The nature of life has excited the interest of human beings from the
earliest times. Although it is still not known what life is, the
characteristics that set living things apart from lifeless matter are
well known. One feature common to all living things, from one-celled
creatures to complex animals like man, is that they are all composed of
microscopic units known as cells.
The cell is the smallest portion of any organism that exhibits the
properties we associate with living material. In spite of the immense
variety of sizes, shapes, and structures of living things, they all have
this in common: They are composed of cells, and living cells contain
similar components that operate in similar ways. One might say that life
is a single process and that all living things operate on a single plan.
The past few years have been a time of rapid progress in our
understanding of the mechanisms that control the function of living
systems. This progress has been made possible by the development of new
experimental techniques and by the perfection of instruments that detect
what happens in the tiny world of molecules. Prominent among the methods
that have contributed to the explosive growth in our understanding of
biology is the use of radioactive isotopes as laboratory tools.
In this booklet we shall attempt to give an account, in chemical terms,
of the materials from which living matter is made and of some of the
chemical reactions that underlie the manifestations and the maintenance
of life. To accomplish this, we have chosen to describe three types of
molecules that have become the basis of modern biology: deoxyribonucleic
acid (DNA), ribonucleic acid (RNA), and proteins. We will show how
radioactive isotopes can be used to pry into the innermost secrets of
these substances. Before we can understand the function of these
precious molecules, however, it will be necessary to review the
structure of a cell and the physical nature of radioactive isotopes.