Replication of DNA in larval salivary glands of Drosophila after in vivo synchronization.

The replication of DNA in the giant chromosomes in different cells of Drosophila larval salivary glands is asynchronous. A method of in vivo synchronization of the nuclei has been successfully devised by a 5'-fluorodeoxyuridine (FdU) block-release-thymidine chase technique, and the patterns of replication sequences have been examined by 3H-thymidine autoradiography. When the larvae of Drosophila melanogaster are fed on FdU for 48 h, and the block is released thereafter, most cells are found in mid-replication phase (termed 3C). When the larvae are subjected to a chase in normal Drosophila medium (or sucrose), a series of cells arrive at 3C phase about every 8 h. When they are chased in sucrose containing thymidine, the number of cells in 3C phase rises to 70%, and then drops rapidly to 1-2% of all labelled cells. The terminal phases (3D, 2D and 1D) reach a peak between 4-8 h. At 12-14 h of chase the 3D-1D peaks decline and a third peak consisting mostly of the initial phases (DD-1C) is found at 14--16 h. The replication of DNA in polytene chromosomes of Drosophila thus seems to proceed in a regular sequence of DD-3C-1D.