"Where a cell exists, there must have been a preexisting cell, just as the animal arises only from an animal and a plant only from a plant." "Omnis cellula e cellula" or "All cells from cells." - Rudolf Virchow (1855)
The ability to reproduce distinguishes living organisms from nonliving entities and this perpetuation of life is based the reproduction of cells or cell division. In unicellular organisms, the division of one cell to form two reproduces an entire organism. In multicellular organisms, cell division allows growth and development plus replacement of damaged or dead cells. Cell division is not just simple pinching in half, but a complex process that passes along the genome from one generation to the next. Cell division involves: precise replication of DNA, allocation of DNA to opposite ends of cells, and separation into two identical daughter cells.
-> Chromosomes: threadlike structures in eukaryotic nuclei composed of DNA & protein; duplicated prior to cell division then distributed by process of mitosis
->Mitosis: Nuclear division during which duplicated chromosomes are evenly distributed into two daughter nuclei; results in two daughter cells that are the genetic equivalent of the parent cell
-> All somatic (nonreproductive) cells have the same number of chromosomes in their nuclei. Sperm & ova have one-half the number of chromosomes as somatic cells. The chromosome number in somatic cells is characteristic of a given species (humans = 46)
->Chromatin: a DNA-protein complex is organized into a long thin fiber that is folded and coiled to form the chromosome. Each chromosome thus contains: A long DNA molecule with thousands of genes and various proteins to maintain structure & control.
-> Before dividing a cell duplicates each chromosome into two identical sister chromatids. For much of mitosis, sister chromatids remain joined a specialized region called the centromere. As mitosis progresses, sister chromatids are pulled apart forming two complete chromosome sets, one at each end of the cell.
-> Mitosis may be followed by cytokinesis: cytoplasmic division that forms two separate daughter cells, each containing a single nucleus.