The Temporal Order of Replication of Some DNA Cistrons

Abstract

An important question concerning eukaryotic cells is the biological significance, if any, of the distinct times at which specific regions of the chromosome replicate their DNA during the S period. For example, heterochromatic regions have been found to be late replicating (for a review see Lima-de-Faria and Jaworska, 1968). Evidence has been presented suggesting that heterochromatin may not be transcribed (Hsu, 1962; Littau et al., 1964; Berlowitz, 1965) and may exert an inactivating influence on other genes. When chromosome rearrangements occur and an active gene is translocated to a heterochromatic region of a chromosome, the gene may become inactivated (for a review see Baker, 1968). It is not known whether the time of replication of this translocated DNA is shifted to that characteristic of the adjoining heterochromatic DNA.