a.多数植物的着丝粒卫星DNA重复单位只对亲缘关系很近的(一般是同一个属的)物种具有专一性，存在广泛的序列变异。

b.而串联重复单位容易发生不等交换。不等交换能够在一个相对较短的时间清除整个阵列中新的多态性，形成着丝粒卫星DNA的均质化。

c.着丝粒卫星DNA 的均质化和变异性之间最终达到一种平衡，既能产生足够的序列分歧以引起物种间交配不相容，同时又具有在一个物种内维持着丝粒活性所需的足够的保守性。

CENP-A:

The primary link between centromeric DNA and the protein components of the

kinetochore is the conserved histone H3-variant, CENP-A. CENP-A localises to the inner kinetochore plate of active centromeres, Thus CENP-A forms specialized centromeric nucleosomes required for the recruitment of other kinetochore components.

1.the a-satellite DNA ,The major DNA component in the centromeric region of human chromosomes, plays an important role in centromere function.

2.The centromeres from budding yeast (Saccharomyces cerevisiae), known as point centromeres, have been well characterized. The genetic information specifying full centromere function in these species is contained within a 125-bp DNA segment.

3.fission yeast (Schizosaccharomyces pombe), Drosophila melanogaster, and mammalian species. The centromeres from these species are much more complex compared with those from budding yeast.

4.These centromeres, called regional centromeres, encompass kilobases or megabases of DNA and include both unique and repetitive DNA sequences.

5.these repetitive elements are essential for full centromere function, satellite DNA

6.Normal human centromeres are characterised by an abundance of 17l-bp a-satellite DNA repeat arrays. These arrays are highly variable in size, ranging from *240 kb on some Y chromosomes to >4000 kb at other centromeres.

7.The primary link between centromeric DNA and the protein components of the kinetochore is the conserved histone H3-variant, CENP-A.CENP-A localises to the inner kinetochore plate of active centromeres (Warburton et al. 1997), Thus CENP-A forms specialized centromeric nucleosomes required for the recruitment of other kinetochore components.