Like twins separated by early evolution, DNA and RNA are the principal molecules for storing and transducing genetic information. Messenger RNAs (mRNAs) are transcribed from genes that constitute as little as 10% of the genome. Most of the remaining noncoding DNA is occupied by a diverse family of repeats including arrays of satellite sequences and transposons. Since the pioneering work of Muller (1) and McClintock (2), these apparently "nonspecific" sequences have been thought of as "epigenetic elements" that control the stability of gene expression programs and organize heterochromatic domains at centromeres and telomeres (3). Although this concept has been instrumental in guiding epigenetics research (4), it has also posed a major paradox: How can stretches of DNA that appear so heterogeneous in sequence organize similar higher order chromatin structures (5)?
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Thomas Jenuwein (2002) studied this question.
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