Do These Sequences Make CENs Yet? (original) (raw)

1. James A. Birchler1
2. Division of Biological Sciences, University of Missouri, Columbia, Missouri 65211-7400 USA

Although there is considerable debate about just what sequences constitute a centromere in multicellular eukaryotes, a picture is emerging that degenerate tandem arrays spanning the megabase range are involved. With the general organization of_Arabidopsis_ centromeres now known (Round et al., this issue), there are three types of centromeres from plants in addition to human and Drosophila for which the physical organization has been studied. A unit repeat present thousands of times at each site is a common theme.

Comparison of Known Centromeric Sequences

The first centromere sequence of a multicellular eukaryote cloned and analyzed was the neocentromere of maize (Peacock et al. 1981). The neocentromeres (or chromosomal knobs) are normally heterochromatic landmarks on several maize chromosomes. There is, in fact, considerable variation for their chromosomal positions. Under normal conditions they are inert, but in the presence of an abnormal chromosome 10, they acquire centromere functions during meiosis (Rhoades and Vilkomerson 1942). These functions supercede those of the normal centromere.

The neocentromere repeat is 185 bp in length and is repeated at one site an estimated 15,000 copies, at a minimum (T. Phelps, pers. comm.). The available evidence suggests that the unit repeat is the sole constituent of neocentromeres, although single- or low-copy interspersions would be difficult to detect.

The sequences required for centromeres in humans are still a matter of discussion, but the alphoid repeat seems to be present in almost all cases (Tyler-Smith et al. 1993; Harrington et al. 1997; Kipling and Warburton 1997). The unit repeat is 171 bp in length. Individual centromeres have clusters of distinguishable alphoid variants and can span stretches of DNA of 2–3 Mb in length (Tyler-Smith et al. 1993).

Analysis of a normal plant centromere from the supernumerary B chromosome of maize (Alfenito and Birchler 1993; Kaszas and Birchler 1996) indicated that …