Banding Patterns

Chromosomes in metaphase can be identified using certain staining techniques, so called banding. Cells are cultured and then stopped in metaphase to maximize the number of suitable cells. They are then spread on a slide, stained with a suitable dye and visualized in the microscope. Most conventional cytogenetic analyses depend on the karyotyping of banded metaphase chromosomes.

A band is defined as that part of a chromosome which is clearly distinguishable from its adjacent segments by appearing darker or brighter with one or more banding techniques. The chromosomes are visualized as consisting of a continuous series of bright and dark bands.

The banding techniques fall into two principal groups: 1) those resulting in bands distributed along the length of the whole chromosome, such as G-, Q- and R-bands and 2) those that stain a restricted number of specific bands or structures. These latter include methods which reveal centromeric bands, C-bands, and nucleolus organizer regions, NOR's (at terminal regions of acrocentric chromosomes). C-banding methods do not permit identification of every chromosome in the somatic cell complement, but can be used to identify specific chromosomes.

G- and R- bands can be bright field or fluorescent.

Bright field G-bands

These G-bands are most commonly used. They take their name from the Giemsa dye, but can be produced with other dyes. In G-bands, the dark regions tend to be heterochromatic, late-replicating and AT rich. The bright regions tend to be euchromatic, early-replicating and GC rich.

Bright field R-bands

These R-bands are approximately the reverse of G-bands (the R stands for "reverse"). The dark regions are euchromatic and the bright regions are heterochromatic.

Fluorescent G- and R-bands

These bands are the photographic negative of the bright field versions. i.e. the reverse of the bright field G-bands and R-bands.

Q-bands are like fluorescent G-bands, but certain heterochromatic regions are more brightly stained with Q-banding.