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 terminal position, always clearly
visible, but extremly fragile (see note).
The short arm is almost completely made up of the NOR.
The cHC on the long arm can be divided in three segmente. The first, proximal segment contains the strong DAPI band (sDB) and e secondary constriction (sES).
The following second segment carries an additional but less intense DAPI band. Distal, divided by further secondary constriction (sES) the cHC ends with the third segment displaying a lower flurescence intensity.
|Euchromatin||No particaluar characterstics.|
|Telomeric heterochromatin||Only the short arm displays dot-like condensations of tHC, and only if the NOR is stretched (Fig. b)|
|NOR||The NOR is located on the short arm.|
|Note||If a chromosome K which is attached to a collecting nucleolus (germ. Sammelnukleolus) is squashed too strong, it breaks off frequently at the centromer position (fragile centromere). In this case the short arm remains usually attached to the nucleous while the long arm can be found at some distance from the nucleolus.|