The Distribution of Newly Synthesized Dna in Mitotic Division* by Walter

The conservative "character of cell reproduction is predicated on the replication of the genetic material and the precise distribution of the products to the daughter cells. The grouping of many genetic units in few chromosomes creates problems in our visualization of the reproductive mechanism as it simplifies the mechanism itself. The evidence of genetics informs us that at least some of the smaller units in the chromosome are duplicated as accurately in their arrangement as in their qualitative activity. The problems of chromosomal reproduction and of the replication of its molecular units are not necessarily mutually inclusive; the extent to which the solution of one includes the solution of the other is an experimental question. I t seems desirable to make this point because the experiments we are reporting were devised to answer a question concerning chromosome duplication, yet are pertinent to the duplication of a molecular chromosomal constituent, the deoxyribonucleic acid molecule. This study was designed to answer this question: When a chromosome reproduces, is the material of the parent chromosome equally distributed between the daughters or does the parent chromosome persist and determine the formation of a substantially new copy of itself? In terms of the distribution of parental and newly synthesized material the question can be formulated experimentally in several ways. A labelling technique can be used. One might label the substance of the parent chromosome and observe how it is distributed between the two products of its reproduction. Conversely, the label might be introduced in the course of the duplication process. Since it is established by other means (1) that those substances that we recognize as being uniquely constituents of chromosomes increase exactly twofold in the course of the reproductive process, the alternative expectations will be the same with either experimental procedure.