G-banding,
G banding, or
Giemsa banding is a technique used in
cytogenetics to produce a visible
karyotype by staining condensed
chromosomes. It is useful for identifying genetic diseases through the photographic representation of the entire chromosome complement. The
metaphase chromosomes are treated with
trypsin (to partially digest the chromosome) and
stained with
Giemsa stain.
Heterochromatic regions, which tend to be rich with
adenine and
thymine (AT-rich) DNA and relatively gene-poor, stain more darkly in G-banding. In contrast,
less condensed chromatin—which tends to be rich with
guanine and
cytosine (
GC-rich) and more
transcriptionally active—incorporates less Giemsa stain, and these regions appear as light bands in G-banding. The pattern of bands are numbered on each arm of the
chromosome from the
centromere to the
telomere.This numbering system allows any
band on the
chromosome to be identified and described precisely. The reverse of G-bands is obtained in R-banding. Banding can be used to identify chromosomal abnormalities, such as
translocations, because there is a unique pattern of light and dark bands for each chromosome.
