18 μm replication units of chromosomal DNA fibers of differentiated cells of Pea (Pisum sativum)

A technique is presented for investigating possible early terminalisation of chiasmata, based on the analysis of labelling patterns in autoradiographs in X₂-labelled diplotene bivalents. Terminalisation is expected to produce unlabelled gaps in otherwise labelled bivalents which, given even a moderate amount of movement, is capable of resolution by this technique. The method has been evaluated using diplotene spermatocytes of Schistocerca gregaria as a test system. Very few unlabelled gaps were actually observed in X₂-labelled bivalents, in fact no more than in X₁-labelled bivalents, where chiasma terminalisation is not expected to produce gaps. Consequently it is concluded that the gaps observed are due to technical causes and that early terminalisation is an unimportant factor determining chiasma distribution in this system.