The cell cycle is a regularly occurring sequence of different phases in a body cell. The cell cycle always begins after a cell divides and ends after the next cell division is completed.
What is the cell cycle?
The cell cycle always starts after a division of the cell and ends after the completion of the next cell division. The cell cycle starts immediately after cell division with interphase. Interphase is also known as G phase. It is composed of the phases G1, G2, S and 0. In the G1 phase, also called gap phase, cell growth is the main focus. Various cell components, such as the cytoplasm and some cell organelles, are added to the cell. Various proteins and RNA, ribonucleic acid, are produced in the cell. RNA plays a role in the cell as a carrier of genetic information. In the G phase, the so-called centrioles divide. Centrioles are organelles of animal cells located near the nucleus. The cell nucleus is now clearly visible. In G1 phase, each chromosome consists of only one chromatid. G1 phase usually lasts from 1 to 12 hours. In degenerate cells, this phase may be extremely shortened. The G1 phase is followed by the S phase. In this phase, replication of DNA takes place in the nucleus, so that at the end of this synthesis phase, the DNA is duplicated and each chromosome is formed from two chromatids. The S phase lasts between 7 and 8 hours. The G2 phase represents the transition to mitosis, the division of the cell nucleus. This phase is also called the postsynthetic or premitotic interval. Cell contacts with neighboring cells are broken up, the cell acquires a roundish shape and becomes larger due to an increased influx of fluid. In addition, increased RNA molecules and proteins are synthesized for cell division. This process takes about four hours. The so-called M-phase stimulating factor (MPF) then leads to a transition into the M-phase, the mitotic phase. In germ cells, mitosis phase is also called meiosis. In M phase, the actual cell division takes place. The chromosomes divide as well as the nucleus and the cell itself. Mitosis phase is further divided into prophase, metaphase, anaphase and telophase. Some cells enter G0 phase after their division. In the G0 phase, no more cells are formed. Nerve cells or epithelial cells are often in the G0 phase. Special growth factors can also reactivate cells from the G0 phase, so that the cell cycle then starts again in the G1 phase for these cells as well.
Function and task
The periodic cell cycle allows the body to replace spent and dead cells with new cells. The lifespan of human cells varies greatly. While nerve cells in the brain are never replaced, some body cells live only a few hours. Scientists estimate that about 50 million cells die every second. In the same time, the same number of cells are newly formed through the cell cycle, directly replacing the lost cells. Thus, the body compensates for the loss of dying cells through the constantly occurring cell cycle. The cell cycle also plays an important role in physical development. Cells can only grow to a certain size. Thus, in order for humans to grow larger, new cells must be formed. The cell cycle is also necessary for the regeneration of damaged body parts or tissues. Here, cell division serves to replace cells damaged by injury. Wounds, for example, can only close if new cells are formed. Therefore, in the course of wound healing, the rate of cell division in the wound area increases sharply.
Diseases and complaints
From a pathological point of view, the cell cycle plays an important role in the development of cancer. In healthy humans, the cell cycle is subject to control by so-called cell cycle checkpoints. They serve to protect DNA and genetic material and to prevent cell degeneration. In addition, they inhibit cell division in cells with DNA damage. The affected cells then have the option of either repairing the damage or, in the case of irreparable damage, initiating programmed cell death. Neoplastic cells, i.e. cancer cells, act autonomously and are no longer subject to these control mechanisms. Two factors now contribute to uncontrolled cell growth. First, so-called protooncogenes mutate into oncogenes.These trigger excessive growth of the affected cell. In addition, the tumor suppressor genes mutate. In their normal state, these actually have a growth-inhibiting effect. After mutation, however, their functions are impaired and apoptosis, i.e. the programmed cell death of damaged cells, is no longer triggered. The cancer cells can thus proliferate unhindered. Disturbances in the phases of meiosis, i.e. the division of germ cells, can lead to a maldistribution of chromosomes. The number of chromosomes in the daughter cells is then pathologically altered. This is also referred to as chromosomal aberration. The best-known chromosomal aberration is certainly Down syndrome, also known as trisomy 21, in which chromosome 21 is present three times instead of twice. Instead of 46 chromosomes, 47 chromosomes are present. Characteristics of trisomy 21 are eyelid axes running outwards, muscle hypotonia and a four-finger furrow. In most cases, the disorder leads to mental retardation. About half of all affected individuals also suffer from a heart defect. Other chromosomal aberrations caused by a defective cell cycle are Turner syndrome or Klinefelter syndrome. Here, the sex chromosomes are affected. Chromosomal aberrations are also often responsible for early miscarriages.
