Incorporation of tritium-labeled thymidine and lysine into chromosomes of cultured human leukocytes.

The incorporation of thymidine-H(3) and lysine-H(3) into human leukocyte chromosomes was studied in order to determine the temporal relationships between the syntheses of chromosomal deoxyribonucleic acid and chromosomal protein. The labeled compounds were incorporated into nuclei of interphase cells. Label from both precursors became apparent over the chromosomes of dividing cells. Incorporation of thymidine-H(3) occurred during a restricted period of midinterphase (S) which was preceded by a nonsynthetic period (G(1)) and followed by a nonsynthetic period (G(2)). Incorporation of lysine-H(3) into chromosomal protein occurred throughout interphase. Grain counts made over chromosomes of dividing cells revealed that the rate of incorporation of lysine-H(3) into chromosomal protein differed during various periods of interphase. The rate of incorporation was diminished during G(1). During early S period the rate of incorporation increased, reaching a peak in late S. The high rate continued into G(2). Thymidine-H(3) incorporated into DNA was distributed to mitotic chromosomes of daughter cells in a manner which has been referred to as a "semi-conservative segregation." No such semi-conservative mechanism was found to affect the distribution of lysine-H(3) to the mitotic chromosomes of daughter cells. Therefore, it is concluded that synthesis of chromosomal protein and its distribution to chromosomes of daughter cells are not directly influenced by synthesis and distribution of the chromosomal DNA with which the protein is associated.