GREAT CHANGES HAVE TAKEN PLACE RECENTLY IN THE TEACHING OF undergraduate biology. To some extent, these changes are due, as they are in many other sciences, to the upgrading of highschool science education. However, the major factor for biology would seem to be the change that has taken place during the last 20 to 30 years in the science itself. Biology, which has in the past been taught primarily as a descriptive subject, is now changing to one that is primarily analytical. One result of this change is that the undergraduate biology courses are becoming more and more dependent on physics, mathematics, and chemistry, to enable the student to achieve a deeper understanding of this subject. The developments in molecular biology and biochemistry have not only become a part of the undergraduate biology curricula, but are inereasingly necessary for a general understanding of our scientific culture.

This series of textbooks is designed to aid those biology teachers who are developing new programs for undergraduate biologists. We do not imagine a program lacking in the subject matter of "classical biology." Rather, we are thinking of a program in which the teaching of many of the more classical biological subjects will be firmly based on the student's background in biochemistry and molecular biologv. Most introductory biology courses already include a section on molecular biologv; however, it is our belief that future developments will see this section expand to the point where it can play a central role in the planning of the rest of biology program. Molecular biology is not a separate subject independent of "real biology"; rather it is fundamental for a serious understanding of any aspect of the subject. cWe expect that this book will be useful as a supplementary text for courses at many different levels in a student's program. But its primary purpose is to enable the beginning students to gain enough background so that it will not be necessary to repeat the general and usually inadequate discussion of molecular biology at the beginning of each of the more advanced courses.

CYRUS LEVINTHAL
Cambridge, Massachusetts lune 1965

PREFACE

THIS BOOK HAS ITS ORIGINS IN A SERIES OF TEN LECTURES WHICH I have given for the past six years to introductory biology students at Harvard. In these lectures, I have not only attempted to convey the excitement of the recent discoveries of molecular genetics, but also to relate this new knowledge to the basic problem of biology - the nature of cells and how they divide. It has, therefore, been necessary to talk about ATP as well as DNA, a feature I have carried over into the text. I am aware, however, that some people may feel that I would have best restricted my discussion to the gene itself, with the expectation that the reader will learn the main principles of intermediary metabolism in another text. This, in fact, was my first intention. When I began to write the first draft, however, I was bothered by the artificial nature of the separation and so decided to start the book with two historically oriented chapters to help the reader see how our ideas about molecular genetics have developed out of the work of the classical geneticists and biochemists.

There may be readers troubled in another way. They could object that, in the later chapters, I have stated the arguments too strongly, in view of the rapidly developing nature of our

ideas about the genetic code, the replication of viruses, and the control of protein synthesis. I do not believe this to be the case. Although a particular fact may later prove to be based on faulty evidence, I firmly believe that almost all the basic concepts I present are sound. This was not true five years ago; the writing of a molecular biology book for introductory students would have been unwise then. Now, however, I believe that biology has as sound a basis as was provided chemistry, about 1932, by the explosive development of the quantum theory of the atom. Therefore, it is time to reorient our teaching and to produce the new texts that will give the biologist of the future the rigor, the perspective, and the enthusiasm that will be needed to bridge the gap between the single cell and the complexities of higher organisms. Then we may expect hard facts about today's most challenging biological problems: the structure of cell membranes, the nature of cancer, the fundamental mechanism ( s ) of differentiation, and how the ability to think arises from the organization of the central nervous system.

It was my original intention to write a small book of about 125 pages. After writing a first draft of this length, I realized that I could not accomplish the task in this short space. Even at four times the original length, I am not completely satisfied. Often I present a fact, and, because of lack of space, I cannot outline the experiments that demonstrate its validity. Given the choice between deleting an important principle or giving an experimental detail, I am inclined to state the principle. This is partly because I have a preference for theory, but, more importantly, because a number of new laboratory manuals now exist which emphasize modern genetics and biochemistry.

Many of my friends have helped me in reading one or more of these chapters. Many useful comments were received from K. Bloch, M. Cohn, L. V. Crawford, J. Darnell, B. D. Davis, G. Edelman, J. T. Edsall, J. Hopkins, H. Latham, S. E. Luria, M. Meselson, N. A. Mitchison, M. Ptashne, H. Rubin, and H. Temin. Most particularly, I wish to thank Dick Roblin for his help with the index and Joan Argetsinger for her consistently intelligent comments throughout the preparation of the manuscript. Whenever possible, I have tried to follow their. advice.

Sometimes, however, I took issue with their comments, and I alone am accountable for the errors in either fact or judgment. I am also greatly indebted to my secretaries, Margrit Hui and Jutta Binstock, for their cheerful competence, and to the Radcliffe students, Dolly Garter and Ellen Glass, for continual advice, not always followed, on what is a grammatical sentence. Most of the original drawings were done bv Keith Roberts of St. John's College, Cambridge, England. I also wish to acknowledge the excellent artistic advice of Bill Prokos, who was responsible for the final version of the illustrations.

J. D. WATSON
Cambridge, Massachusetts