Question

For each phase of the cell cycle,: G1 Phase - Growth, S PHase - DNA Replication, G2 - Growth, M Phase - Mitosis) state what the overall (general) histone methylation state OR acetylation state of  the inactive X (Xi) and active X (Xa) chromosomes in human females.

Xi Xa

G1

S

G2

M

Answer 1

Okay, so remember that females randomly inactivate an X chromosome and activate the other to work with, this occurs through the methylation of histones that will hold the inactive X chromosome into packed heterochromatin. But such packing unables the DNA from being read, thus neither transcription nor replication can occur.

There is no problem with blocking transcription, that's actually the goal, to silence this specific X chromosome while using the other. But blocking replication is no good for the cell when it needs to undergo division. That's why the histones need to not be methylated during S phase, as the cells is replicating the whole genome.

Acetylation has the opposite effect on DNA, acetylated histones will promote DNA expression, so the inactivated X chromosome will have low acetylations and high methylations, but during S phase methylation is required to be lowered, while acetylation is not needed to rise.

Now, let's fill our table:

	Xi	Xa
G1	High methylation, low acetylation	Low methylation, high acetylation
S	Low methylation, low acetylation	Low methylation, high acetylation
G2	High methylation, low acetylation	Low methylation, high acetylation
M	High methylation, low acetylation	Low methylation, high acetylation

As you can see, the active X chromosome (Xa) is always active so the genes needed for life are being expressed. On the other hand, the inactive X (Xi) is only allowed to lose its methylation when the genome is needed for replication (S stage)