One of the most remarkable characteristics of cells is their ability to
grow and divide. New cells come from preexisting cells. When a cell
reaches a certain stage in its life, it divides into two parts. These
parts, after another period of growth, can in turn divide. In this way
plants and animals grow to their normal size and injured tissues are
repaired. Cell division occurs when some of the contents of the cell
have been doubled by replication, or copying (to be discussed later).
The division of a cell results in two roughly equal new parts, the
daughter cells. The process of cell division is known as mitosis and is
diagrammed in Figure 6.
Mitosis is a continuous process; the following stages of the process are
designated only for convenience. During _interphase_ the cell is busy
metabolizing, synthesizing new cellular materials, and preparing for
self-duplication by synthesizing new chromosomes. In _prophase_ the
chromosomes, each now composed of two identical strands called
chromatids, shorten by coiling, and the nucleolus and nuclear membrane
disappear. During _metaphase_ the chromosomes line up in one plane near
the cell equator. At _anaphase_ the sister chromatids of each chromosome
separate, and each part moves toward the ends, or poles, of the cell.
During _telophase_ the chromosomes uncoil and return to invisibility; a
new nucleus, nucleolus, and nuclear membrane are reconstituted at each
end, and division of the cell body occurs between the new nuclei,
forming the two new cells. Each daughter cell thereby receives a full
set of chromosomes, and, since the genes are in the chromosomes, each
daughter cell has the same genetic complement.
[Illustration: Figure 7 _Photomicrograph of cells of the Trillium plant,
which has five chromosomes, in anaphase. Note the duplicate sets of
chromosomes moving to opposite poles of the cell._]
All life processes use up energy and therefore require fuel. The
mitochondria have a central role in the reactions by which the energy of
sugars is supplied for cellular activity. The importance of this vital
activity is obvious. In this booklet, however, we are concerned with the
processes, involving nucleic acids and proteins, that can be described
as making up “the gene-action system”. The gene-action system is the
series of biochemical events that regulate and direct _all_ life
processes by “transcription” of the genetic “information” contained in
molecules of DNA.