x-ray diffraction photographs of sperm heads , extracted nucleoprotamine , calf thymus nuclei and extracted nucleohistone , and of chicken erythrocyte nuclei , are not all as well defined as those given by extracted dna , but it is clear from the general characteristics of the pattern that much of the dna bound to protein in these nuclei has the usual helical configuration , and that the double-helical structure of dna exists in the cell and is not an artifact .
C0007610
nuclei
C0007634
cell