You know something I love about biology? The depth. The complexity that we don't always recognize at first.
I remember when I first learned about Punnett squares. I was in 9th grade and there were practice problems with eye color as the trait. But then, as we would explore more depth in biology, we learned eye color is actually very complex.
It's a polygenic trait with many genes and that trait is so complex that it really doesn't work like a simple Punnett square. In fact, even traits that we often treat like a one gene trait, like attached or free earlobes, even that trait isn't quite that simple, so we even mention that in our pedigree video when we use it as an example. There are so many exceptions and details in biology, and they are fascinating to discover. They are still there waiting for you to explore them, even when you think you've mastered general biology. For a perfect example, something I not too long ago learned in a graduate level biology course, the topic of epigenetics.
And I have to share it with you now, just like I did when I called my sister at 1102 PM on a Wednesday night when I was covering it as part of my course. It's that interesting. Epigenetics. Consider identical twins.
They have the same DNA code, right? But if one of them develops a disease that has a major genetic component, it does not necessarily mean the other one will also develop it. Even though the identical twins have a genetic component.
the same genetic code. Why? Recall your DNA has bases, and those bases code for physical traits like your eye color, your hair color, your height, and more. We learn about this in biology. But there's something else at work too.
Epigenetic factors. I don't mean to sound so dramatic with my voice when I say epigenetic, but I really wish this had been something that I had learned about even when I was in high school. Although, to be fair, scientists are still learning a lot.
about epigenetic factors, and this field is already more advanced than it was when I was in high school. The root epi can mean above, which is fitting for the word epigenetic as above genome. You can visualize this word as factors that work on the genome and change how the genome is expressed. Let's talk about epigenetic marks.
These can influence gene activity. One example is methylation, methyl groups that are attached to DNA bases in certain locations and they keep transcription from occurring. You remember transcription, right?
It's part of protein synthesis. You have the DNA and you transcribe it to mRNA. That's transcription.
You have to do that because the other part of protein synthesis is translation. And that's when the mRNA is used as a template so the correct amino acids will be brought in to eventually make a protein. But DNA methylation can prevent transcription, so the DNA that is methylated will not be actively transcribed. Demethylation can remove methyl groups where those areas of DNA then could be actively transcribed. Epigenetic marks don't have to be on the DNA directly.
They can also impact the histones. Histones are proteins that DNA will be rolled up and wound around, often compared to a spool for the DNA. Histones, too, can have methyl groups attached, which would be histone methylation, and this generally tends to prevent transcription.
And we also want to mention histone acetylation, which is where acetyl groups are attached to certain parts of the histones. Now when acetyl groups are attached, this causes a change to how tightly histones grip the DNA. The packing is more loose, and it allows transcription factors to access the DNA.
There is also histone deacylation, which removes acetyl groups and this will cause the DNA to be more tightly wrapped around the histones. This intense packing will limit the access of transcription factors and enzymes involved in transcription, and this can be used along with methylation. Epigenetic marks, like the ones mentioned, can impact which genes are expressed, and that can make very big differences.
Like what kind of difference? There are several examples in rodents that we have referenced in our video details. One example is how parental care that female rats have on their offspring, specifically whether female rats lick and groom their young or not, can affect the control of the expression of the GR gene.
The GR gene codes for receptors that are involved in regulating the stress response in rats. Epigenetic marks, which in this case are influenced by the mother rat's behavior, can influence the transcription of the GR gene in the offspring. Due to whether this GR gene is frequently expressed or not, This can affect the baby rat's stress response and behavior even when they reach adulthood and have their own offspring.
Another example is the agouti trait in mice. Consider two mice that have the same DNA. Only, they don't look alike because one of them is a brown mouse with an average mouse size and the other is a yellow mouse with obesity.
In the yellow mouse, a gene called the agouti gene is being actively transcribed. The gene is NOT being actively transcribed in its twin. It is actually methylated.
And it turns out the active agouti gene can affect more than the color as it also makes this mouse prone to being obese and potentially prone to specific diseases. Scientists found that the environment that a pregnant mouse is in and the diet that the pregnant mouse has can impact whether the offspring will have this agouti gene active or not. And if a pregnant mouse has fed foods that are high in methyl groups? Well, it turns out this can decrease the expression of the agouti gene in her offspring even if the pregnant mouse herself fully expresses the agouti gene.
Before I move on, two quick points I want to mention. First point is about inheritance. So when an organism's body cells divide in mitosis, the new daughter cells can inherit the same epigenetic marks as the original cells they came from. This can be inherited from cell to cell.
Now, when a sperm cell and an egg cell combine to form a fertilized egg, which occurs in humans and many other organisms, epigenetic marks are often cleared. Except, some epigenetic marks are not cleared and can therefore be inherited from parent to offspring in this way. See the further reading details. 2. I don't want to create a misconception that epigenetic marks are only something influenced by the external environment. So for example, you have many different types of specialized cells.
After all, most of your human body cells contain your entire DNA code, but generally your cells are actively transcribing certain genes based on the cell type. Meaning your eye cells and stomach cells aren't actively transcribing the exact same genes. Many of these genes are actively transcribed or not actively transcribed.
due to epigenetic marks from internal cues in addition to external ones. It's critical for human development too. One last thing.
You may wonder why is the field of epigenetics so important? Well, as we mentioned earlier, many diseases that have a genetic component can involve epigenetics. Cancer can be an example, and cancer is one that personally has affected our family and many other families.
We talk about cancer in our cell cycle video. And yes, with cancer, there is an uncontrolled growth of a person's own cells. And also, as we mentioned in that video, those cells are not functioning like they should. And by not functioning like they should, it can mean that there are genes that are actively transcribed that should be turned off, or genes that have been inactivated that should be actively transcribed.
In cancer cells, the epigenetic marks are often not arranged as they should be. In a fascinating field known as epigenetic therapy, researchers are looking to advance cancer treatment options and treatment for a lot of other medical conditions too by targeting those epigenetic marks acting on the genome. Overall, increasing our understanding of the epigenome, and by that I mean the genome and all of its epigenetic marks, will help us all better understand how our cells and bodies function.
Well, that's it for the Amoeba Sisters, and we remind you to stay curious.