Barr body in mitosis

Question

In early embryonic development, some female cells pass through a process called lionization and one of the X chromosomes get condensed and began to be called Barr body.

What I don't understand is how the Barr body behaves during mitosis. Specifically, if a cell had condensed a paternal X chromosome, all of its children will have a paternal Barr body. But aren't the barrbodies supposed to decondense to be able to be copied during mitosis? If so, when they recondense, why isn't the choice of which chromosome will condense random?

Thank you!

Answer 1

X inactivation is mediated by several epigenetic marks on DNA that cause condensation into heterochromatin. These include DNA methylation and histone modifications: https://en.wikipedia.org/wiki/X-inactivation#Silencing

As you suggested, a newly copied DNA strand would be free of the epigenetic marks that mediate heterochromatin condensation. Yet epigenetic modifications are inherited, along with DNA molecules, by the daughter cells during cell division. That is, cells have ways of copying these modifications from the old DNA strands to the new ones. These mechanisms are not all known, and are the subject of much research.

In this fashion, each cell division produces two cells that inherit the X-inactivation status (i.e. heterochromatic state) of the parental cell.

This cycle of epigenetic inheritance only ends when the chromosomes are reactivated in primordial germ cells before meiosis and the production of eggs or sperm. The embryo of the animal's offspring can then make that initial, random X chromosome inactivation that is inherited by all of the cells in its body.