and this DNA is also identical to that of the parent cell.
Mitosis and Chromosomes
During cell division, two nuclei must form during the process of mitosis, so that one nucleus
can be given to each of cells that form from cytokinesis. In the nucleus, the genetic infor-
mation of the cell, DNA, is stored. The copied DNA needs to be moved into a new nucleus
for the new cell to have a correct set of genetic instructions.
The DNA in the nucleus is condensed intochromosomes, structures composed of DNA
wrapped around proteins. Each organism has a unique number of chromosomes; in human
cells our DNA is divided up into 23 pairs of chromosomes. When a cell is not undergoing
division, such as during interphase, the complex of DNA and proteins is a tangled mass of
threads known as chromatin. As mitosis begins, however, the DNA becomes tightly coiled
into the chromosomes which become visible under a microscope.
Figure 5.3: The DNA double helix wraps around histone proteins (2) and tightly coils a
number of times to form a condensed chromosome (5). The chromosomes contains millions
of nucleotide bases. This figure illustrates the complexity of the coiling process. The red
dot shows the location of the centromere, where the microtubules attach during mitosis and
meiosis. ( 16 )
As mentioned previously, the DNA is replicated during the S stage of interphase. Each
chromosome now has two identical molecules of DNA, called sisterchromatids,forming
the ”X” shaped molecule depicted inFigure5.3. During mitosis, the two sister chromatids
must be split apart to give rise to two identical chromosomes (in essence, each resulting
chromosome is made of 1/2 of the ”X”). Through this process, each daughter cell receives
one copy of each chromosome.
Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. During
prophase, the chromosomes become tightly wound and become visible under the micro-
scope. Also, the nuclear envelop dissolves, and the spindle begins to form. Thespindleis
a structure containing many fibers that helps to move the chromosomes. By late prophase,