Long interspersed repeat sequences LINEs

All of the mammalian transposable elements that have been characterized to date seem to be the result of transpositions that proceeded through an RNA intermediate. This process is known as retrotransposition or retroposition. Three classes of these retrotransposable elements are known in mammals (1) SINEs, or short interspersed repeated sequences such as the human Alu family and rodent Bl (2) LINEs, or long interspersed repeated sequences such as LI in a variety of mammalian species and (3) retrovirus-like elements, such as THE 1 in humans and mys and IAP in rodents. Retrovirus-like elements have long terminal repeats (LTRs) that often surround two open reading frames (ORFs) like those of retroviruses, but they lack the ability to leave one cell and enter another. LINEs also have two ORFs, but have no LTRs. SINEs have no LTRs and no ORFs. Transposition of all of these elements must involve reverse transcription of the RNA intermediate in some cases the required reverse transcriptase is apparently encoded by the element itself. 

There are several large classes of DNA sequences which are not translated, including those for structural RNAs [ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs)], pseudogenes, and repetitive DNAs [e.g., short and long interspersed repeated sequences (SINES and LINES)]. Ribosomal... 

Repetitive sequences are also defined by the number of base pairs in each repeated segment. Sequences that have from 100 to 500 bp are referred to as SINES (short interspersed repeated sequences) and sequences that have several thousand base pairs are referred to as LINES (long interspersed repeated sequences.) Thus, repetitive DNA sequences can be described both by the length of the segment and the degree to which it is repeated. 

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. 

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