Positive and negative birefringence in chromosomes

By using the optical properties of birefringence of DNA, the arrangement of these molecules has been studied in Dinoflagellate chromosomes and Dipteran polytene chromosomes. These latter are used, here, as a reference material. These observations have been made under a polarizing microscope on intact and stretched chromosomes. — Intact Dinoflagellate chromosomes show a positive birefringence, in contrast with polytene chromosomes bands which are negatively birefringent. From these observations one can deduce the preferential orientation of DNA filaments, in Dinoflagellates, normal to the chromosome axis, and in polytene chromosomes parallel to the same axis. — After stretching, these two kinds of chromosomes are negatively birefringent. In both cases, DNA molecules have been aligned along the stretch axis. — In Dinoflagellate chromosomes the passage from a positive to a negative birefringence is realized without any isotropic stage. The intermediary state presents a biaxial structure.