184 



( HAPTER 13 



the ends produced h\ separate breaks exist 

 simultaneously and are able to cross-unite. 

 But when the same dose is given more 

 slowly, the pieces of the fust break ma\ 

 restitute before those o\ the second arc pro- 

 duced, thus eliminating the opportunity for 

 cross-union. In this event, the same dose 

 produces fewer gross rearrangements when 

 given in a protracted manner than when 

 given in a concentrated manner. Although 

 this dose-rate dependence for X rays is true 

 for most cells — at least during part of the 

 interphase stage — it does not apply to ma- 

 ture sperm of animals, probably including 

 man. In these gametes and during most of 

 nuclear division in other cells, the broken 

 pieces cannot join each other (sec p. 166) 

 and. therefore, accumulate. For this rea- 

 son, it makes no difference how quickly 

 or slowly the dose is given to the chromo- 

 somes in such a sperm head, since the breaks 

 remain unjoined at least until the sperm 

 head swells after fertilization. 



As already mentioned, the spatial arrange- 

 ment of chromosomes with respect to each 

 other influences the number of breaks and 

 the kinds of structural changes they produce. 

 It should be noted that the possibilities for 

 multiple breakages and for joinings are quite 

 different for chromosomes packed into the 

 tiny head of a sperm than they are for chro- 

 mosomes located in a large nucleus. But 

 even within a given type of cell, a number 

 of other factors can influence breakage or 

 rejoining, such as the presence or absence 

 of a nuclear membrane, the degree of spiral- 

 ization 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, centrif- 

 ugal force, and vibration. 



In cells whose chromosomes have just 

 replicated and in somatic or meiotic cells 



where homologS arc synapscd, a special re- 

 striction on the movements of the pieces is 

 produced when only some of the apposed 

 strands are broken (see p. 166). In this 

 situation, the forces which keep parts of one 

 strand adjacent to the corresponding parts 

 of its sister or homolog may prevent the 

 broken pieces from moving apart freely, so 

 that the unbroken strand or strands serve as 

 a splint for the broken one(s) and reduce 

 the opportunities for cross-union. Many 

 factors exist, therefore, which determine to 

 what degree chromosome and chromatid 

 fragments can move or spring apart; those 

 affecting the distances between different 

 chromosomes or the parts within a chromo- 

 some also affect chromosome and chromatid 

 breakability. 



The frequencies and types of structural 

 changes depend also upon the total amount 

 of chromosomal material present in the nu- 

 cleus and the number and size of the chro- 

 mosomes into which this material is divided. 

 The rearrangements that occur in different 

 cells of a single individual depend upon 

 whether the cell is haploid, diploid, or poly- 

 ploid, and whether or not the chromosomes 

 are polynemic, are in the process of replica- 

 tion, or are otherwise metabolically active. 



Radiation can produce important non- 

 mutating effects upon the chromosomes by 

 damaging nonchromosomal cellular compo- 

 nents which, in turn, affect chromosomal be- 

 havior and function. If the cells are capable 

 of repairing such nonchromosomal, struc- 

 tural or functional damage, they will have a 

 longer time in which to repair when a radia- 

 tion dose is given slowly than when given 

 quickly. The most obvious example is the 

 effect of radiation upon mitosis (and prob- 

 ably meiosis). Cells at about midprophase 

 or a later stage in nuclear division usually 

 complete the process even though irradiated. 

 Cells no farther advanced than about mid- 

 prophase often return to interphase when 

 irradiated. For this reason, ionizing radia- 



