The process of mitosis
Before mitosis starts, all the chromosomes in the nucleus The doubling of the chromosomes resulting from the copying of DNA to produce chromatids before mitosis. 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.
Stage one
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 This is the replicated copy of a chromosome visible during mitosis..
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.
Stage two
The nuclear membrane (the boundary of the nucleus) breaks down.
Stage three
Rope-like structures called Protein threads or fibres produced to pull the chromatids apart during mitosis.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 The middle position in a cell where the chromosomes line up and attach to the spindle fibres during mitosis..
Stage four
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 The opposite ends of a cell to which chromatids are pulled during mitosis..
Stage five
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.