单位。染色体联会及重组可能发生于该级结构水平上。（50圈/环）

二、染色单体纤维（chromatid fiber） 染色体的五级结构

以细胞核基质为支撑基础的侧环结构进一步螺旋化，形成18个侧环组成的圆盘。许多圆盘

上下重叠形成粗度约为200~300nm、中空的管状结构，内侧为染色体骨架，称染色单体纤

维。

三 、 中期染色体

染色单体纤维进一步螺旋化。主缢痕和次缢痕处未发生这种螺旋化。

第四节 高等生物染色体的端粒和端粒酶

TEL，端粒序列，telomere DNA sequence

CEN，着丝粒序列，centromere DNA sequence

ARS，自主复制DNA序列，autonomously replicating DNA sequence，

1983年，A. W. Murray等人成功构建了包括ARS、CEN、TEL和外源DNA，总长度为

55kb的酵母人工染色体(yeast artificial chromosome, YAC)。YAC可用于转基因和构建基因文库，

容纳插入片段的能力远高于质粒。

端粒（telomere）是指真核生物染色体线性DNA分子末端的结构部分，通常膨大成粒状。

端粒是线性DNA分子末端的特殊结构，由规则或不规则的双链重复序列和最末端的

一条单链重复序列以及端粒蛋白质构成。

端粒结合蛋白分为两类：端粒单链结合蛋白和端粒双链结合蛋白。

前者在端粒末端提供帽状结构以稳定端粒；

后者可能直接参与端粒长度平衡的维持，是端粒延伸的负调节因子。

Double-strand（双链） telomere-binding proteins（端粒结合蛋白）

Budding yeast出芽酵母 Rap1p is a major component of telomeric调聚 chromatin

that binds the telomeric repeats with high affinity亲和力 through a Myb-like DNA

binding domain. Rap1p negatively affects telomere length.

It recruits, through a C-terminal domain, two proteins, Rif1p and Rif2p, which mediate

the telomere length control.

Deletion of either RIF1 or RIF2 leads to substantial elongation of telomeres establishing

their function as negative regulators of telomere length.

Telomere elongation analysis of single-chromosome end molecules in rif1 and

rif2-mutant cells reveals that these proteins downregulate the frequency of

telomerase-mediated telomere elongation.

Single-strand telomere-binding proteins

Single-strand telomere-binding proteins regulate telomerase access to the telomeric

3′end.

In S. cerevisiae, Cdc13p is the major single-strand DNA-binding protein. Cdc13p’s

functions include telomerase recruitment, telomerase repression, coordination of G- and

C-strand synthesis, and protection of telomere ends from nucleolytic degradation.

Loss of CDC13 leads to degradation of the telomeric C-rich strand, causing a

Rad9-mediated cell-cycle arrest.

Cdc13p is present at telomeres throughout the cell cycle, but the levels of Cdc13p at

telomeres peak in late S phase, consistent with the idea that Cdc13p assembles along

the extended telomeric 3′overhang present at that time.