Chapter *39 



REGULATION OF GENE 



ACTION-DOSAGE 



COMPENSATION 



Mammals 



Because DNA synthesis occurs almost with- 

 out interruption in the relatively uncoiled 

 chromosome of E. coli, we may suppose 

 that, in general, uncoiled chromosomes 

 can synthesize complementary DNA and 

 RNA. On the other hand, DNA synthesis 

 does not occur during mitosis or meiosis 

 when the chromosomes are highly con- 

 densed. Consequently, we may hypothesize 

 that a chromosome cannot function in DNA 

 or RNA synthesis while coiled. If these 

 premises are acceptable, we may consider 

 that during interphase the presence of heav- 

 ily Feulgen stained, clumped, chromosomal 

 material (chromatin knots or karyosomes) 

 is an indication of coiled chromosomal ma- 

 terial and. therefore, the total absence of 

 genetic activity in such bodies. 



That certain chromosomes or chromo- 

 somal regions are highly clumped or coiled 

 while others are not is probably correlated 

 with their different times for DNA replica- 

 tion. Chromosomes or chromosomal re- 

 gions which are normally coiled and stained 

 (eupycnotic), relatively overcoiled and over- 

 stained (hyperpycnotic), and relatively un- 

 dercoiled and understained (hypopycnotic) 

 differ in their time of DNA replication. 

 Abnormal staining, heteropyknosis, is one 

 of the characteristics of heterochromatin (p. 

 155). 



The gene action hypothesis presented 

 484 



above can be tested by comparing the in- 

 terphase activity of genes when a given 

 chromosomal region is normally coiled and 

 when it is overcoiled. Manx of the diploid 



interphase nuclei in human males show a 

 small karyosome touching the nuclear mem- 

 brane; in the human female the same cells 

 show a similar but much larger karyosome. 

 Because the size of this chromatin knot dif- 

 fers in each sex. the extra karyosome ma- 

 terial in the female is called sex chromatin, 

 or the Barr body (after its discoverer). 

 The presence of sex chromatin in individ- 

 uals aneusomic with regard to sex chromo- 

 somes has been investigated. The maxi- 

 mum number of separate Barr bodies found 

 are: none in XY and XO individuals; one 

 in XX, XXY, XXYY; two in XXX, 

 XXXY; and three in XXXX individuals. 

 Cells with less than the maximum number 

 have fewer and, accordingly, larger Barr 

 bodies. We may conclude, therefore, that 

 the maximum number of Barr bodies is one 

 less than the number of X chromosomes in 

 a diploid individual. Tetraploid cells of a 

 male probably have no valid Barr body; 

 the larger karyosome reported is attributed 

 to the union between the small karyosomes 

 of the two X's. That tetraploid cells of a 

 female have two Barr bodies suggests the 

 maximum number of Barr bodies is deter- 

 mined by the balance between the number 

 of X chromosomes and the number of auto- 

 some sets. One X chromosome balanced 

 by two sets of autosomes does not give rise 

 to a Barr body. However, each X in ex- 

 cess of this balance clumps and is either 

 detected as a separate Barr body or is fused 

 with other excess X's to form larger bodies. 

 Hence the maximum number of Barr bodies 

 is equal to x-(p 2). where x is the num- 

 ber of X chromosomes, and p is the ploidy 

 or number of sets of autosomes.' Note that 

 the Y chromosome has no influence upon 



1 According to D. G. Harnden. 



