Genetic Material and Mitosis 



tion arise? If a pure line of large beans is 

 bred for many generations, one finds, on rare 

 occasions, a very small bean which gives 

 rise to offspring beans ranging from tiny to 

 small, and which clearly make up a new. 

 different, pure line (Figure 1-1C). What 

 has apparently happened is that the genetic 

 material in the pure line of large beans 

 somehow changed to another transmissible 

 form which henceforth caused the produc- 

 tion of beans which are, on the average, very 

 small. Such a change in the genotype that 

 is transmitted to progeny may be attributed 

 to a process called mutation. The result of 

 mutation is a mutant, a term which is ap- 

 plicable to either or both the genotype and 

 the phenotype of the new kind of individual. 



Just as it is easy to ascribe differences be- 

 tween dogs and cats to genetic differences, 

 so it is often simple to tell that certain dif- 

 ferences between lines of the same species 

 have a genetic basis. There are many strains 

 or breeds of pigeons, dogs, cattle, and of 

 other domesticated animals, each of which 

 differs from the other in phenotype. That 

 many of these differences are due to genetic 

 differences is proved by finding that the 

 phenotypic differences are retained even after 

 the different breeds are raised together gen- 

 eration after generation in essentially iden- 

 tical environments. Revealed in this way, 

 the genotypes within a species are of im- 

 mense variety. This already-present genetic 

 variation should be kept in mind in seeking 

 to learn more about the nature of the genetic 

 material. 



In order to learn more about the genetic 

 material, the material things comprising or- 

 ganisms can be examined more closely, par- 

 ticularly those substances transmitted from 

 parent to offspring. Most types of organ- 

 isms are composed of (usually microscopic) 

 building blocks, or cells, plus substances that 

 have been manufactured by cells. Such an 

 organism begins life either as a single cell, 



or by the fusion of two cells into one, or as 

 a group of nonfusing cells derived from the 

 parents. The cell serves as the link or bridge 

 between generations. In those cases where 

 the new individual begins life as one cell or 

 as a group of nonfusing cells derived from 

 a single parent, reproduction is asexual, 

 whereas in cases where two parents con- 

 tribute cells, reproduction is sexual. In sex- 

 ual reproduction two mature sex cells, or 

 gametes, fuse in the process of fertilization 

 into one new cell, the zygote, which is the 

 start of a new individual. In higher animals 

 the gametes are called egg (female) and 

 sperm (male), and the zygote the fertilized 

 egg. In the bean plant, as already men- 

 tioned, male and female gametes are pro- 

 duced in the same individual and self-fertili- 

 zation normally occurs; in human beings the 

 two kinds of sex cells are produced in sepa- 

 rate individuals of different sex, so that cross- 

 fertilization always occurs. 



When might the hypothesized genetic ma- 

 terial be transferred from parent to offspring? 

 Consider certain organisms, composed of 

 only a single cell, which reproduce asexually 

 by dividing into two cells. In this process 

 the parent becomes extinct, so to speak, its 

 individuality being replaced by two daughter 

 cells of the same kind. Once formed, the 

 two daughters often separate, never to meet 

 again. In such a case, the genetic material 

 must have been transmitted before the com- 

 pletion of cell division. Accordingly, the 

 cell and this process of cell division should 

 be studied in some detail for clues concern- 

 ing the physical basis and transmissive char- 

 acteristics of the genetic factors. 



Mitosis 



Attention has already been called to the 

 cellular bridge between generations. It is 

 only via this bridge that genetic transmission 

 may take place, at least in single-celled or- 

 ganisms for which cell division is equivalent 



