Units of Replication 35 



This then is the tremendous significance of mciosis and crossing- 

 over. It provides a continual reshuffling of the genetic material in 

 reproduction. New gene combinations are continually being pro- 

 duced, and essentially random union of gametes makes it unlikely 

 that any two individuals will have the same genetic make-up. This 

 cytological mechanism of the organism is part of its genetic svstcm, 

 the system that determines the amount of recombination a popula- 

 tion will produce and that will be available for the operation of 

 selection. The organisms most familiar to us are diploid, sexual out- 

 crossing organisms such as cats and dogs, oaks and pines. In later 

 chapters (8,9) other genetic systems will be discussed as examples 

 of the ways in which the amount of recombination produced bv this 

 familiar genetic system may be modified (usually decreased, per- 

 haps to zero ) . 



SUMMARY 



The cell is a metabolic unit composed of large and small molecules 

 associated in specific ways, commonly as membrane svstems, to 

 form subunits or organelles of specialized function. The nucleus of 

 the cell initiates and controls protein synthesis through the function- 

 ing of its chromosomes. When somatic cells divide, the cvtoplasmic 

 organelles are apportioned between the daughters in roughlv equal 

 quantitv. By means of mitosis, the chromosomes are di\ided equa- 

 tionallv between the daughters. Meiosis reduces the ninnber of chro- 

 mosomes in cells that will produce gametes. In the first division of 

 this two-stage process, homologous chromosomes first synapse and 

 then disjoin without division of their centromere. Cytological cross- 

 ing-over takes place during the first division, and, when the centro- 

 meres divide in the second division, four daughter cells with re- 

 combined chromosomes and chromosome segments result. Meiosis 

 provides the recombination that results in the variation upon which 

 selection acts. 



REFERENCES 



Swanson, C. P. 1957. Cytology and Cytogenetics. Prentice-Hall, Engle- 

 wood Cliffs, N.f. A rather detailed and excellent discussion of c\to!ogy 

 and its bearing on genetics and evolution. The author's introductory 

 The Cell (1960. Prentice-Hall, Englewood Cliffs, N.J.) provides an 

 elementary description. 



Wilson, G. B., and J. H. Morrison. 1961. Cytology. Reinhold, New York. 

 An excellent modern cytology text which relates structure and cell 

 physiology. The illustrations are generally first-rate. 



