Assortment of chromosomes during cell division
Cell division is an important process in every organism's life. Cell division also enables sexually reproducing organisms to develop from the single-celled zygote, where it was produced by cell division from gametes. After its growth, cell division helps to function in renewal and repair, replacing the dead cells from normal wear and tear or accidents in a fully grown organism.
Cellular organization of the genetic material
A cell's endowment of DNA, its genetic information, is called its genome. Although a prokaryotic genome is often a single long DNA molecule, eukaryotic genomes usually consist of a number of DNA molecules. The overall length of DNA in a eukaryotic cell is enormous. A typical human cell, for example, has about 2m of DNA- a length about 250,000 times greater than the cell's diameter. Before the cell divides, all of this DNA must be copied and then the two copies separated so that each daughter cell ends up with a complete genome. The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into chromosomes, so named because they take up certain dyes used in microscopy. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.
When a cell is not dividing, and even as it duplicates, its DNA is in preparation for cell division. Each chromosome is in the form of a long, thin chromatin fiber becomes densely coiled
and folded, making the chromosomes much shorter and so thick, that we can see them
with a light microscope.
Each duplicated chromosome has two sister chromatids. The
two chromatids, each containing an identical DNA molecule, are initially attached
by adhesive proteins all along their lengths. In its condensed form, the duplicated
chromosome has narrow “waist” at a specialized region called the centromere where
the two chromatids are most closely attached.
Later in the cell division process, the two sister chromatids of each duplicated
chromosome separate and move into two new nuclei, one at each end of the cell. Once
the sister chromatids separate, they are considered individual chromosomes. Thus,
each new nucleus receives a group of chromosomes identical to the original group
in the parent cell.