DNA sequence at end of chromosome
111.12 DNA at the ends of eukaryotic chromosomes consists of telomeric sequences.
The G-rich strand often protrudes beyond the complementary C-rich strand at the end of the chromosome (< Figure 11.12). The length of the telomeric sequence varies from chromosome to chromosome and from cell to cell, suggesting that each telomere is a dynamic structure that actively grows and shrinks. The telomeres of Drosophila chromosomes are different in structure. They consist of multiple copies of the two different retrotransposons (discussed later in this chapter), Het-A and Tart, arranged in tandem repeats. Apparently, in Drosophila, loss of telomere sequences during replication is balanced by transposition of additional copies of the Het-A and Tart elements.
Farther away from the end of the chromosome, from several thousand to hundreds of thousands of base pairs form telomere-associated sequences. They, too, contain repeated sequences, but the repeats are longer, more varied, and more complex than those found in telomeric sequences.
A telomere is the stabilizing end of a chromosome. At the end of each telomere are many short telomeric sequences. Longer, more complex telomere-associated sequences are found adjacent to the telomeric sequences.
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