The amount of time it takes for one cell to completely divide in most mammals, including the human body, is about 24 hours. Some cells take more or less time, depending on their purpose; a fly embryo takes only eight minutes to divide, for example, while a human liver cell could take more than a year. Gametes, cells used in sexual reproduction, can take decades to divide. Abnormal cell division can also occur, resulting in cancerous cells that multiply much more rapidly.
Average Cell Division Times
|fly embryo||mitosis||8 minutes|
|human skin||mitosis||20 - 24 hours|
|human sperm||meiosis||about 64 days|
|human liver||mitosis||1 year or more|
|human egg||meiosis||up to 40 years or more|
|human nerve||mitosis||never, once mature|
There are two primary types of cell division: mitosis and meiosis. Mitosis allows for both an entire unicellular organism to reproduce and the tissues in multicellular animals to be repaired. In this process, a "parent" cell doubles its chromosomes and then divides itself into two identical "daughter" cells. In the case of bacteria and rapidly growing organisms like a fetus or a young animal, cell division takes place very rapidly so the organism can grow and thrive. Most of the cells in fully-grown organisms divide on a much slower scale to slowly renew the body. The exception to this is in areas like the skin and bone marrow, which are constantly generating new cells so that, if an injury is sustained, the cells can divide quickly to heal the area.
Though there are many types of cells, the same general cell cycle takes place during mitosis for most mammals. During the cycle, the cell grows and rests, copying its DNA and dividing into new daughter cells. Each step must be complete before the next one can begin. There are also a variety of checkpoints built into the process that ensure that the proper changes have taken place before the signal to start the next stage is sent.
The cell cycle has four main stages:
- The first gap, or G1, during which the cell grows and develops, takes from zero to five hours. Not all cells progress from this stage; if not, they move into a state called G0, where they are not able to divide. Most cells in the body are actually in G0 phase.
- The next stage is called the synthesis, or S, phase when DNA is replicated. It takes from six to eight hours to complete.
- The cycle then enters the second gap phase (G2), which takes from two to five hours. As in G1, this is when the cell grows and produces proteins as it prepares to divide.
- Finally, the mitosis, or M, phase is when the DNA is divided and separates, and the cell divides. It lasts only about one hour.
These time estimates are averages for most cells in the body as not all cells divide at the same speed. Some cells, such as brain and liver cells, do not typically divide at all. Cells in the liver can go through mitosis to repair minor damage, but may take a long time to complete the process.
The Phases of Meiosis
Meiosis, which takes place only in multicellular animals, involves forming eggs and sperm, or gametes, for sexual reproduction. The end product is four daughter cells, each of which contains a unique combination of half of the original chromosomes. In humans, for example, an egg and sperm each contain 23 chromosomes, which when combined, make up the 46 found in most cells.
Although each is made up of several stages, there are three main phases of meiosis:
- Interphase is the first phase, and is similar to the first two stages of mitosis, with the cell growing and replicating its DNA.
- During the next phase, meiosis I, the homologous chromosomes — those that come from each parent and have genes for the same characteristics — are paired and DNA is exchanged to create new combinations. The pairs of chromosomes then line up and are moved to opposite sides of the cell, and the cell divides into two daughter cells.
- The meiosis II phase repeats many of the steps from phase I; however, the DNA is not replicated. As a result, when the two daughter cells split, the four resulting cells contain only half of the chromosomes, and the DNA in each is unique.
Each of these phases can take different amounts of time, depending on the species and gender of the organism. In human women, the meiosis process starts before birth as the oogonia — immature eggs — develop in a fetus. The development process stops in the middle of the meiosis I phase, however, until the body needs the cell to become an egg. This means that this type of cell division can take decades to be completed, as a woman's eggs continue to develop until menopause, usually in a woman's 40s or 50s. In men, meiosis to produce sperm does not begin until puberty, and takes about 64 days.
Meiosis is a one-way process that must occur alongside of mitosis. In prenatal development, the sperm fertilizes the egg to form a zygote. Within 24 to 30 hours, the zygote then goes through its first cell division using mitosis; one cell becomes two, and then those two divide into four, and so on. This process continues to replicate cells in a mitotic cell cycle as the organism develops.
Cells that divide too quickly, to the point where their growth becomes out of control, are called cancer. Abnormal rapid cell growth can result in damage or mutation to the genes needed for a mature cell to function. There are many reasons that cells can divide and grow out of control, including genetic causes, nutritional deficiencies, some infections, or exposure to environmental causes, such as carcinogens or ultraviolet (UV) radiation from the sun. Cancer can begin with a single cell that starts to multiply too rapidly, with the cells then unable to receive the proper instructions to make proteins in order to grow, multiply, and, in general, survive.