Chromomeres and genes.

Abstract

The giant cross-banded chromatin “threads” found in the nuclei of various Dipteran cell types had caught the imagination of many cytologists even before their chromosomal nature was established almost forty years ago (Heitz and Bauer, 1933; Painter, 1933; King and Beams, 1934). The revelation that these structures simply presented an enormously magnified but faithful picture of the linear subdivision of the mitotic interphase chromosomes suddenly changed mere curiosity into enthusiasm: In the words of Painter (1934) “it was clear that we had within our grasp the material of which everyone had been dreaming. It was clear... that the highway led to the lair of the gene.” We must admit today that the goal defined in these prophetic words has not yet been reached, and we cannot even claim that the highway towards this goal has been found, in spite of the concerted efforts of two generations of cytologists, cytochemists, and geneticists to localize and characterize genes in polytene chromosomes. This is not the fault of the material itself, nor is it due to lack of skill on the part of the investigators. Progress has been blocked both by problems of methodology and by the difficulty in defining the basic questions of chromosomal and genetic subunits in physical-chemical terms. Adequate micromethods to study the composition and activity of individual genetic units in polytene chromosomes are only now beginning to become available.