By sequencing, fingerprinting and in situ hybridization of a centromere-specific large insert clone (BAC 7), the sequence organization of centromeric DNA of barley could be elucidated. The 23 kb clone insert harbours three copies of the Ty3/gypsy-like retroelement cereba and G+C-rich satellite sequence. Two cereba elements (~7 kb) are arranged in tandem and include LTRs (~1 kb) similar to the 'cereal centromeric sequence' family, as well as the complete Ty3/gypsy-like polygene region. The high density (~200 elements/centromere) and completeness of cereba elements represent unique features of the barley centromeres as compared to those of other cereals. Obviously, the conserved cereba elements together with barley-specific G+C-rich satellite sequences constitute the major components of centromeric DNA in this species. As to the upper chromosome size limit in barley, the influence of recombinantly elongated chromosome arms on nuclear divisions in barley was tested and confirmed a rule according to which half the length of the average spindle axis defines the upper tolerance limit for chromosome arm length. A slightly longer chromosome arm caused incomplete separation of sister chromatids (~30%) of mitotic telophase cells and >2.5% of daughter cells showing a micronucleus, due to disruption of non-separated sister chromatids by the newly forming cell wall. In homozygous condition, this elongated chromosome mediated a slower growth and reduced fertility of the carrier plants. Its meiotic transmission was not impaired because of the larger spindle dimensions in meiocytes as compared to those in mitotic cells.