An integrated description of polytene chromosome structure

An integrated description from nucleohistone fiber level to light microscopic level of polytene chromosomes is presented. The integration is based on the results obtained by different new applications of electron microscopic techniques and on the analysis of earlier data of chromatin structure. A model for the organization of polytene chromosomes in vivo is extrapolated from the EM‐data by approximating the effects of the physico‐chemical parameters changing during preparative procedures. The nucleosome unit fiber forms loops, which are organized into about 500Å diameter coils with 24 nucleosomes per turn of the double fiber, leading to an overall packing ratio of about 1:78. The coils are packed differentially into structural units forming the polytene bands, which are separated from each other by interbands of unit fibers with no higher order coiling evident. For the general higher order structure of chromatin a model capable of explaining former observations in other eukaryotic chromosome systems is proposed. A whole mount method suitable for nucleosome fiber examination in polytene chromosomes is introduced. A sectioning method preserving the nucleosomal structures and a quick method for whole mounting polytene chromosomes retaining their light microscopic morphology are also described.