Giemsa banding (G-banding) to form a karyogram Giemsa banding (G-banding) to form a karyogram (A) Metaphase spreads like this are obtained from cultured cells arrested in metaphase using colcemid, followed by Giemsa staining to create characteristic light and dark bands. Generally the dark bands represent regions which are AT-rich and gene-poor. (B) The chromosomes from the spread are arranged in pairs to view the karyotype, often using specialist software like Cytovision. (C) Diagrammatic representations of the G-banding patterns, called ideograms, are used as a reference. The ideograms have been aligned at the centromere (dotted line); blue shaded regions are highly variable – note for example the variation between p arms of chromosome 13, 14 and 15 in (B). In fact the p arms of the acrocentric chromosomes (13, 14, 15, 21, 22) all have very similar content, which includes the nucleolar organiser regions or NORs. Each NOR contains a tandem repeat of ribosomal DNA (rDNA) which encodes the rRNAs. Between all five acrocentrics there are approximately 300–400 rDNA repeats, though the actual number varies between individuals. Chromosome ideograms from NCBI Genome Decoration Page. Maria Jackson et al. Essays Biochem. 2018;62:643-723 ©2018 by Portland Press Ltd