Chromosomal replication of Drosophila virilis. II. Organization of active origins in diploid brain cells.

DNA fiber autoradiography was used to determine parameters underlying the DNA replication of the eukaryotic chromosome in Drosophila diploid brain cells organ culture. The average rate of fork movement, estimated from 4 different labelling intervals, is 0.35 micron/min at 25 degrees C. Of the tandem arrays 93% show patterns which are compatible with bidirectional replication, 7% show unidirectional replication. The unidirectional mode of replication is interpreted as being a consequence of the experimental schedule (using hot-cold pulse labelling) combined with the occurrence of termination signals. - Some autoradiograms showed the expected two grain tracks of different densities; others showed only a high density track. The latter were most prominent in arrays of short replicons (less than 10 micron) which correlate with replicating satellite sequences. - The majority of replicons fall into size classes less than 100 micron. The frequency distribution is skewed towards larger replicon sizes; it spans 2-238 micron, has a mean of x = 25.6 micron and a median of approximately 21.0 micron. If the distribution is corrected for supposed satellite replicons, the median increases of approximately 31.0 micron. - In experiments using warm-hot pulse labelling, arrays were scored which must have been a consequence of fixed termination signals. Furthermore, grain tracks diverging from weak labelled centers often have different lengths, indicating that these replicons contain two diverging replicating sections of unequal length.