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The Cell Cycle: Mitosis

The part of the cell division cycle that gets the most attention is called the M phase or mitosis. Mitosis is the process by which a single cell divides into two daughter cells. The two cells have identical genetic content of the parent cell. As we will see later, cancer cells don't always follow this rule. Mitosis is further broken down into sub-phases based on visible changes within the cells, especially within the nucleus.

The first step is prophase. In prophase, the nuclear envelope dissolves and the chromosomes condense in preparation for cell division. Just like winding up thread on a spool, the condensation of the chromosomes makes them more compact and allows them to be more easily sorted into the forming daughter cells. Also in prophase, protein fibers ( spindle fibers ) form and reach from one end of the cell to another. This bundle of fibers give the dividing cell the structure it needs to push and pull the cell components and form two new cells.

The protein strands that reach from one end of the cell to the other are called microtubules. These proteins are assembled and disassembled during the cell division process. They are the target of several different chemotherapy agents. Taxol®, a chemical derived from an extract of the yew tree, binds to the microtubules and does not allow them to disassemble. This causes the cells to fail in the mitosis process and die. Another class of chemotherapy agent, represented by vinblastine, has the opposite effect. These drugs don't allow the spindle fiber to form. The result is the same, as the cell division process is inhibited. More on Cancer Treatments.


A Closer Look at Human Chromosomes

The image below shows the chromosomes from a human cell. The depiction of all of the chromosomes in this manner is known as a karyotype. Karyotypes are often performed on fetal tissues during pregnancy to detect chromosomal abnormalities in the unborn child. These chromosomes have been colored by the binding of fluorescent dyes. Notice that there are two copies of each chromosome. The wide range in sizes of the chromosomes is also apparent. The chromosomes are numbered in the inverse order of their size. Chromosome 1 is the largest chromosome and the smallest chromosomes are those numbered 21 and 22. The karyotype shown below is from a male and contains one X and one (much smaller) Y chromosome. The DNA in even the smallest of the chromosomes contains millions of basepairs.

chromosome set

In many cancer cells the number of chromosomes is disturbed so that there are either too many or too few chromosomes in the cells. Cells with too many or too few chromosomes are said to be aneuploid. More on mutation and cancer.

The image above is courtesy of Applied Imaging, Santa Clara, CA.

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