Let’s reiterate from the previous post, just in case you need a recap:
All cells are made from other cells; we start with a few that are the same, and as the number of cells increases, they begin to differentiate and become cells for specific body parts.
DNA is the blueprint for the final product (a sexually mature adult human being, for illustration purposes.) RNA is a segment of DNA that begins the process of cell differentiation, but the mechanism that actually creates the proteins that build cells is the epigenetic process, which depends upon histones interpreting the genetic instructions.
Once we are full grown, our bodies are almost always replacing cells rather than making new ones, and the new cells may not be exact duplicates of the cells that created them.
Yes, there is such a thing as histone modification. Yes, gene expression (in the form of cells that follow specific genetic instructions) can be changed during the epigenetic process. Yes, it’s possible for some of these changes to become heritable (passed on from parent to offspring.) But let me explain what’s reasonable and rational about these possibilities.
You’ve heard of this, but usually in the form of “you can change your DNA by doing this thing or eating that thing” which is, essentially, not true. Histone modification takes place on a cellular level, and changes in different ways depending on what the chemicals that can modify histones are doing. I’ll save the technical terms and illustrations for another time. Baby steps.
What happens is that while a cell is preparing to replicate itself, a chemical can make the histones do something differently from the way they were instructed, and that makes the resulting copied cell different from the cell that created it. Right now, we have some very specific examples of changes that depend on specific chemical exposures (some from external environment, some from internal environment.) DNA is huge. We have a hundred trillion or so cells in our bodies. The genome is almost infinitely diverse. There are very few examples right now of direct cause and effect, and each one we discover in the future will be just as limited.
The number of possibilities alone makes it pure speculation to assume that a food given to a pregnant mouse that changes her babies’ fur color and body shape is going to do the same thing for a fully-grown adult, or even something similar!
Now the add another layer of complexity, these are the things that can happen when you modify the histones in a cell:
*a beneficial gene is suppressed
*a detrimental gene is suppressed
*a beneficial gene is activated
*a detrimental gene is activated
So if someone claims that a food or something “methylates” your genes (besides being wrong) it could easily be a bad thing!
Changing Gene Expression
I mentioned the prenatal modification above, and that’s because it’s an important thing to study. Why? Because in order for histone modification to have any observable and verifiable effect, it needs to happen early. Think about it. If you modify the histones of a four or eight celled creature, then a lot more cells are going to be made not according to plans. In an adult, modifying a single cell, or even a few cells, out of all the cells in our bodies, is going to have minimal impact. In order to change gene expression in an adult, exposure needs to be intense enough or prolonged enough to influence a large number of cells.
I like to use the example of skin, partly because it’s a cell type that’s replaced frequently, and partly because we can see a lot of the possible changes to it. It’s a good way to illustrate that an environmental factor can produce a change that does not alter gene expression, and how the level of exposure can make a difference in whether an epigenetic change is even possible.
If you go out into the sun, your skin changes color. It could get burned, it could get tanned. But when those darker skin cells make their replacements and die, the replacements are your original skin color. You have exposed yourself to an environmental factor that has an obvious effect on your body, but it doesn’t change your gene expression. Why? Because the exposure was not prolonged enough that the visible change was messing around with histones while the replacement cell was being created.
On the other hand, if you’re out in the sun all the time so that your skin is constantly in a damaged state, then those cells are more likely to be in that damaged state when they’re replicating themselves. This could still even be temporary, but it could change gene expression so that the replacement cells are cancerous, for example. (Cancer is epigenetic – but it could be caused by environment *or* part of the plan all along.) So you need to expose the same group of cells to the same environmental factor for long enough that most of the cells begin reproducing with the alteration in gene expression. . .and that is not guaranteed to be a good thing, so don’t buy into the hype.
Heritability of Epigenetic Changes
Yep, this has been studied, too, and it does sometimes happen. The most repeatable changes happen when the fathers’ bodies have changed. I credit that to the fact that sperm are constantly being made, and things like stress hormones or chemical exposures, or starvation, can change what genes go into what chromosomes in the sperm cells at that time. Give the dads some time to recover, you get a completely different result.
Keep in mind that the normal set of instructions is the default. If you look at plants or other animals who’ve been genetically altered, a lot of times you’ll find that their offspring regress to the original, dominant form. In both human and animal studies, most of the epigenetic changes that were brought about by environmental exposure get passed down to the next generation, maybe the generation after that, and in a few cases, the third generation. Then things go back to normal.
I probably missed a few things, but I hope this is clear. Ask me stuff, tell me stuff. Thanks!