Double-stranded DNA with the size expected of replicons can be released from Chironomus polytene chromosomes

The effect of the drug 5-fluorodeoxyuridine on DNA synthesis in Chironomus polytene chromosomes was investigated. The DNA was labelled by injection of radioactive precursor into living animals pre-treated with the drug, extracted with an neutral non-denaturing buffer at 25 degrees C and then characterized by gel electrophoresis. After a short pulse a heterogeneous double-stranded DNA population is released from the polytene chromosome. These fragments are later jointed together to produce a double-stranded DNA with a size ranging between 8-13 X 10(6) D. The release of both types of fragments from the polytene chromosome is prevented by lysing the cells at 0 degree C instead of at 25 degrees C. The larger double-stranded DNA has the size expected of replicons in Chironomus. The results are discussed in relation to the organization of replication in polytene chromosomes.