Pattern of Segregation of Labeled Dna at the Chromatid Level of Chromosomes With Diffuse Centromeres

Abstract

After pulse labeling IL7~zzdln pzlr/litrrn shoot meristems \vith "H-thymidine, the cell cycle and its subdivisions \\rere determined autoradiographically using the labeled mitoses method. T h e cycle time was 20 hr. GI = 4.3 hr; C = 9.5 hr; G, = 2.9 hr; and A4 = 3.3 hr. In subsequent experiments seedlings were labeled for 4 t o 5 hr, long enough to label the chron~osomcs' full lengch, eliminating problems associated with asynchronous DhTA replication. T h e longer labeling period did not alter the cell cycle parameters.Cells were designated as being in the first or second division after labeling by the time from the midpoint of the labeling period. Seedlings were placed in 0.1% cycloheximide solution to cause sister chromatids in metaphase cells to separate from each other. Autoradiographs were then made to determine segregation of labeled D N A at the chromatid level. A t the first division after labeling all chromatids appearcd to be labeled. A t the second division after labcling \\.it11 "H-thymidine the labcling pattern was consistent \\.it11 serniconscrvative segregation of labeled D N A and with sister chromatid exchange.These results are discussed in relation to cytological and photometric studies on chromosome strandedness in L71-rrIn and in other organisms with chromosomes \\it11 diffuse ccntronlcrcs.

ASPECTS OF THE MECHANISM OF COLCHICINE RESPONSE IN ALLIUM CEPA AND LUZULA PURPUREA. I. CYTOLOGY AND 3H-COLCHICINE AUTORADIOGRAPHY

“…However, it was also observed (Nelson 1969) that in the determination of the duration of the cell cycle by the labeled mitoses method, the curve of per cent labeled metaphases vs. time never reached 100%. This suggests a population of cells in the meristem with a cycle longer than 20 hr.…”

“…The cell cycle in Luzula purpiirea shoot meristems is 20 hr long (Nelson, 1969); 72 hr represents about three and one-half cell cycles after colchicine treatment. Arrested metaphases with as many as 48 chromosomes (ploidy level = 16x) have been seen at this time.…”

“…Luzula and Allium responded very differently to identical treatments with colchicine, although both have total cell cycles of about 20 hr as determined by the labeled mitosis method ( Van't Hof, 1965;Nelson, 1969). In Allium, concentrations ranging from 1.25 X lo-' to 1.25 X lo-' M produced some mitotic arrest after a 2-hr treatment, the effect being greatest at the highest concentration.…”