Natural and Induced Chromosomal Changes 



181 



arrangement of chromosomes relative to 

 each other will influence the number of 

 breaks and the kinds of structural changes 

 these produce. It should be noted, specifi- 

 cally, that when the chromosomes are packed 

 into the tiny head of a sperm, the possibilities 

 for multiple breakages and for later joinings 

 may be quite different than they are for chro- 

 mosomes located in a large nucleus. But 

 even within a given type of cell, there are a 

 number of other factors which may influence 

 breakage or rejoining. These include the 

 presence or absence of a restrictive and in- 

 sulating nuclear membrane, the degree of 

 spiralization of the chromosomes, the stress or 

 tension under which the parts of a chromo- 

 some are held, the degree of hydration, the 

 amount of matrix in which the genes are 

 embedded, protoplasmic viscosity and the 

 amount of fluid and particulate movement 

 around the chromosomes, gravity effects, 

 centrifugal forces, and vibrations. One 

 special restriction to the movements of 

 broken pieces occurs in cells whose chromo- 

 somes have just divided, and in meiotic cells 

 where homologs synapse. For under these 

 conditions, the forces, which keep like parts 

 of one strand apposed to the like parts of its 

 sister or homolog, may prevent the pieces 

 newly produced by breakage from moving 

 apart freely, so that the unbroken strand or 

 strands serve as a splint for the broken one, 

 reducing the opportunities for cross-union. 

 All these factors determine to what degree 

 chromosome fragments may move or spring 

 apart; those affecting the distances between 

 different chromosomes, or the parts within a 

 chromosome, may also affect chromosome 

 breakability. 



The frequencies and types of structural 

 changes will depend also upon the total 

 amount of chromosomal material present in a 

 nucleus and the number and size of the chro- 

 mosomes into which this is partitioned. The 

 rearrangements occurring in different cells of 

 a single individual will depend upon whether 



the cell is haploid, diploid, or polyploid, and 

 whether or not the chromosomes are in the 

 process of replication and are metabolically 

 active. 



Finally, there are two different types of 

 chromosomal material and each has a differ- 

 ent capacity to produce structural changes. 

 Most of the chromosomal material in the 

 genome reacts similarly to certain staining 

 procedures, and is said, therefore, to be 

 euchromatic (truly or correctly colored). 

 Other portions of the chromosomes stain 

 either darker or lighter than this, and are said 

 to be heterochromatic. Although some hetero- 

 chromatin may be located at various places 

 along the chromosome arm, it is normally 

 found in both arms adjacent to the centro- 

 mere and, to a lesser extent, near the telo- 

 meres. The genes in heterochromatin func- 

 tion to produce large blocks of chromosomal 

 material in mitotic and meiotic chromosomes, 

 so that their contribution to the size of the 

 chromosome, as seen at metaphase, is rela- 

 tively greater per gene than it is for genes 

 located in euchromatic regions. 



Besides its special stainability, location, 

 and function, heterochromatin has a fourth 

 characteristic, in that it is less specific in 

 synapsis than is euchromatin; heterochro- 

 matin located at different places along a 

 chromosome or in nonhomologous chromo- 

 somes is often found synapsed. (Thus, for 

 example, in the salivary gland nuclei of Dro- 

 sophila larvae, the heterochromatic regions 

 proximal to the centromeres of all chromo- 

 somes synapse to form one mass, called the 

 chromocenter. Also, the heterochromatic 

 regions near the telomeres are sometimes 

 found in synapsis with the chromocenter.) 

 Relative to the number of loci present, the 

 fifth fact is observed, that radiation-induced 

 structural changes more frequently involve 

 heterochromatic reg'ons than they do eu- 

 chromatic ones. Whether this excess is 

 to be attributed to a greater breakability or 

 lesser restitutability, or both, of heterochro- 



