Tightly wrapped histones

No, not really. That’s a misleading title, but my hope here is that I can explain this in terms that are simple enough for people who aren’t scientists to understand. I’m hoping that because I’m not a professional scientist but am really, really into this stuff, the language and illustrations I use serve as a bridge for the gap in understanding.

So let’s start with the cell, and let’s use humans as an example. Even though epigenetics happens in every living thing, even plants, I want you to be able to identify personally with this so the information takes hold a little better. What do we know about genetics and conception and fetal development? Well, we start off with an assortment of genes and 46 chromosomes. We got all of them from our parents and grandparents and so on down the line, but it’s a mix between Mom’s side and Dad’s side, because her eggs start off with a random selection of 23 chromosomes (see my previous post about what random means) and his sperm also start off with a random selection of matching chromosomes.

Sperm meets egg, and there you go, 46 chromosomes in a single cell, and a complete, unique strand of DNA that has all the information needed to build a human body.

If you’ve watched videos of human development, you’ve seen how that one cell splits into two, two into four, four into eight, and then things really start to happen. In the beginning, each of those cells is exactly the same. Each time they split, they’re making another cell that’s just like they are. Remember this, because I’m going to mention it again later. . . Here’s how it looks, in case you haven’t actually seen it, in a video on in-vitro fertilization:

After this point, the cells begin to differentiate. Instead of simply reproducing copies of themselves, they start to become more specialized. They still contain all the DNA, but some of the instructions will be used, and some will be silenced. This starts with the transcription from DNA to RNA. What we used to believe (or at least what I was taught in school days in ancient times) was that the RNA was the sole messenger, containing only the information needed to make cells. That’s only kind of sort of true, and doesn’t explain a lot of confusing things that happen to human bodies. You see, it is part of the picture in cell differentiation, which is, to put it in simple terms, the process that makes one cell be a bone cell and another be a heart cell and another be a brain cell and so on. The RNA puts this in process by taking the pieces of the DNA that are needed to make a specific call and creating the proteins that manufacture that cell. With these instructions, cells continue to divide, but they’re not just making carbon copies of themselves.

We see this in fetal development because parts of the body from the brain, the eyes, the internal organs, to the fingers and toes, go from being kind of blobby and alien-looking, to functional and human-like. The manufacturing of differentiated cells continues throughout fetal development, and the differentiation is pretty much complete by the time a baby is born.

But there’s a piece missing – we know that RNA has instructions for making the proteins that manufacture differentiated cells, but it doesn’t make those proteins all by itself. This is where epigenetics comes in. The actual work of taking the orders from the RNA and making the proteins is done by histones. The DNA has the construction diagrams, the RNA is barking orders, the histones are doing the work.

This is still happening inside a cell. The cells are still dividing. It’s just that this epigenetic process is making two different cells out of one cell instead of two identical cells. The new cells aren’t coming out of nowhere, they’re coming from existing cells that are multiplying.

As we get older, we tend to go back to more of a model of cell replication. A cell duplicates itself, then dies after the new cell has been made. The epigenetic process takes place then, as well. Sometimes the cells won’t necessarily die, because we’re growing and need more cells. That’s done epigenetically, too, because the blueprint from the DNA says what the final adult product is supposed to be like, not just the infant version. As we get really older, the cells are trying to replace themselves, but they don’t do quite as good a job, and that’s an epigenetic process as well, because the instructions are getting messed up *after* the RNA. The histones just aren’t doing such a great job after a while.

The point here is that epigenetics is part of the process of cell development that is already written out in the DNA. The way it works without interference is genetic and heritable, and every single one of the many trillions of cells in your body was created the same way. The DNA has the plans, the RNA is the subcontractor, the histones are giving the orders to the proteins based on the instructions from the higher-ups.

Keep this in mind when you hear things about the amazing effects of environment on epigenetics. Yes, this is the part where things can get screwed up, because, yes, histones can be modified. But I’m going to save that for later, because this is a lot to absorb. I hope this makes sense, and if anyone has questions or corrections, please comment – I want to hear from you.