Dicentric Chromosome

 Dicentric chromosomes have two centromeres. Such chromosomes are products of chromosomal damage, breakage, and fusion. Dicentric chromosomes normally break at anaphase, when the centromeres have separate attachments to the spindle and are subject to forces pulling them apart.

 The definition of a chromosome breakage-fusion-bridge cycle in maize by McClintock in 1951 was an important step in defining chromosomal dynamics (1). The origin of chromosomal breakage in this case in maize is the transposable element Dissociation (Ds), which causes chromosomal instability in the presence of another element called Activator (As). Ds is present at specific sites in the chromosome and transposes to a different site only in the presence of As. Events such as these led to the discovery of transposable elements. Two forms of this cycle have been described resulting in the appearance of either dicentric chromosomes or chromatids.

 One type of cycle is shown in Figure 1. Several convenient markers are present on chromosome 9 of maize, which include colored (C1), shrunken (sh1), bronze (bz1), and waxy (wx1). Each of the two chromosomes has a Ds element located at the site indicated (Ds). When the Ds element is activated, chromosome breakage may occur. The acentric fragment without a centromere is lost. The ends of the two sister chromatids fuse to form a bridge, so the resulting single DNA molecule contains two centromeres. At the anaphase, the dicentric fused chromosomal arms break, so that segments of the chromosome are either duplicated or deficient in daughter cells. This cycle occurs either at meiosis or mitosis. 

Figure 1. Chromosome breakage-fusion-bridge cycle in maize. (a) A break in maize chromosome 9 at the site of the transposable element Ds is indicated. (b) At the end of the S-phase and in G2, two sister chromatids are formed that have a break at this site (sticky ends). (c) The sticky ends of the two chromatids fuse to form a chromosome containing two centromeres—a dicentric chromosome. (d) At the anaphase, the two centromeres may be forced to move in opposite directions, leading to chromosomal breakage and forming two chromosomes, one containing a short duplication and the other a deficiency.

References

1. B. McClintock (1951) Cold Spring Harbor Symp. Quant. Biol. 16, 13–47.

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