This unit follows the format of the Prince Georges County Framework for Learning. The unit is structured in the 5 E's format of Engagement, Exploration, Explanation, Elaboration, and Evaluation

Before teaching the unit on mitsosis, the students should be able to:

use a microscope properly

prepare a wet mount slide

identify and define the purpose of a cell and major cell parts

All living things start life as a single cell. The cell grows and eventually splits in two. This process is called mitosis. Mitosis is the process by which the nucleus of a cell divides to produce two nuclei, each with the same type and number of chormosomes that the parent cell had. In order to divide, the cell, must follow a cell cycle. The cell cycle is a continuous process. For the middle school student, it is important that the students recognize several phases in each stage of cell division/mitosis. The students also need to realize that some cells do not divide. Nerve cells do not divide, although individual cells grow, lengthening as the body grows. Red blood cells do not divide. New cells are constantly being produced by cells in the bone marrow.


Periods of the cell cycle. Most cells spend about 90% of their time in interphase and about 10% of their time in mitosis and cytokinesis.

Moore, clark, Vodopich, Botany, WCB/McGraw-Hill, 1998

The two main parts of the cell cycle are cell growth and cell division. Cell growth is the longest process.This phase is called interphase. In interphase the cell grows and duplicates the chromosomes.

The second main part of the cell cycle is cell division which is called mitosis. The phases in mitosis are prophase, metaphase, anaphase and telophase. After telophase, the cell cycles back to interphase.


Chromosome condensation :prophase

Moore, Clark, Vodopich, Biology, WCB/McGraw-Hill, 1998

Prophase is the first stage of mitosis in which the nuclear membrane breaks down and the chromosomes become short and thick. By late prophase, individual chromosomes are visible, appearing as two parallel threads attached at a constriction point called the centromere. In the double-thread from of the chromosome, each thread is called a chromatid. However, the double chromatid is still considered to be one chromosome because it has just one centromere. At the end of prophase the nuclear membrane disappears. This allows the chomosomes to move and protrude into the cytoplasm.



Chromosome alignment: metaphase

Moore, Clark, Vodopich, Biology, WCB/McGraw-Hill, 1998

In early metaphase, after the nuclear membrane dissolves, parallel fibers called spindle fibers appear. The spindle fibers attach to each chromosome on either side of its centromere. The spindle fibers force the chromosomes into a tight plane which is a circle around the circumference of the spindle apparatus. The chromosomes are now ready for the splitting of their centromers and the separation of their chromatids.


Anaphase is the shortest stage of mitosis and is the separation of the chromosomes. The chromatids that were attached to the same centromere now separate and move to opposite poles of the spindle. When the centromere is near the center of a chromosome, the two chromosome arms from aV as they are dragged through the cytoplasm. Now each structure is considere a separate chromosome because it has its own centromere. By the end of anaphase, chromosomes have been separated into two genetically identical nuclei.


The next phase, telophase, seems to be prophase in reverse. The spindle fibers disappear, a nuclear membrane forms around each of the two sets of chromosomes, and a nucleoli appears in each new nucleus. The chromosomes become longer.


The final stage of the division of cells, is cytokinesis. Cytokinesis splits the cytoplasm of dividing cell into daughter cells.

 Every time a cell divides, it must first replicate all its DNA. Since chromosomes are simply DNA wrapped around protein, the cell replicates its chromosomes also.

The rate at which mitosis occurs in different types of cells may vary.



Diagram of metaphase chromosome with spindle fibers from opposite ends of the cell

[all illustrations above were taken from Moore, Clark, Vodopich, Botany, WCB/McGraw-Hill, 1998]

Diagram of metaphase chromosome with spindle fibers from opposite ends of the cell attached to opposing kinetochore. (A kinetochore is a complex of proteins that binds to the centromere.



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