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Lecture9DSXinact

Puberty generally occurs at the normal time but the

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these two patients. Puberty generally occurs at the normal time, but the testes remain small. Abnormal body proportions (long limbs, narrow shoulders and chest); sparse or absent facial, axillary, pubic, and body hair, breast enlargement, decreased libido, decreased muscle mass and strength, and osteoporosis.
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The failure of normal testes development, particularly of the endocrine Leydig cells, results in a low serum testosterone level Leydig cells synthesize and secrete testosterone
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Klinefelter’s Syndrome Treatments Between 15% and 20% of genes escape inactivation and are expressed from the inactive and active Testosterone replacement: testosterone injections at puberty, every two weeks – increase in muscle mass, masculine apearance Surgery for gynecomastia (breast enlargements) if needed Psychological help: low self esteem due to learning disabilities, language problems, lack of strength, agility
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How is it possible that a double dosage of a chromosome (X) in Klinefelter s syndrome leads to a relatively mild phenotype? In mammals, in every cell, where more than one X chromosomes are present (XX, XXX, XXY), only one X chromosome remains active, while others undergo inactivation. The X chromosome inactivation occurs very early in development. As a result, female and male cells contain approximately similar levels of X-encoded gene products. This equalization is called dosage compensation.
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50 years since X chromosome inactivation discovery (2011) by Mary Lyon The Gift of Observation: An Interview with Mary Lyon http://www.plosgenetics.org/article/info%3Adoi %2F10.1371%2Fjournal.pgen.1000813
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Characteristics of the inactivated X chromosome Highly condensed chromatin (heterochromatin) - a Barr body Nuclear envelope association Late replication Lack of transcription from most (~85%) of the genes XIST RNA production Barr body
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XIST is a non-coding RNA that associates with and initiates the inactivation of, the X chromosome from which it is produced X chromosome inactivation in mouse Light blue - XIST RNA, red - an X-linked gene (Pgk), dark blue - nuclei
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