Figure 4–50 The entire set of polytene chromosomes in one Drosophila salivary cell. In this drawing of a light micrograph, the giant chromosomes have been spread out for viewing by squashing them against a microscope slide. Drosophila has four chromosomes, and there are four different chromosome pairs present. but each chromosome is tightly paired with its homolog (so that each pair appears as a single structure), which is not true in most nuclei (except in meiosis). Each chromosome has undergone multiple rounds of replication, and the homologs and all their duplicates have remained in exact register with each other, resulting  in huge chromatin cables many DNA strands thick.
The four polytene chromosomes are normally linked together by heterochromatic regions near their centromeres that aggregate to create a single large chromocenter (pink region). In this preparation, however, the chromocenter has been split into two halves by the squashing procedure used. (Adapted from T.S. Painter, J. Hered. 25:465–476, 1934. with permission from Oxford University Press.)
Figure 4–51 Micrographs of polytene chromosomes from Drosophila salivary glands. (A) light micrograph of a portion of a chromosome. The DNA has been stained with a fluorescent dye, but a reverse image is presented here that renders the DNA black rather than white; the bands are clearly seen to be regions of increased  DNA concentration. This chromosome has been processed by a high-pressure treatment so as to show its distinct pattern of bands and interbands more clearly.
(b) An electron micrograph of a small section of a Drosophila polytene chromosome seen in thin section. bands of very different thickness can be readily distinguished, separated by interbands, which contain less condensed chromatin. (A, adapted from D.v. Novikov, I. kireev  and A.S. belmont, Nat. Methods 4:483– 485, 2007. with permission from  macmillan Publishers ltd; b, courtesy  of veikko Sorsa.)
Figure 4–52 RNA synthesis in polytene chromosome puffs.
An autoradiograph of a single puff in a polytene chromosome from the salivary glands of the freshwater midge Chironomus tentans. As outlined in Chapter 1 and described in detail in Chapter 6, the first step in gene expression is the synthesis of an RNA molecule using the DNA as a template. The decondensed portion of the chromosome is undergoing RNA synthesis and has become labeled with 3H-uridine, an RNA precursor molecule that is incorporated into growing RNA chains. (Courtesy of José bonner.) whereas the so-called Polycomb form of heterochromatin contains a similar number of proteins of a different set (PcG proteins). In addition to the five major chromatin types, other more minor forms of chromatin appear to be present, each of which may be differently regulated and have distinct roles in the cell.
The set of proteins bound as part of the chromatin at a given locus varies depending on the cell type and its stage of development. These variations make the accessibility of specific genes different in different tissues, helping to generate the cell diversification that accompanies embryonic development (described in Chapter 21).
Chromatin loops Decondense when the genes within Them Are Expressed
When an insect progresses from one developmental stage to another, distinctive chromosome puffs arise and old puffs recede in its polytene chromosomes as new genes become expressed and old ones are turned off (Figure 4–52). From inspection of each puff when it is relatively small and the banding pattern is still discernible, it seems that most puffs arise from the decondensation of a single chromosome band.