10 



CHAPTER 1 



read) been accomplished. Thus, the chem- 

 ical replication that takes place in a given 

 interphase is not visible in chromatid form 

 until the succeeding telophase. 



What are the consequences of mitosis'.' 

 Speaking in terms of visible structures, the 

 chromosomal content of the parent nucleus 

 has become repeated in each daughter nu- 

 cleus, so that the subsequent division of the 

 cytoplasm produces daughter cells whose 

 chromosomal composition is identical to each 

 other's and to that of the parent cell from 

 which they were derived. Mitosis merely 

 provides the cellular machinery for the exact 

 partitioning of previously replicated chromo- 

 somal material. The cells of different species 

 are different in that they have different num- 

 bers of chromosomes per nucleus, or the 

 chromosomes differ in appearance, or both. 

 One chromosome may differ from another 

 in size, in stainability with various dyes, and 

 in the position of the centromere. Most 

 chromosomes have a single centromere which 

 is not located terminally, i.e., at an end, and 

 therefore separates the two arms of a chro- 

 mosome; all chromosomes and chromatids 

 are unbranched fibers. 



Examination of the kinds of chromosomes 

 present at metaphase in sexually reproducing 

 organisms typically reveals that for each 

 chromosome which arrives at the equatorial 

 plane, there is another chromosome very 

 similar or identical in appearance which also 

 takes a position independently in this plane. 

 Chromosomes thus occur as pairs; the mem- 

 bers of a pair are called homologous chro- 

 mosomes, or homologs, whereas chromo- 

 somes of different pairs are nonhomologous, 

 or nonhomologs. It should be repeated that 

 the members of a pair of homologs take 

 their positions at mitotic metaphase inde- 

 pendently of each other. 



The number of chromosomes seen in 

 typical mitosis of the garden pea is 7 pairs; 

 in Indian corn (maize) there are 10 pairs, 



in the domesticated silkworm 28, and in 

 human beings 23; thus the chromosomes of 

 a species are characteristic in number - as 

 well as in form. Whatever the number o\ 

 chromosomes present in the zygote, then, 

 other things being equal, the same number 

 of chromosomes will be found in every cell 

 of a multicellular organism descended from 

 the zygote by cell divisions in which mitosis 

 has occurred. 



Chromosomes as Genetic Material 



The chromosomes are one of the character- 

 istic components transmitted by all cells to 

 daughter cells. Chromosomes reproduce 

 themselves and are transmitted in mitosis 

 equally to the daughter cells so that these 

 are identical, in this respect, to each other 

 and to their parent cell. Let us make the 

 additional reasonable assumptions that ge- 

 netic material arises only by the replication 

 of pre-existing genetic material, and also that 

 different genotypes arise only from each 

 other by mutation, that is, by a genotype's 

 changing to an alternative mutant form which 

 in turn is involved in reproducing the alter- 

 native form until it undergoes mutation. A 

 chromosome may occasionally become visi- 

 bly altered in certain ways; in these cases all 

 chromosomes ordinarily derived from such a 

 modified chromosome via mitosis have ex- 

 actly the same alteration. Therefore, both 

 genetic material and chromosomes are con- 

 sidered capable of mutation and are subse- 

 quently involved in replicating their new 

 form. On this basis, then, we can hypoth- 

 esize that the chromosome is, or carries, the 

 genetic material. 



It has been implied that the genetic ma- 

 terial routinely retains its individuality or 

 integrity regardless of the nature of the en- 

 vironment. One indirect piece of evidence 

 has already been cited for believing this is 



-'See S. Makino (1951) and C. D. Darlington and 

 E. K. Janaki-Ammal (1945). 



