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| Modern Genetic Analysis Chapter 3. Gene Function Making Functional Transcripts Figure 3-13. Lariat, or loop, formed by a category of introns as they are removed from their RNA molecules. (a) One of the many 2 hydroxyl groups on the intron attacks the 5 splice site. (b) Thus the subsequent reaction joins the 5 end of the intron not to the 3 end but to the point of the 2 hydroxyl attack a short distance away, yielding a branched structure with a loop: the lariat. (c) The branching is accomplished by the formation of a novel 2-5 phosphodiester bond that enables one adenosine nucleotide to form phosphodiester links with three other nucleotides rather than with the usual two. Ligation of the exons frees the lariat from the remainder of the RNA. (From T. Cech, "RNA as an Enzyme." Copyright © 1986 by Scientific American, Inc. All rights reserved.)

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