DNA replication of a polytene chromosome section in Drosophila prior to and during puffing.

Polytene chromosome sections 63E1-6 of 3L in Drosophila melanogaster were studied by 3H-uridine and 3H-thymidine autoradiography in late third instar larvae and prepupae. In late third instar larvae 63E does not incorporate 3H-uridine. In prepupae, however, a large puff is formed in 63E which is most active in RNA synthesis. 3h-thymidine labeling patterns and frequencies of regions 61A-64C were analysed and the nonpuffed and puffed 63E sections were compared with reference sections. Both in late third instar larvae and in prepupae 63E shows late replication behavior. It is concluded that the decondensation of chromosome bands does not necessarily entail earlier and/or faster DNA replication.