RNA Synthesis on Giant Chromosomes

Differential gene activity can be demonstrated on the chromosomes themselves provided suitable objects, such as giant chromosomes and lampbrush chromosomes, are chosen and adequate methods are used. 

Giant chromosomes are found, inter alia, in the salivary glands of Dip-/emsuch as Drosophila, Chironomus, Acricotopus, and Sciara. They are bundles of a few thousand homologous chromatids laid down in parallel and, at the same time, slightly skewed like the strands of a rope (Fig. 143). Each chromatid appears to be divided lengthwise chromomeres appear to be strung on the chromonema like a set of beads. 

Puffs are not fixed structures. In the course of development puffs that are already in existence on a given giant chromosome contract and new ones are formed. Differences in puff formation on any given giant chromosome are also found between one tissue and another. In puff patterns of this kind, which are specific for the tissue and stage of development, differential gene activity becomes microscopically visible. 

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