EARLY HISTORY OF LIFE 



33 



Even with this start it is a long trek to an 

 animal cell, to the simple single-celled ani- 

 mal living in stagnant water. 



Genes and gene strings 



All animals and plants are composed of 

 cells, the details of which we shall go into 

 later, but for the moment in order to con- 

 tinue our story of the origin of life, it is 

 necessary to know something about certain 

 vital parts of these cells. The cell is com- 

 posed of a limiting membrane, cytoplasm, 

 and a nucleus (see Fig. 3-1). Within the 

 nucleus are dark-staining bodies called 

 chromosomes, which are made up of nucleo- 

 protein molecules or aggregates of such 

 molecules called genes (Fig. 2-9). These 

 are capable of reproducing themselves 

 precisely, obtaining material from the sur- 



Fig. 2-9. Chromosomes are composed primarily of 

 nucleoprotein molecules which show up in these 

 stained fruit-fly salivary gland cells as dark bands or 

 discs. These molecules or aggregates of these mole- 

 cules probably constitute the genes. 



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Fig. 2-10. A picture of a bacterium (Pseuc/omonas f/oo- 

 rescens) taken with an electron microscope (X22,000). 

 Note the long hair-like flagella that are used to 

 propel the cell through the water. 



rounding cytoplasm for that purpose. In 

 a sense, then, they resemble viruses, at 

 least in being nucleoprotein in nature and 

 being able to reproduce themselves. They 

 also could be considered similar to the 

 original protein molecules that formed in 

 the early history of life. Here we see coun- 

 terparts of these early substances in both 

 the viruses that live at the expense of ani- 

 mal and plant cells and in the genes which 

 are an integral part of all cells. 



Let us consider these independent, self- 

 sustaining protein molecules as free-living 

 genes. They could then aggregate into long 

 strino;s resembling chromosomes, but of 

 course they would be merely free-floating 

 gene strings or chromosomes. Sometime 

 later, because of their physical nature they 

 could gather about themselves a semi- 

 liquid, semi-solid mLxture of substances 

 which would set them apart from others 

 in the same vicinity. Somehow a membrane 

 could have formed, and a cell not unlike 

 certain specialized cells alive today could 

 have evolved. Some of the single-celled 

 plants, such as the bacteria (Fig. 2-10) and 

 certain blue-green algae, possess dispersed 

 chromosomes without nuclear membranes. 

 In fact, it is possible to select a whole series 

 of algae which show succeeding stages 

 from completely dispersed chromatin to 



