Pattern of chromosomal replication in synchronized lymphocytes. I. Evaluation and application of methotrexate block.

The BrdUrd-Hoechst 33258-Giemsa technique was employed to study patterns of chromosomal replication in human lymphocytes synchronized by Methotrexate (MTX). It is proposed that in the presence of MTX, a major portion of the cell population is blocked in an advanced stage of the S-phase and not in the G1/S border of the cell cycle. At this point, the replication of the chromosomal segments corresponding to the R-bands is terminated, and the replication of the G-bands and the inactive X-chromosomes is being initiated. The use of this method in the study of higher resolution patterns of chromosomal replications is proposed.