Karyotype Analysis and Chromosome Banding

Nearly all methods of chromosome banding rely on harvesting chromosomes in mitosis. This is usually achieved by treating cells with tubulin inhibitors, such as colchicine or demecolcine (colcemid), that depolymerize the mitotic spindle and so arrest the cell at this stage. Excessively long incubations with Colcemid result in overcondensed chromosomes that band poorly and moreover some cell types, especially those from the mouse, eventually escape theColcemid block and proceed through the cell cycle. Chromosome banding methods are either based on staining chromosomes with a dye or on assaying for a particular function. The most common methods of dyebased chromosome banding are G(Giemsa), R(reverse), C(centromere) and Q(quinacrine) banding. Bands that show strong staining are referred to as positive bands; weakly staining bands are negative bands. However the staining patterns are not black and white, different bands stain to different intensities (Francke, 1994). G-positive bands are usually just called G-bands and likewise for Rpositive (R-) bands. Positive C-bands contain constitutive heterochromatin. Q-bands are considered equivalent toGbands. Themost widely used function-based bandingmethod is replication banding and is based on the fact that different bands replicate theirDNAat different times during Sphase of the cell cycle. Generally, R-band DNA is replicated earlier thanG-bands (Dutrillaux et al., 1976).G-bands also correspond to the condensed chromomeres of meiotic chromosomes and R-bands to the interchromomeric regions.