The Z‐conformation, recently elucidated in crystals of synthetic alternating dG‐dC polymers, is a dramatically different structure for DNA. Despite suggestive locations of alternating purine‐pyrimidine tracts in chromosomes and intriguing functional hypotheses, unequivocal demonstrations of the Z‐conformation in vivo are not proving easy. Perhaps the Z‐conformation should be considered largely as a dynamic structure transiently forming in torsionally stressed chromatin.

Cracks in the one‐gene, one‐band paradigm for polytene chromosome organization are widening. At the same time evidence is accumulating suggesting that decondensed regions of the chromosomes (puffs, diffuse bands, interbands and possibly vacuoles within some bands) are generally associated with gene transcription. A model, now on the ascendancy, is based on the proposal that the band‐interband pattern is primarily a reflection of local transcriptional state, rather than the distribution of genic and non‐genic material.