为了帮助考生对
Introduction：
Telomere
端粒(Telomere)是真核生物染色体末端的一种特殊结构,实质上是一重复序列,作用是保持染色体的完整性。细胞分裂一次,由于 DNA 复制时的方向必须从 5'方向到 3'方向,DNA 每次复制端粒就缩短一点,所以端粒其长度反映细胞复制史及复制潜能,被称作细胞寿命的“有丝分裂钟”。当细胞分裂一次,每条染色体的端粒就会逐次变短一些,构成端粒的一部分基因约 50~200 个核苷酸会因多次细胞分裂而不能达到完全复制(丢失),以至细胞终止其功能不再分裂。因此,严重缩短的端粒是细胞老化的信号。在某些需要无限复制循环的细胞中,端粒的长度在每次细胞分裂后被能合成端粒的特殊性 DNA 聚合酶-端粒酶所保留。
Background information：
A chromosome is packaged and organizedchromatin, a complex of macromolecules found in cells, consisting of DNA,protein and RNA. The main information-carrying macromolecule is a single pieceof coiled double-stranded DNA, containing many genes, regulatory
elements and other non-coding DNA. TheDNA-bound macromolecules are proteins, which serve to package the DNA andcontrol its functions. Chromosomes vary widely between different organisms.Some species also contain plasmids or other extra chromosomal DNA.
A telomere is a region of repetitivenucleotide sequences at each end of a chromatid, which protects the end of thechromosome from deterioration or from fusion with neighbouring chromosomes.
During chromo some replication, the enzymesthat duplicate DNA cannot continue their duplication all the way to the end ofa chromosome, so in each duplication the end of the chromosome is shortened(this is because the synthesis of Okazaki fragments requires RNA primersattaching ahead on the lagging strand). The telomeres are disposable buffers atthe ends of chromo somes which are truncated during cell division; theirpresence protects the genes before them on the chromosome from being truncatedinstead.
Over time, due to each cell division, the telomereends become shorter.They are replenished by an enzyme, telomerase reversetranscriptase.
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