In order to identify each polytene chromosome the visible features of the centromeric heterochromatin (cHC) were used as crucial characteristic. Other structures like e.g. the NOR, telomeric heterochromatin or secondary constrictions in the euchromatin represent additional features. The length of chromosome arms does not play an important role. Note that the arm ration of the length of the chromosomes with median centromer position can be inverted due stretching during chromosome squashing.
The following images describe each polytene chromosome showing on the left side (a) two representative chromosomes after DAPI-staining and on the right side (b) two polytene chromosomes after DAPI/PI-staining. On each side the region of the cHC is indicated by a bracket while the position of the centromere is labled by a angular line. The nucleolus organizing regions (NOR) is marked by a bracket with a double line. Both sides of the images (a, b) display on the left the chromosome images and on the right the schematic representation as andidiogramm for the given chromosome. The following represenations have been used for presentation of the visual features:
- Chromatin after staining with DAPI (figure a): the intensity of the DAPI fluorescence within the cHC is represented by the relative brightness of gray while the areas of euchromatin are displayed in dark grey. The strong DAPI bands (sDB) have a whilte color corresponding to their high brightness.
- Chromatin after staining with DAPI/PI (figure b): like under the fluorescence microscope the cHC and the telomeric heterochromati (tHC) have a pink color, euchromatic areas a dark blue and the strong DAPI bands, corresponding to their brightness light gray or white.
- Secondary constrictions (sES) are labled by notches on both sides
- Structures that are not visible on a regular base are marked by brackets or in case of the centromer by a dashed line.
- Dot-like structures of condensed tHC are represented by three big dots.
- The NOR is hatched in diagonal and additionally subdivided in a white and a dark region. This division indicates that very often only a part of the NOR is condensed while the other part is decondensed.
The scale bar in all images corresponds to 10 µm.
Features of Chromosome
|Centromeric heterochromatin||The centromeric constriction (Cen) is situated in a median position. Very characteristic for the cHC on the short arm is the large and strong DAPI band (sDB). The cHC on the long arm shows distal a secondary constriction (sES).|
|Euchromatin||The euchromatic part of the short arm usually shows a secondary constriction (sES).|
|Telomeric heterochromatin||Although dot-like condensations of tHC is present on both arms, the one on the long arm is visible most easily if the chromosome is stretched. (compare Fig. a, left chromosome at the bottom).|
|NOR||The nucleolus organizing region is located terminal on the long arm. In most cases, part of the NOR is condensed so much that it is mistaken for heterochromatin (compare also to chromosome A and K).|