Scanning electron microscopy of polytene chromosomes (I).

The structure of polytene chromosomes has been observed by conventional scanning electron microscopy and also with a high resolution 'in lens' field emission instrument. Surface imaging with secondary electron emission has characterized condensed chromatin and regions known for their RNA synthetic potential (nucleoli, Balbiani rings and puffs). In DNA-rich bands pairing of chromatids appears so perfect that individual chromomeres cannot be visualized as discrete units. In the interbands only chromatid bundles appear as elements, but not individual chromatids. High resolution scanning electron microscopy allows resolution at the nucleosome level. Improved localization of chromosomal structures is demonstrated, as in the case of the proposed separation of the prepupal 'Balbiani ring 1'. Surface images of the RNA synthetic centres of the salivary gland cell are presented. Transcripts of the Balbiani ring template can be observed in the condensed and gradually unfolded state, allowing measurement of unit ribonucleoparticles. The surfaces of puffs have been visualized and are characterized by multiple supercoiled loop structures, which are described according to size, conformation and distribution.