Chromosome Banding

By Levi Clancy for Student Reader on
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The 24 types of human chromosome can be distinguished by different staining procedures. Each chromosome has a unique banding pattern, a distinctive pattern of dark bands (stained regions) and light bands (unstained regions).

Banding of condensed metaphase chromosomes reveals about 450 different bands. Based on the banding pattern and the location of the centromere, chromosomes can be readily identified. High-resolution banding is G- or R-banding of less condensed chromosomes, revealing 550 to 850 bands (as opposed to 450, as mentioned above). Usually involving prophase or prometaphase chromosomes, high-resolution banding helps pinpoint structural abnormalities.

G banding (Giemsa staining)

G Banding (aka Giemsa Staining) is the treatment of chromosomes with trypsin (to remove chromosomal proteins) and then staining with giemsa. Each chromosome pair stains in a distinctive pattern of light and dark bands. The dark bands are called G bands, and roughly correlate to base-pair composition (GC or AT) and repetitive DNA sequences.

Q banding (Quinacrine staining)

Q Banding involves staining a chromosome with quinacrine mustard, and then examination by fluorescent microscopy. There are bands which brightly fluoresce and bands which are dim. The dim bands are Q bands, and correspond almost exactly to G bands. Q Banding is useful for detecting heteromorphisms.

R banding

If the chromosomes are treated (using heat or special chemicals) before staining, their dark and light bands are reversed. When G and Q bands stain poorly, then R banding is used to increase contrast and provide better readability.

C banding

C banding stains centromeres and constitutive heterochromatin. Heterochromatin is chromatin which remains condensed and stains darkly even in interphase (nondividing) cells.

Chromosome painting

Not actually a form of banding, chromosome painting involves hybridization of each chromosome using a chromosome-specific with a unique combination of fluorescent dies. The provides a colorful array of chromosomes, each one painted a different color.