The Principles of Clinical Cytogenetics - Extra Materials - Springer

72 Martha Keagle and Steven Gersen
Fig. 3. Q-Banding. The fluorescence banding pattern is essentially the same as with G-banding. Note, however,
the bright fluorescence on the long arm of the Y chromosome (arrow).
Techniques that Stain Selective Chromosome Regions
C-Banding (Constitutive Heterochromatin Banding)
C-banding techniques selectively stain the constitutive heterochromatin around the centromeres,
the areas of inherited polymorphisms present on chromosomes 1, 9, and 16, and the distal long arm of
the Y chromosome. C-band-positive areas contain highly repetitive, late replicating sequences of
α-satellite DNA. The function of constitutive heterochromatin is not understood, but it is stable and
highly conserved evolutionarily.
With CBG banding (C-bands, by barium hydroxide, using Giemsa), the DNA is selectively
depurinated and denatured by barium hydroxide, and the fragments are washed away by incubation
in a warm salt solution. Constitutive heterochromatin resists degradation and is, therefore, the only
material left to bind with the Giemsa stain. The result is pale, almost ghostlike chromosomes with
darkly stained areas around the centromeres, at the pericentromeric polymorphic regions of chromosomes
1, 9, and 16, and at the distal Y long arm (see Fig. 5). C-banding is useful for determining the
presence of dicentric and pseudodicentric chromosomes, and also for studying marker chromosomes
and polymorphic variants.
T-Banding (Telomere Banding)
T-banding is an offshoot of R-banding that results in only the terminal ends or telomeres of the
chromosomes being stained. A harsher treatment of the chromosomes diminishes staining except at
the heat-resistant telomeres. There are fluorescent and nonfluorescent T-banding techniques.