Chapter II 



The Replication and Organization 

 of DNA in Chromosomes 



J. HERBERT TAYLOR 



Department oj Botany and 

 Department of Zoology, 

 Columbia University, 

 New York, New York 



I. Introduction 65 



II. A Morphological Picture of the Chromosome 67 



A. Mitotic Chromosomes 67 



B. Giant Chromosomes 68 



III. Replication of DNA 72 



A. Chromosomal Level 72 



B. Molecular Level 77 



IV. Exchanges between DNA Subunits and Genetic Recombination . . 82 



A. Chromosomal Level 82 



B. Molecular Level 86 



V. Secjuences of Replication of DNA in Chromosomes 88 



VI. Chromosome Reproduction in Relation to DNA Replication 



and Protein Synthesis 89 



A. General Comments 89 



B. Types of Chromosomal Aberrations Produced at Various 



Stages of the Cell Cycle 90 



C. Healing and Reunion of Broken Chromosomes 92 



VII. Molecular Models for Organization of DNA into Chromosomes ... 96 



References 108 



I. Introduction 



Reproduction is a unique characteristic of living systems. Obviously 

 it can occur at many levels of organization. The largest hiatus in our 

 understanding of reproduction involves problems of the establishment 

 and maintenance of organization above the ordinary molecular level. 

 Most of the enzymatic reactions of the cell can be demonstrated in 

 cell-free systems and at least one virus (tobacco mosaic) can be dis- 

 sociated and reassembled. However, we are far from being able to 

 reconstruct the simplest cell, much less a multicellular organism. In 

 principle the problem is not beyond the realm of possibility for we have 

 the concept that all of the complexity is traceable to properties of the 



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