![]() Alu is particularly prolific, having originated in primates and expanding in a relatively short time to about 1 million copies per cell in humans. In contrast, Alu elements average only a few hundred nucleotides, thus making them a short interspersed transposable element, or SINE. L1 elements average about 6 kilobases in length. Both the LINE1, or L1, and Alu genes represent families of non-LTR TEs. There are two major types of class 1 TEs: LTR retrotransposons, which are characterized by the presence of long terminal repeats (LTRs) on both ends and non-LTR TEs, which lack the repeats. In other words, class 1 TEs do not encode transposase rather, they produce RNA transcripts and then rely upon reverse transcriptase enzymes to reverse transcribe the RNA sequences back into DNA, which is then inserted into the target site. Unlike class 2 elements, class 1 elements-also known as retrotransposons-move through the action of RNA intermediaries. Approximate full-length element sizes are given in parentheses. Alus contain two similar monomers, the left (L) and the right (R), and end in a poly(A) tail. An Alu element is an example of a nonautonomous retrotransposon. The RT, EN, and a conserved cysteine-rich domain (C) are shown. L1s are usually flanked by 7- to 20-bp target site duplications (TSDs). L1s consist of a 5'-untranslated region (5' UTR) containing an internal promoter, two ORFs, a 3' UTR, and a poly(A) signal followed by a poly(A) tail (An). ![]() L1 is an example of a non-LTR retrotransposon. These elements are not present in humans, and essentially all are defective, so the source of their RT in trans remains unknown. Other LTR retrotransposons that are responsible for most mobile-element insertions in mice are the intracisternal A-particles (IAPs), early transposons (Etns), and mammalian LTR-retrotransposons (MaLRs). Also shown are the reverse transcriptase (RT) and endonuclease (EN) domains. They produce target site duplications (TSDs) upon insertion. These elements have terminal LTRs and slightly overlapping ORFs for their group-specific antigen (gag), protease (prt), polymerase (pol), and envelope (env) genes. Examples of LTR retrotransposons are human endogenous retroviruses (HERV) (shown) and various Ty elements of S. ![]() Common autonomous retrotransposons are (i) LTRs or (ii) non-LTRs. ![]() Retrotransposons are divided into autonomous and nonautonomous classes depending on whether they have ORFs that encode proteins required for retrotransposition. They are flanked by short direct repeats (DRs). DNA transposons (e.g., Tc-1-mariner) have inverted terminal inverted repeats (ITRs) and a single open reading frame (ORF) that encodes a transposase. ![]()
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