CHAPTER 7 



GENES, MOLECULES AND PROCESSES 



Chromosome Structure Chromomeres and Proteins HctcrocJiromatin and Polygenes 



What Major Genes do The Interaction of Genes The Expression oj Genes 



Synthetic Seq^uences: Supply and Demand 



We have learnt something of the resemblances and differences 

 that occur between parents and their offspring, and of the 

 materials and processes which are ultimately responsible for these 

 relationships. But we also have to discover how these materials, 

 lying in the cells of animals and plants, are so organized that they 

 determine the effects we observe, and by what steps they do so. How 

 does the cell nucleus with its outfit of genes and chromosomes act 

 on the cell with the constancy that we recognize in heredity; 

 repeating in each generation the elaborate processes of development 

 which seem to contradict all possibility of a constant basis ? What 

 part does the cytoplasm play in this paradox ; To these questions we 

 must now address ourselves. 



Chromosome Structure 



The nucleus, as we have seen, is a body constituted of chromo- 

 somes. The chromosome is a body constituted of a protein fibre to 

 which active groups or genes are attached. Pepsin disintegrates this 

 fibre and it must therefore be based on a polypeptide chain. This 

 chain may be single or multiple but it must be highly folded in the 

 ordinary resting nucleus since, when the nucleus grows (as in the 

 salivary glands), the chain itself stretches to many times its early 

 prophase length. 



During mitosis, nucleic acid is attached to the genes which, owing 

 to the folding of the thread between them, form an almost con- 

 tinuous chain. The nucleic acid consists of an unlimited polymeriza- 

 tion of desoxyribose nucleotides to form a colunm. The nucleotides 

 are spaced at the same distance of 3 • 3 Angstrom units apart as are 

 the repeats in the stretched polypeptide chain. Desoxyribose nucleic 



Elements of Genetics 145 K 



