Mitosis 21 



cells derived from the original zygote, and number. Instead each contains a complete, 

 since the zygote of any generation combines unpaired, haploid, set of chromosomes. The 

 two gametes, the number of chromosomes zygote therefore has the diploid chromosome 

 should increase in the zygotes of successive constitution restored because each gamete 

 generations. Yet we know that paired genes furnishes a haploid set of chromosomes, one 

 in one generation remain paired genes in the set contributed by the sperm from the father, 

 next sexual generation, so that their diploid and another set by the egg from the mother, 

 condition is maintained generation after In this way chromosomes remain as pairs, 

 generation via segregation and fertilization. sexual generation after sexual generation. 

 We are led to assume, therefore, that some- and the number of chromosomes remains 

 thing paralleling segregation of genes must unchanged. Clearly, then, the cell divisions 

 take place for chromosomes. This is reason- preceding gamete formation cannot be invari- 

 able in view of the statement already made ably mitotic, but must involve at some point 

 that all individuals of a species have a char- a special mechanism for reducing chromo- 

 acteristic chromosomal composition. In some number. The nature of this special 

 actual fact, as expected, the gametes do not kind of nuclear behavior is investigated in 

 contain the paired, diploid, chromosome the next Chapter. 



SUMMARY AND CONCLUSIONS 



Studies of cell division in which nuclei divide mitotically reveal that, of all cellular com- 

 ponents, the chromosomes are the structures most likely to serve as the material basis for 

 genes. This hypothesis receives support from several of the properties of chromosomes 

 which parallel established or assumed properties of genes. Chromosomes come only from 

 pre-existing chromosomes; chromosomes can mutate on occasion, the mutant chromosome 

 then replicating the mutant form; different species have different chromosomal composi- 

 tions; the chromosome content is identical both quantitatively and qualitatively in each 

 cell of a line produced by asexual reproduction; chromosomes are unpaired in gametes 

 and paired in zygotes; each chromosome retains its individuality, mitotic cell generation 

 after mitotic cell generation, regardless of the nature of its homologous chromosome. 



REFERENCES 



Flemming, W., 1 879. "Contributions to the Knowledge of the Cell and its Life Phenomena," 

 as abridged and translated in Great Experiments in Biology, Gabriel, M. L., and 

 S. Fogel (Eds.), Englewood Cliffs, N.J., Prentice-Hall, 1955,'pp. 240-245. 



Schrader, F., Mitosis: the Movement ofCliromosomes in Cell Division, New York, Columbia 

 University, 1953. 



Scientific American, Sept. 1961, Vol. 205, No. 3, "The Living Cell," articles by J. Brachet 

 and by D. Mazia. 



Swanson, C. P., Cytology and Cytogenetics, Englewood Cliffs, N.J., Prentice-Hall, 1957. 



QUESTIONS FOR DISCUSSION 



3.1. What are the consequences of mitosis? 



3.2. Including those properties of chromosomes listed in the Summary and Conclusions, 

 state the corresponding gene property and whether it is one which is proved or as- 

 sumed. Give evidence or reasons for accepting or rejecting these as genie properties. 



