The nucleus is a membrane bound organelle that contains the genetic information
in the form of chromatin, highly folded ribbon-like complexes of deoxyribonucleic acid
(DNA) and a class of proteins called histones.
When a cell divides, chromatin fibers are very highly folded, and become visible in
the light microscope as chromosomes. During interphase (between divisions), chromatin is
more extended, a form used for expression genetic information.
The DNA of chromatin is wrapped around a complex of histones
making what can appear in the electron microscope as "beads on a string" or
nucleosomes. Changes in folding between chromatin and the mitotic chromosomes is
controlled by the packing of the nucleosome complexes.
DNA or deoxyribonucleic acid is a large molecule structured
from chains of repeating units of the sugar deoxyribose and phosphate linked to four different
bases abbreviated A, T, G, and C. We will later show how the simple structure of DNA contains
for specifying the proteins that allow life. The process of mitosis is designed to insure that
exact copies of the DNA in chromosomes are passed on to daughter cells.