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|The following slides are of the
embryos if whitefish (an early stage of embryonic development called the
blastula stage). When cells are not dividing, they are in a part of the
cell cycle known as Interphase. The nucleus is clearly surrounded by a nuclear
membrane, the DNA is visible as chromatin rather than the densely packed
chromosomes, and a nucleolus may be visible (see onion slides). There is
no sign of the centrosome and the developing spindle.
| In prophase , the centrosome duplicates
and begins to organize the spindle. On the next page you will see the two
centrosomes separate and move to opposite sides of the cell. The nuclear
membrane begins to break down and will eventually disappear completely.
The DNA condenses to form chromosomes.
|In metaphase, the spindle fibers
pull on the chromosomes until they are aligned in the center of the cell
(along an imaginary line which is referred to as the metaphase plate).
|In anaphase, the centromeres separate
and the chromosomes (which existed as pairs of sister chromatids until they
separated) move to opposite poles of the cell.
|TELOPHASE AND CYTOKINESIS|
|Once the chromosomes have been separated,
they unwind and return to the form of chromatin. The nuclear membrane reforms
and the spindle is broken down. In cytokinesis, the cytoplasm is pinched
to form two separate cells.