The nuclear DNA content and chromatin ultrastructure of the coelacanth Latimeria chalumnae

By measurement of 731 erythrocytes by Feulgen cytophotometry, the nuclear DNA content of the coelacanth Latimeria chalumnae Smith is determined to be 7.22 picograms (pg). This value is high among fishes but is closely comparable to that of man and most other mammals. The average mass of erythrocyte nuclear chromatin, measured by quantitative electron microscopy, is 15.2 pg. This chromatin is in the form of fibers having a mean diameter of 202 Å. The average weight of the chromatin fiber is 6.75 × 10^?16 g/μm. Thus, the nucleus contains 22 500 μm of chromatin fiber. Dividing the nuclear DNA content of Latimeria by the known mass of the DNA double helix (3.26 × 10^?18 g/μm) gives a total length of 2 215 000 μm of DNA double helix. In comparing these two measurements of structural length, it is found that 98.4 lengths of double helix are packed into one length of chromatin fiber. This packing ratio is over three times greater than that of human G1 lymphocytes. The difference may be attributable to the difference between the two tissues and thus reflect a functional distinction, or it may be due to the difference between the two species and reflect an evolutionary distinction.