Before mitosis starts, all the chromosomes in the nucleus replicate their DNA to make identical copies. The nucleus now contains two sets of replicated chromosomes, twice as many as a normal body cell.
The stages outlined in the following example show a cell with a diploid chromosome number of four (two sets of two chromosomes) undergoing cell division.
The replicated chromosomes are not visible as individual units - their DNA is loosely coiled.
The replicated chromosomes 'condense'. Their DNA becomes tightly coiled up and the replicated chromosomes become visible as separate units. The chromosomes shorten and thicken and can now be seen as pairs of chromatids.
Replicated chromosomes remain attached to each other by their central region: the centromere. As long as they remain attached to each other the two identical copies of the chromosome are known as sister chromatids.
The nuclear membrane (the boundary of the nucleus) breaks down.
Rope-like structures called spindle fibres attach themselves to the centromere region of the sister chromatid pairs. The spindle fibres line sister chromatid pairs up in the centre of the cell. This location is known as the equator.
The spindle fibres contract and separate the sister chromatids from each other. The sister chromatids are pulled to opposite ends of the cell - the cell poles.
The separated sister chromatids are now known as chromosomes. Their DNA becomes more loosely coiled (decondensed), and new nuclear membranes form around them.
Two new nuclei are formed. Each nucleus now contains four chromosomes (two sets of two chromosomes) exactly the same complement as the original parent cell.
The cytoplasm of the cell then divides in two to produce two daughter cells. The daughter cells are genetically identical because they each contain the same diploid chromosome complement as the original parent cell.
It can be seen in the stages shown above that mitosis maintains the chromosome number or complement of a cell.
Mitosis therefore maintains the chromosome complement and ensures that each daughter cell receives all the genetic information needed to carry out its activities and functions.