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--- dna [2026/08/04 09:24] – [DNA replication] gary1
+++ dna [2026/08/04 10:04] (current) – [DNA activation] gary1
@@ Line 61: / Line 61: @@
     *the signature of an active gene, marked by the pattern of histone modifications clustered around its start and along its body, was nearly identical in every species the team examined(([[https://www.nature.com/articles/s41588-026-02672-1|2026: Diversity and evolution of chromatin regulatory states across eukaryotes]]))
     *in contrast, evolution has created many ways to turn off these genes via diverse configurations of repressive heterochromatin states associated with silenced genes and transposable elements
+  ***X chromosome inactivation (XCI)**
+    * X chromosome inactivation is a natural process that occurs in female mammals to balance gene expression between individuals with two X chromosomes and those with one.
+    * during early embryonic development, one X chromosome is randomly silenced in each cell, creating a stable pattern that is maintained throughout life.
+    * this process is controlled primarily by the long non-coding RNA XIST, which recruits multiple epigenetic regulators that compact chromatin and suppress gene expression on the inactive X chromosome.
+    * it is not always perfectly balanced - in some individuals, skewed XCI occurs, where one X chromosome is preferentially inactivated in a greater proportion of cells.
+      * this phenomenon can significantly influence disease severity in women who carry mutations in X-linked genes.
 =====DNA to RNA=====
   *RNA is usually a single-stranded molecule (ssRNA) and unlike DNA, its sugar backbone is ribose, not deoyxribose, and whereas the complementary base to adenine in DNA is thymine, in RNA, it is uracil