Differential Deoxyribonucleic Acid Replication in the Giant Foot-pad Cells of Sarcophaga bullata

Abstract

GIANT polytene chromosomes from the salivary glands of various fly larvae have been extensively examined; because of their enormous size they have become one of the most important tools in investigations of gene activity and nuclear-cytoplasmic relationships. Malpighian tubules and certain other tissues of flies also possess polytene chromosomes, but they show a lower degree of polyteny. It is generally accepted that in most animal and plant cells there is a constant relation between the DNA content and the number of chromosome ‘sets’ it contains. Synthesis of DNA results in a doubling of this pre-existing amount; a dipteran polytene chromosome may result from more than a thousand such replications¹. Recently it has been suggested² that differentiation in a multicellular organism involves the progressive inactivation of DNA, and that, in the polytene chromosome, three conditions characterize these inactivated regions: first, inactivity with respect to RNA synthesis; secondly, late replication, replication being slower and continuing longer than over the rest of the chromosome; thirdly, the appearance of dense packing. Heavy bands and heterochromatic regions are given as typical examples of such regions.