Repetitive element sequenced-based

Studies of overall genome composition based on reassociation kinetics (Simpson et ai, 1982 Cox et ai, 1990 Marx et a/., 2000) and analysis of fully sequenced bacterial artificial chromosome (BAC) clones from the 5. mansoni genome project show that platyhelminth genomes contain abundant highly and moderately repetitive sequence (Fig. 2.1). Much of the repetitive DNA comprises two classes of integrated mobile elements class I elements, which include long terminal repeat (LTR) retrotransposons and retroviruses, non-LTR retro-transposons and short interspersed nuclear elements (SINES) and transpose via an RNA intermediate, and class II elements (trans-posons), which transpose as DNA (Brindley et ai, 2003). Additionally, small dispersed or tandemly repeated sequences are common. A wide variety of these sequences have been isolated and characterized from a variety of taxa (Table 2.4). 

Telomeres. The DNA sequences at the chromosome ends have a TG-rich strand, such as the (TTGGGG)5o 7o Tetrahymemy and the (TTAGGG) of both human and trypanosome chromosomes. The complementary DNA strand is CA-rich. The S. cereaisiae telomers have 350 base pairs containing the sequences (TGj 3 / C3 3A) as well as one or more copies of a 6.7-kb nonrepetitive sequence and other elements. In many species the repetitive telo-meric sequences have 3 poly(G) tails at the ends of the DNA molecules. These tails are able to form G quartet structures (Fig. 5-26 and Chapter 5, Section C,4). A variety of telomere-binding proteins have been isolated. " Some of these bind to G quartet struc-... 

The intruder elements undergo random, single base substitutions these mutations are frequent (one to 10/10 base pairs), heterogenous, and become clonal. Best known are these mutations in the hereditary non-polyposis colon cancer Lynch syndrome [2098]. These somatic point mutations appear as repetitive sequences, which are mismatch-repaired. While the initial mutations as oncogenic pathways were of definitive causative effect, the additional somatic point mutations occur... 

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