Published in: Proc. Natl. Acad. Sci. USA, vol. 49, pp. 581-589 (May 15, 1963):

"Human Sex Chromosome Abnormalities in Relation to DNA Replication and Heterochromatinization".

Melvin M. Grumbach, Akira Morishima, and J. Herbert Taylor

Department of Pediatrics, College of Physicians and Surgeons, Columbia University, and Departments of Botany and Zoology, Columbia University, New York, NY USA


Summary:

Tritiated thymidine and autoradiography were utilized to study the sequence of DNA replication in X-chromosomes of the human complement. Five individuals with extra X-chromosomes were found to have one X which replicates with the majority of the complement while all others replicate late. Evidence is presented which indicates that these late-replicating X-chromosomes are heteropycnotic in interphase and their genetic expression is repressed. Two patients with a structurally abnormal X were found to have this chromosome consistently heteropycnotic and late-replicating. These observations are consistent with the "fixed differentiation hypothesis" of X-chromosome behavior.


Additional References:

1. "Mechanisms of Repression and De-Repression within Interphase Chromatin".

2. "Activation of DNA Transcription within Repressed Chromatin".


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