|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
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 the information
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.