Long interspersed repetitive elements

Additional analysis of DNA sequences in the human genome has revealed that large blocks of human genes are filled with repeated elements, including long interspersed repetitive elements (LINEs) and short interspersed repetitive elements (SINEs). Short interspersed repetitive elements such as Alu sequences are often used as target sequences for DNA fingerprinting. 

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). 

This method is useful for the isolation of interspersed repetitive DNA elements whose copy number is on the order of tens of thousands, but it will not be practical for the isolation of less abundant elements. As long as the copy number is high enough, SINEs, LINEs, and the retroviral-like elements can all be isolated using this approach. 

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