Polytene structures

Transversal banding pattern

A Transversal banding pattern of the polytene chromosomes could not be observed regularly but only rarely in about one to two chromosomes per nucleus. The pattern usually does not expand the entire length, but was restricted only to part of the chromosome (figure a) and variable size.

Longitual structure

The typical composition of polytene chromosomes consisting of numerous chromatids laying densely and more or less parallel side by side is in general not visible in the light microscope due to it's low resolution. Sometimes, however, when the euchromatin in very decondensed, and especially the one of chromosome A, it can let imagine the bundling of many chromatids (figure b).

Figure: Transversal banding pattern and longitual structure of the polytene chromosomes of Phaseolus coccineus cv. Preisgewinner after fluorchrome staining with DAPI. (a) Transversal banding pattern in part of a polytene chromosome. The magnification shows the area with the banding pattern both in phase contrast as well as after stainging with DAPI. (b) Euchromatin of chromosome A that appears very decondensed and let imagine the longitual structure that constist of many chromatids laying side by side.
Scale bar represents 10 µm.