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.
