The double-stranded DNA replication intermediates formed in Chironomus polytene chromosomes consist of two populations.

DNA replication in Chironomus larvae that 7 days earlier passed the red-head stage is investigated. When pulse-labelled polytene chromosomes are lysed at 25 degrees C there is an enzymatic release of double-stranded DNA replication intermediates from the chromosomes. The released DNA molecules can be divided into two populations by gel electrophoresis in 1.5% agarose gels, one population is centered around slice 18 and the other population os centered around slice 27. Moreover, it is possible to chase with cold thymidine the population of labelled DNA molecules centered around slice 27 into the population centered around slice 18. The intermediates are after a time-lag joined together to produce a high molecular weight DNA which is not released from the chromosomes during cell lysis. Furthermore, in animals treated with 5-fluorodeoxyuridine the intermediates are joined together to produce a double-stranded DNA with the size expected to replicons. In contrast it is known that in late fourth instar larvae there is no subfractionation in 1.5% agarose of the DNA replication intermediates. This indicates that there is a difference in the formation of intermediates in the same tissue at two developmental stages.