Eukaryota: Life History and Ecology

The eukaryotes are so diverse that it isn't easy to generalize about their life histories and ecological roles. However, a particular method of cell division is found in most eukaryotes: this is mitosis. The pictures below show the stages of mitosis in the cells of an onion root tip.

Eukaryote DNA is divided into linear pieces called chromosomes (unlike bacterial DNA, which forms loops). In the non-dividing phase of a eukaryotic cell's life cycle, known as interphase, the chromosomes are uncoiled. When a eukaryotic cell prepares to divide, first the DNA in the chromosomes replicates, creating doubled chromosomes, and then the chromosomes begin to coil and supercoil upon themselves, in the stage known as prophase. At this point the chromosomes become thick enough to be seen under the light microscope. Next, in metaphase, the chromosomes line up in the center of the nucleus (in some eukaryotes the nuclear membrane disappears, in others it does not). The chromosomes then are pulled apart to opposite sides of the cell, in anaphase. Lastly, in telophase, the chromosomes uncoil, and the nuclear membrane re-forms if it disappeared. At this time, cytokinesis - the actual splitting of the whole cell - takes place. In eukaryotes with no cell wall, the daughter cells generally pinch off from each other; in eukaryotes with cell walls, like plants, a new cell wall forms and partitions the parent cell into two.

These pictures of cells dividing are from the collection of mitosis images available through the University of Wisconsin.