Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert & Mary Jean Holland

CELL CYCLE AND MITOSIS

Nuclear and Cytoplasmic Division in the Eukarya

Mitosis is the name for the kind of cell division that produces a greater number of cells = cell multiplication; after division, the daughter cells are about half the size of their parent, and they grow before division occurs again. Genetic content (chromosome number) of parent and daughter cells is the same. Cells multiply to make an organism bigger, to repair damage, or to multiply the number of organisms of that kind.

The cell cycle refers to the continuing series of divisions alternating with cell growth. Most of the time a cell is in interphase, the growth and preparation stage of the cycle. Mitosis, the actual process of dividing has four defined phases: Prophase, Metaphase, Anaphase, and Telophase; then the daughter cells enter interphase. An acronym for the cell cycle is ...IPMATIPMATIPMATI...

Mitosis is a continuous process, and the phases blend into one another; it can often be hard to tell if an image is in the late part of one phase or the early part of another. We see them shown in books as snapshots of a particular instant in the process; you have to judge what was happening "when the music stopped."

Phases of the Cell Cycle:

Interphase: The nucleus of the cell is clearly stained and appears to have tiny dots and one or more dark nucleoli inside. What you can't see are the phases of interphase:

G1—period of cell growth before (G = gap); ribosomes and organelles are being duplicated.
S —New copies of DNA are synthesized--DNA replication (S = synthesis).
G2—DNA has been replicated; cell is preparing for mitosis.

Mitosis:

Prophase:

Chromosomes become visible and nucleoli disappear (DNA + associated proteins become tightly organized).
Chromosomes consist of two sister chromatids (DNA replicas + associated proteins) attached together at a specialized region called the centromere.
Nuclear membrane breaks down and disappears, and chromosomes spread out.
Spindle fibers (microtubules) appear. They radiate out to the plasma membrane at the poles in animal cells. These radiations, called asters, are absent in plant cells.

Metaphase:

The chromosomes assemble on the equatorial plate (an imaginary disc that crosses the center of the 3-dimensional cell). Some part of each chromosome is on this plate. Metaphase ends when the centromeres of each pair of chromatids split apart.

Anaphase:

Sister chromatids separate from each other and move to opposite poles of the cell. A portion of the centromere, called a kinetochore, of each new chromosome moves along a spindle fiber.
The free ends of each chromosome trail back toward the equatorial plate and indicate movement.

Telophase: (This is like the reverse of prophase.)

Chromosomes (now single molecules of DNA with associated proteins) have reached opposite poles of the cell.
Spindle fibers disappear.
Nuclear membrane forms around the chromosome clusters.
Chromosomes disappear from view as DNA re-extends, and nucleoli appear.
The cell divides into two daughter cells (cytokinesis). In animal cells the plasma membrane forms a neck; in plant cells vesicles deposit new cell wall material in a line at the equator to form the cell plate.

Interphase: Daughter cells grow in size and prepare for renewed mitosis.

Slides:

Allium root tip. Examine the square cells just inside the root cap. This is the root meristem (embryonic tissue) where mitosis is occurring. Farther up the root is the elongation zone, where cells are long rectangles; these cells are not undergoing mitosis. Identify cells in each of the phases of mitosis. The presence of the cell plate in telophase indicates that these are plant cells.

Whitefish blastula (demonstration). The developing embryo of any organism is a good tissue to examine for mitosis, since cells must divide at a high rate to transform a fertilized egg (single cell) into the trillions of cells of a viable organism. In telophase the dividing cells form a neck and pinch off from each other; this is characteristic of animal cell division.

Return to index.

Last updated 22 September 2007 (JHW)