The composition and structure of isolated chromosomes.

1. The preparation of isolated chromosomes from liver, kidney, and pancreas has been described. 2. It has been shown that there is no gross cytoplasmic contamination in these preparations. 3. In a microscopic study of isolated chromosomes the same chromosomes have been found in different tissues of the same organism. Since individuality is one of the main characteristics of chromosomes, there can be little doubt that the preparations do, in fact, contain isolated chromosomes. 4. A quantitative study of staining with crystal violet shows that this basic dye competes with histone for the phosphoric acid groups of the DNA in chromosomes. The displacement of histone by protamine has been demonstrated. 5. Preparation of histone-free chromosomes has been described. Removal of histone does not affect the microscopic appearance of chromosomes. 6. The non-histone or residual protein has been prepared from histone-free chromosomes. The quantity of residual protein in a preparation of chromosomes is correlated with the amount of cytoplasm in the cells from which the chromosomes were prepared. 7. The microscopic appearance of chromosomes depends upon the association of DNA with residual protein. 8. Evidence has been given that in a chromosome there are two DNA-containing nucleoproteins; in one DNA is combined with histone, and in the other it is combined with residual protein.