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THE PLAN OF 

 CELLULAR REPRODUCTION* 



Daniel Mazia 



UNIVERSITY OF CALIFORNIA, BERKELEY 



A cell is at once tlie most perishable and the most enduring of natural 

 objects. As an individual, its allotment of time may be much shorter 

 than that of a mountain or a continent, but its character merges with a 

 future and a past which may be very long even when measured by the 

 geologists' time scales. The secret of biological immortality, which pro- 

 vides not merely for survival but also for evolution, is not the durability 

 of the substances on which it is based, but rather the abihty of or- 

 ganized living units, of which the simplest is the cell, to recreate their 

 characteristic structure and composition more rapidly than the dumb 

 forces of thermodynamics can destroy them. 



With high fidelity, cells can reproduce their characteristic kinds of 

 molecules, can reproduce strange molecules which are introduced to 

 tliem as infectious entities, and, above all, can reproduce themselves. 

 I define the reproduction of a cell as a qualitatively precise doubling— 

 a cycle beginning with a cell of a given kind and ending with two 

 which are identical to the original. The completion of a full reproduc- 

 tive cycle, archetypically at least, must include both division and 

 growth. In practice these are separated in time; the daughters of a 

 division grow and then divide. As we shall see, the period of growth 

 includes events which are prerequisite to division, and division re- 

 freshes the capacity for growth. 



" The author's own researches on this subject have been supported for 

 various periods by the American Cancer Society, the University of California 

 Cancer Research Coordinating Committee, the Office of Naval Research, and the 

 National Institutes of Health. He has had the benefit of discussions with many of 

 those who have built our present knowledge of the cell cycle, notably Dr. Erik 

 Zeuthen and Dr. J. M. Mitchison. 



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