells with all the tools they need for metabolism and development. After cytokinesis is complete, the daughter cells enter interphase, starting the cell cycle again.
