Cells are comprised of very complex networks. One of the most important structures having to do with cell division is the chromatin network, and it contains all the genetic material that will be passed on to the new cells.
When the cell receives instructions from the nucleus to divide, the chromatin network changes into long thin bodies called chromosomes. The chromosomes line up in the center of the cell and then divide equally, each half going to opposite ends of the cell. When this is completed, they dissolve to form a new chromatin network for each new nucleus. The rest of the cell divides as the nucleus splits and two new cells result. This process is called mitosis.
When a male and female cell come together to form a new organism (i.e. human reproduction), the full number of chromosomes is again restored in the fertilized cell. This is called meiosis.
Chromosomes are filamentous bodies which are typically present in the nucleus and which become visible during cell division. They are the carriers of the genes or units of heredity. Certain segments of the chromosomes, or the entire chromosomes, are more condensed than the rest of the karyotypes during various stages of the cell cycle.
Chromosomes which remain condensed during interphase are called heterochromosomes, e.g. the sex chromosomes of insects. The non condensed chromosomes which extend during interphase are called euchromosomes. Chromatin material is of two types, heterochromatin and euchromatin. In the early stages of mitosis or meiosis (cell division), the chromatin strands become more and more condensed. They cease to function as accessible genetic material and become a compact transportable form.