Figure 1.

Diagrammatic representations of McClintock's observations that defined a specific locus on maize chromosome 6 as the nucleolus organizer region. A. Chromosomes 6 and 9 and their reciprocal translocation products. In wild type maize, a single nucleolus is associated with chromosome 6 on the distal side of a dark knob of heterochromatin known as a chromomere. A secondary constriction is adjacent to this chromomere at metaphase. A reciprocal translocation resulting from double-strand breaks in both chromosomes 6 and 9 produced chromosomes 9⁶ and 6⁹. Nucleoli are associated with both translocated chromosomes, which suggested to McClintock that the breakage site in chromosome 6 must have occurred within a nucleolar organizer whose genetic information was redundant. B. When together in the same microspore (shown at prophase), the 9⁶ chromosome forms a larger nucleolus than does the 6⁹ chromosome, which suggested to McClintock that the two NORs compete for a limiting substance. The graphics are adapted from McClintock's drawings (McClintock, 1934). This figure is reprinted, with permission from the publisher, from (Pikaard, 2000b).