Heterochromatin - heterochromatin The phenomenon of X...

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Heterochromatin During interphase- some of the chromatin (~10% of total chromatin) remains highly condensed and is termed heterochromatin - this DNA is transcriptionally inactive . Two types: constitutive heterochromatin Contains DNA sequences which are never transcribed [such as highly repetitive satellite DNA sequences]. facultative heterochromatin Contains sequences that are not transcribed in the particular cell under examination but are transcribed in other cell types. Examples: B lymphocytes are the only cells which express the immunoglobulin genes. During interphase, the Ig genes would be in euchromatin form in B lymphocytes. In all other cell types, the Ig genes would be in the form of
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Unformatted text preview: heterochromatin.] The phenomenon of X chromosome inactivation also provides an example of the role of heterochromatin in gene expression. In many animals, including humans, females are XX and males are XY. The X chromosome contains thousands of genes that are absent from Y. Thus, females have twice as many X chromosome genes as males. However, males and female cells have equal amounts of the proteins encoded by the X chromosome. One of the 2 X chromosomes in female cells is inactivated by being converted to heterochromatin early in development....
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