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12.18: La inactivación del cromosoma X

JoVE Core
Molecular Biology

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12.18: X-Inactivation

The human X chromosome contains over ten times the number of genes as in the Y chromosome. Since males have only one X chromosome, and females have two, one might expect females to produce twice as many of the proteins, with undesirable results.

Instead, in order to avoid this potential issue, female mammalian cells inactivate nearly all the genes in one of their X chromosomes during early embryonic development. In the nuclear envelope surrounding the cell nucleus, the inactivated X chromosome condenses into a small, dense ball called a Barr body. In this state, most of the X-linked genes are not accessible to transcription.

In placental mammals, the inactivated X chromosome—maternal or paternal⁠—is randomly determined (marsupials, however, preferentially inactivate the paternal X chromosome). X inactivation in one cell is also independent of X inactivation in other cells. Thus, about half the embryonic cells inactivate the maternal X copy; the remaining half inactivate the paternal copy, producing a mosaic. When these cells replicate, they produce cells with the same X chromosome inactivated. Notably, Barr bodies get reactivated in cells within the ovaries that become eggs.

X inactivation accounts for the appearance of female tortoiseshell and calico cats. These cats are heterozygous for a gene with alleles for black fur and orange fur located on the X chromosome. Their mottled coats result from random inactivation of the black and orange fur alleles in groups of cells (calico cats also have white fur patches that are caused by a different chromosome). While male tortoiseshell and calico cats exist, they have an extra X chromosome and are generally infertile.

X inactivation reduces the severity of conditions caused by extra X chromosomes. Males with Klinefelter syndrome form Barr bodies to inactivate their extra X chromosome. Females with Triple X syndrome form additional Barr bodies for their excess X chromosome or chromosomes.

Lectura sugerida


X-inactivation Is A Process That Occurs In Female Mammals Where One Of The Two X Chromosomes In Each Cell Is Randomly Inactivated. This Process Ensures Dosage Compensation Between Males And Females As Males Have Only One X Chromosome While Females Have Two. During Development X-inactivation Occurs Early In Embryonic Development And Is Maintained Throughout The Lifetime Of The Individual. It Involves The Silencing Of One Of The X Chromosomes By The Formation Of A Condensed Structure Called A Barr Body. The Choice Of Which X Chromosome Is Inactivated Is Random And Occurs Independently In Each Cell. Once Inactivation Has Occurred All Descendant Cells Will Have The Same X Chromosome Inactivated. This Leads To A Mosaic Pattern Of Gene Expression In Females Where Some Cells Express Genes From The Paternal X Chromosome And Others Express Genes From The Maternal X Chromosome. X-inactivation Is Crucial For Normal Development And Function In Females. It Ensures That The Expression Levels

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