Chronology and Pattern of Human Chromosome Replication. II. Autoradiographic Behavior of Various Y and X Chromosomes2

Abstract

Auto-radiographic techniques were used for the study of the deoxyribonucleic acid (DNA) replication pattern of sex chromosomes in normal and abnormal human cells. In normal male cells the Y was the last chromosome to replicate its DNA, the grain content being 4 to 5 times that of the G-group chromosomes, with which the Y chromosome may be confused. In subjects with 2 Y chromosomes, 1 replicated early and the other very late. The autoradiographic behavior of a long Y in a subject with acute leukemia was similar to that of the normal Y. Analysis of the length and arm ratio of the X chromosome revealed it to be similar in length to autosome C8 and not C6, as has been generally held. These studies may serve as a possible model for a more rigorous morphologic definition of the various chromosomes and may explain the asynchronous DNA replication of chromosomes when more than one of a type is present in a cell.