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Cell Division: Eukaryotes
The Cell Cycle

the events

The three main phases of a single cell cycle are: interphase, nuclear division and cytoplasmic division.

Cytoplasmic Division

With the right techniques, the final stage in the cell cycle, mitosis (M), can be observed using a good light microscope.

Cytoplasmic division or Cytokinesis separates the original cell, its organelles and its contents into two more or less equal halves. While all types of eukaryotic cells undergo this process, the details are different in animal and plant cells.

Animal cells

The original cell now has two, equal, daughter nuclei at roughly opposite poles of the cell. A circle of microfilaments begins to form (during anaphase) at right angles to these nuclei which stretches all around the circumference of the cell.

This circle of microfilaments, called a contractile ring contains actin (a protein found in muscle) and starts to contract as the daughter nuclei become visible again in telophase. This contraction, as it tightens, pinches the cell around the middle, forming a furrow ring, which deepens and deepens as the contraction proceeds.

Eventually the cleavage furrow tightens to the point that the original cell can separate into two, new, daughter cells. These cells, after a short pause, re-enter G1 phase and can start the cell cycle all over again.

Plant cells

Plant cells have additional protection in the form of heavily cellulose strengthened walls around them. These are rigid structures that do not easily distort. During the later parts of the cell cycle, therefore, plant cells do not change their shape and as they enter telophase they are more of less the same size as at the beginning of M.

During telophase small, membrane bound sacs, or vesicles start to appear and line up at right angles to the daughter nuclei all across the center of the cell. These vesicles contain all the materials needed to synthesize a new cell wall.

These cell wall precursors were made in the plant equivalent of the Golgi apparatus (for some reason, this is called a dictyosome in plant cells), packaged into the vesicles and moved to the central region of the cell.

During telophase, smaller vesicles start to fuse into larger and larger vesicles and gradually they all fuse, forming a continuous structure, called a cell plate all the way across the cell from wall to wall.

The cell plate is made of membrane (next to the cytoplasm) and cell wall materials (in the center). This effectively cuts the original cell into two new daughter cells. These daughter cells are identical in genetic content and can now, after a short pause, re-enter G1 phase.

events of Cytokinesis

© 2002, Professor John Blamire