Dark bands on chromosomes, often referred to as G-bands, are caused by the differential staining of chromosomal regions. These bands appear when chromosomes are treated with a dye called Giemsa stain. The staining process reveals patterns of light and dark bands that are characteristic of each chromosome.
The dark bands correspond to regions of the chromosome that are rich in adenine (A) and thymine (T) base pairs. These regions are typically more condensed and contain fewer genes. In contrast, the light bands are regions that are rich in guanine (G) and cytosine (C) base pairs, which are less condensed and contain more genes.
The specific pattern of these bands helps scientists identify and study chromosomes, as each chromosome has a unique banding pattern. This technique is widely used in karyotyping, which is the process of examining chromosomes to detect genetic abnormalities.