GENES, MOLECULES AND PROCESSES 



acid is tound only attaciicd to chroinosonics and in bacteria and 

 certain animal and bacterial viruses. In all of these it must indicate 

 the presence of genetically analogous structures. 



At low temperatures in plants, or with low feeding in animals, 

 the chromosomes can be starved of nucleic acid. Two consequences 

 follow. First the chromosomes, especially their end genes, fail to 

 reproduce, or at least to separate the products of their reproduction, 

 in mitosis. The daughter chromosomes consequently sometimes 

 stick together at anaphase. Secondly, certain short segments of the 

 chromosomes are undercharged and therefore fail to stain. In 

 extreme cases the coiling or spiralization of the chromosome thread 

 fails. It remains uncoiled at metaphase. Such a failure of coiling is 

 usual during rapid mitoses in certain protozoa but would be highly 

 inconvenient in the specialized cells of higher organisms if it 

 affected the whole chromosomes. Thus the attachment of nucleic 

 acid regulates the reproduction of the chromosomes, possibly, as 

 Astbury suggests, by acting as a template. It also locks them up 

 in spiral bundles; and these are not merely neat bundles for 

 movement and distribution. They are strong bundles protected 

 against the dangers of extra-nuclear life, even, as experiment 

 shows, against being broken by X-ray ionization or chemical 

 poisons. 



The uncoiling of the chromosomes at telophase depends on their 

 throwing off this coat of nucleic acid. How do they do this? 

 Evidently the nucleic acid is depolymerized and broken down. At 

 the same time the products of action of the genes, now uncovered, 

 fill the nucleus; and the simple proteins produced, together with 

 the ribosc nucleic acid derived by reconstruction from the 

 desoxyribose, are assembled by a particular gene or organizer to 

 reconstitute a nucleolus. We now fmd, however, that in some cells 

 of some organisms certain constant parts of the chromosomes do 

 not throw off their coat. The nucleic acid becomes sticky, owing 

 perhaps to depolymerization, but remains attached throughout the 

 resting stage. These parts are known as heterochromatin as opposed 

 to the normally behaving euchromatin. It is these parts or blocks 

 which in some organisms, under the conditions we saw, are liable to 

 be starved of nucleic acid at mitosis. 



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