What do the dark bands produced by routine GTG banding represent?

The dark bands produced by routine GTG (Giemsa-Trypsin-Giemsa) banding represent regions that are rich in adenine-thymine (AT) base pairs, which often correlate with gene-poor areas of the chromosome. During the banding process, chromosomes are treated with trypsin and then stained with Giemsa dye; the AT-rich regions absorb the dye more strongly, leading to the formation of these dark bands.

These bands are important for the identification and classification of chromosomes during cytogenetic analysis, as they provide a framework for recognizing specific chromosomal abnormalities. The presence of dark bands typically indicates less transcriptionally active regions, which contributes to the assessment of gene density and overall genomic structure.

In contrast, the other choices relate to regions that are not characterized by the dark bands produced by GTG banding. Gene-rich regions and euchromatin regions are usually lighter in appearance due to their composition (higher GC content or being more transcriptionally active), whereas gene-poor regions, while they may seem similar to dark bands, specifically refer to the lower density of genes rather than the AT richness that characterizes the dark bands.