In mitosis, the cell nucleus divides once and produces two identical nuclei. The new daughter cells are genetically identical to the parental cell (unless their DNA has changed in some way, e.g. mutated). So mitosis doubles the number of cells without changing the genetic information, and it is an extremely accurate process - the error rate is only 1:100 000 cell divisions! Mitosis produces new cells for e.g. growth of multicellular organism, asexual reproduction and wound healing.
The cell division cycle
The cell cycle is the sequence of events that occurs between one cell division and the next. It consist of three main stages.
1. In the first stage, called the interphase, the cell grows, carrying out its functions and replicating its DNA. After the replication of the DNA, new protein becomes attached to it. The chromosome then consists of two strands called sister chromatids which both have identical genetic information. The chromatids are joined at a point along their length by a centromere. These centromeres become visible under a light microscope only during mitosis. Usually, interphase takes about 90% of the cell cycle.
2. The second stage, called nuclear division, takes place during mitosis. The chromatids which contain the replicated DNA are separated and redistributed as chromosomes in the nuclei of the new daugher cells.
3. The third and the final stage is called cell division (or cytokinesis, for more fancy, scientific linguistics). In the phase the cytoplasm divides to finally form the two daughter cells.
Mitosis is a constantly ongoing process, but it is simplified to four most important stages:
In prophase, the chromosomes condense and become more tightly coiled and folded, seemingly becoming shorter and fatter as the process progresses. The DNA becomes inactive when the chromosomes start condensating. By condensating, the chromosomes enables them to be moved easily without being entangled when moved around the nucleus. At the later stages of prophase the sister chromatid pairs are visible, attached to the centromere. The nucleoli disappear, the nuclear membrake breaks down and a spindle apparatus appears.
The centromeres of all the chromoses line up on the equator and divide. The spindle apparatus forms fully. The apparatus is made up from microtubules, controlling the chromosome movement. One of the sister chromatids of each chromosome is attached to one pole by the microtubules. The other attaches itself to the opposite pole.
First, the centromeres separate. The sister chromatids are drawn apart the opposite poles of the cell. After separation the sister chromatids are referred to as daughter chromoses. Not that it matters to them, anyway. The movement and separation is cause by the shortening mictrotubules, connecting the chromosomes to the poles, and the lengthening of the pole-to-pole microtubules. As the poles move further apart, they lengthen the cell.
As the two sets of daughter chromosomes reach the poles of the cell, telophase begins. The nuclear membrane and nucleoli reform, and the chromosomes become less visible. The spindle apparatus disappears at the end of telophase. Mitosis is finished when two identical daughter nuclei are formed.