In mammals, Xist RNA is transcribed only from the inactive X, with which it associates at its site of synthesis and then spreads over the entire chromosome through an unknown mechanism. These RNAs, Xist (X-inactive specific transcript) in mammals, and roX1 and roX2 (RNA on the X) in Drosophila, are structurally unrelated, yet they share the intriguing property of remaining tightly associated with the X chromosome. Īt least one common theme has emerged in Drosophila and mammals of how the dosage-compensated chromosomes are marked and eventually recognized by regulatory proteins: the use of non-coding RNAs transcribed from genes located on the X chromosome itself. Much attention has been dedicated to the elusive question of how the compensated or inactivated chromosomes are recognized by these proteins, and three recent papers investigating some of these aspects in Drosophila have provided new insights into this mystery. elegans and Drosophila, several proteins with different molecular functions involved in dosage compensation have been identified. In Drosophila, dosage compensation is achieved in the heterogametic male by a twofold chromosome-wide up-regulation (hypertranscription) of essentially all genes on the single X chromosome. Consequently, each gene on the single active X chromosome in female cells and the corresponding gene on the single X chromosome in male cells are expressed at equal levels. In mammals, dosage compensation also occurs in the homogametic sex (the female) by X inactivation, whereby an entire X chromosome forms a distinct, heterochromatic, transcriptionally inactive nuclear structure known as the Barr body. For example, in Caenorhabditis elegans, dosage compensation occurs in the homogametic hermaphrodite (XX) by the down-regulation of virtually all genes on the two X chromosomes by about 50%. Because sex chromosomes appeared relatively late in evolution, dosage compensation evolved independently in different organisms and is accomplished by distinct mechanisms. Dosage compensation, which ensures that the expression of X-linked genes is equal in males and females, is an essential process in organisms that have sex chromosomes.
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