What occurs with Barr Bodies during mitosis?

[jaumzaum]

At a certain point during embryonic development (late blastula), one of the X chromosomes in the female condenses and become a Barr body. It's random if this X chromosome will be paternal or maternal, creating some cells with paternal X chromosome and some cells with maternal X chromosome. We know that all children of these cells will keep the rule, and if the paternal chromosome was initially condensed, all the children will have this chromosome condensed.

My question is, why does this happen? Because, in my understanding, for the polar body do duplicate it has first to decondensate, and then, when the lyonization occurs again, isn't it random which X chromosome will condense?

@Ygggdrasil

 

[Ygggdrasil]

X-chromosome inactivaton (XCI) occurs via the epigenetic silencing of one X-chromosome. This epigenetic silencing is associated with deposition of specific histone modifications (e.g. tri-methylation of histone H3 at lysine 27, H3K27me3) along the inactive X by a enzyme called Polycomb Repressive Complex 2 (PRC2). These histone modifications are maintained though DNA replication and mitosis, allowing the cell to re-establish the active and inactive X after these processes. The mechanism by which the cell maintains these epigenetic states thoughout the cell cycle is still an area of active research, but here's a review on the topic: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3852060/

In other words, the cell is able to chemically mark which X-chromosome is the active chromosome and which is the inactive chromosome, and these chemical marks persist throughout DNA replication and mitosis.

 

[BillTre]

My question is, why does this happen?

The traditional explanation, from a evolutionary perspective, would involve the need for a complex genome (with many different interacting genes/gene products) to keep predictable relative expression levels, and in this case (mammals) doing it by "inactivating" (with respect to gene expression) one of two X-chromosomes.

In this specific case (mammals), there would be a difference in the numbers of genes/genome between males and females, for those genes located on the X-chromosome. Not all genes on the X-chromosome are involved in sexual differences. Some will be involved in everyday functions.
Males would have one copy of those genes on the X-chromosome, while females would have two copies of genes on the X. This is thought to throw off the normal relationships a gene and it products has with those of many other genes.
In many cases, differences of gene have be manipulated by changing the number of genes in a cell. A male and female will have a 2x difference the in genes on the X-chromosome, of maybe 4% of the genome (one of about 25 chromosomes).

Biology has found different ways to resolve this problem in different organisms, but in the mammals, after a certain age of development, in females, one of the X-chromosomes are randomly inactivated (WRT gene expression) as described by @Ygggdrasil, which make the male and female X-chromosome expressible gene counts equivalent.
The mammalian female, after this stage, is therefore a mosaic (a mix of cells expressing either genes on he paternal X-chromosome or genes on her maternal X-chromosome) and in this manner, would be unique even among a genetic clone (but not a homozygous clone).

Why does it (genomic transcription) turn on when it does? you might ask:

Transcription only turns on at certain stages (the mid-blastula transition) in "lower" vertebrates, don't know about mammals. As development proceeds,
Controls on gene expression, like described above, may not be necessary until some particular later stage. Once the genome started running (expressing its genes).

Choosing randomly which chromosome's genes are repressed, rather than eliminating the X-chromosome derived from one or the other parent (probably at an earlier ties in development), would provide the female with a wider diversity of gene expression in her whole body (considering the expression, in all of her cells).

The inactivated X-chromosome state, is inherited in all of the cells progeny, generally speaking, but
Is it turned off at some time in the reproductive cycle?
It (X-chromosomal inactivation) would have to be either:
Turned off in the germ cells (cells that are or will form the reproductive cells: either sperm or eggs)
Or not ever form in the germ cell lineage because the germ cell lineage has already separated from the lineages of all other cells (the somatic or non-reproductive cells) before a X-chromosome is mosaically inactivated. These germ cell lineages may operate on their own rules.

 

[Ygggdrasil]

The inactivated X-chromosome state, is inherited in all of the cells progeny, generally speaking, but
Is it turned off at some time in the reproductive cycle?
It (X-chromosomal inactivation) would have to be either:
Turned off in the germ cells (cells that are or will form the reproductive cells: either sperm or eggs)
Or not ever form in the germ cell lineage because the germ cell lineage has already separated from the lineages of all other cells (the somatic or non-reproductive cells) before a X-chromosome is mosaically inactivated. These germ cell lineages may operate on their own rules.

XCI is present in primordial germ cells but the inactive X is reactivated during oogenesis around the onset of meiosis (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC98919/).