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I am reading Albert et. al’s Molecular Biology of the Cell and at one point the authors discuss the idea of “position effect variegation.” They mention that through events of DNA relocation, it’s possible for portions of echromatic DNA to become heterochromatic by being positioned next to heterochromatic DNA. They stress that “remarkably,” this heterochromatic state of DNA often gets inherited by the cell progeny. This phenomenon of inheritance is what they call, or I guess is called, position effect variegation. I don’t quite see what’s so “remarkable” about this, which is making me wonder if there’s something I’m underappreciating or misunderstanding. In particular, if the underlying DNA structure is changing, then wouldn’t we expect the progeny to inherit these epigenetic changes? Why is it so remarkable?

Here's the actual quote from the book:

In chromosome breakage-and-rejoining events of the sort just described, the zone of silencing, where euchromatin is converted to a heterochromatic state, spreads for different distances in different cells in the fly embryo. Remarkably, these differences then are perpetuated for the rest of the animal’s life: in each cell, once the heterochromatic condition is established on a piece of chromatin, it tends to be stably inherited by all of that cell’s progeny (Figure 4–31). This remarkable phenomenon, called position effect variegation, was first recognized through a detailed genetic analysis of the mottled loss of red pigment in the fly eye

and later on:

But in at least some cases, the covalent modifications on nucleosomes can persist long after the transcription regulator proteins that first induced them have disappeared, thereby providing the cell with a memory of its developmental history. Most remarkably, as in the related phenomenon of position effect variegation discussed above, this memory can be transmitted from one cell generation to the next.

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Re:

In particular, if the underlying DNA structure is changing, then wouldn’t we expect the progeny to inherit these epigenetic changes? Why is it so remarkable?

Given the context of the quote, the "variegation" part of the phrase refers to gene expression sometimes being turned off by influence of newly-nearby heterochromatin. The extent to which this suppression of expression extends into the euchromatic region from the adjacent heterochromatin varies from cell to cell, but once established is stably inherited through further divisions of that cell. It is unknown what molecular mechanisms initiate and maintain the suppression of expression in the formerly euchromatic genes.

Since we do not fully understand the molecular nature of what is required to initiate and maintain heterochromatin, especially in a section of DNA that was formerly euchromatin and so presumably does not contain any global "make me heterochromatin" sequence signals, the "remarkable" likely means "we don't yet understand the details" -- it's like magic.

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  • $\begingroup$ I added the quote $\endgroup$ Aug 9 at 13:08
  • $\begingroup$ There's no evidence the actual DNA is changing. The molecular mechanism of this "creeping heterochromatinization" is unknown as far as I know. I think we would guess it might have to do with nucleosome positioning or state. $\endgroup$
    – Armand
    Aug 10 at 13:20

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