Quadriradial formation and gene density. A. Data from a study of 481 quadriradials found in BS lymphocytes  is shown as the number of quadriradials observed relative to the length of each chromosome. These values are compared to gene density on each of the 22 human autosomal chromosomes as calculated from the public human genome database . The number of each chromosome is shown above the gene density bar. B-D. Model of origin of quadriradial composed of two acrocentric chromosomes that have replicated in the presence of BrdU and stained for SCE analysis. The rDNA repeats are in close physical proximity, possibly in the nucleolus, and have numerous single-stranded invasions that are not resolved. During DNA replication the sister chromatids can develop entanglements as well. These unresolved invasions combined with the attachments between the sister chromatids lead to mitotic chiasmata. Quadriradial image from Schonberg .
Schawalder et al. BMC Cell Biology 2003 4:15 doi:10.1186/1471-2121-4-15