DNA content and base composition of human chromosomes

Abstract

The relative DNA content and base composition of the 24 human chromosomes have been measured by flow cytometry. Isolated human metaphase chromosomes flowing through a finely focused laser beam were analyzed singly at a rate of 200 per s. The relative chromosomal fluorescence of the DNA specific dye propidium iodide correlated strongly ( r = 0.99) with the DNA-based human karyotype determined by image cytometry. The relative fluorescence of chromosomes stained with the dye combination Hoechst 33258 (AT binding specificity) and chromomycin A 3 (GC binding specificity) was compared with independent measurements of chromosomal base composition. The results correlated positively with quinacrine brightness ( r = 0.88) and autoradiographic determination of base-specific radioactive DNA precursor incorporation ( r = 0.55). The larger chromosomes were found to have a higher AT content than the average base composition of total human genomic DNA and the smaller chromosomes a higher GC content. Hoechst-chromomycin fluorescence depends primarily on chromosomal variation in base composition, but may also be influenced by chromosome-specific classes of repetitive DNA.