Because it has several important implications in biology and medicine,
it is important to remember that DNA synthesis occurs only during the
short, well-defined S period of the cell cycle. Other synthetic
processes go on throughout the cycle. We mentioned, for instance, that
all cells can be labeled by a brief exposure to a radioactive amino
acid, a precursor of proteins; this means that protein synthesis occurs
throughout the entire cell cycle, including mitosis. When we use a
radioactive RNA precursor, all cells except those in anaphase and
metaphase are labeled; this means that RNA synthesis occurs throughout
the entire cycle except during anaphase and metaphase. But a radioactive
tag on a DNA precursor reveals that only during the S phase is there DNA
synthesis.[9]
It is also important to remember that a cell that has synthesized DNA is
a cell that, with a few exceptions, will divide in the very near future.
Thus, for an understanding of the mechanisms that control cellular
proliferation, it is important to investigate the factors that control
DNA synthesis. Our recent knowledge of the cell cycle has therefore led
to a shift in the focus of investigation from mitosis to DNA synthesis.
Another point to remember is that not all cells keep going through the
cell cycle indefinitely. As shown in Figure 25, when a cell divides, the
daughter cells have two alternatives, either to go through another cycle
or to leave it altogether. Cells that leave the cycle are called
differentiated cells and will eventually die without any further
division. Many cells in an adult organism also have lost the capacity to
make DNA and therefore the capacity to divide. These cells often have
other specialized functions in the body; examples are nerve cells and
muscle cells.
The synthesis of other macromolecules (giant molecules, like DNA)
connected with the gene-action system is another field of active
investigation. We have described how we can investigate the synthesis of
proteins and RNA with radioactive isotopes, and we have given some
information on the gene-action system, which is also shown in Figure 26.
The genetic material of a cell is DNA. The DNA molecule is in the form
of a double-stranded helix that is supported by a protein backbone.
Genes are often described as simply segments of DNA. They differ from
each other only in the order in which the four nucleotide bases that
make up DNA are arranged. (Look at Figure 13 again.) Since a single gene
is usually made up of several hundred bases, it is easy to imagine the
infinite variety of genes that could exist by simply changing the order
of the four bases several hundred times.
[Illustration: Figure 26
THE GENE-ACTION SYSTEM]
Not all genes in the cells of a living organism are active. In fact,
most of them are inactive, or, as geneticists say, repressed. What
represses genes to make them inactive is not known, but many
investigators believe the activity, or lack of it, is regulated by
proteins called histones. If a gene is repressed, nothing happens; it
remains inactive, presumably until something removes the repressing
factor. But an active gene sets in motion a train of events that results
in activation of one of the processes of life: The gene’s DNA directs
the manufacture of RNA, which in turn brings about the synthesis of a
specific protein to carry out a specific metabolic process. In other
words, all the activities of the cell are dictated by active genes (the
DNA molecules) through the mediation of RNA and are executed by
proteins.
Here is what happens as nearly as scientists can reconstruct it: