In this video, we're going to introduce a specific type of chromatin modification, which is histone acetylation. Histone proteins that are found within nucleosomes of chromatin actually contain a long polypeptide tail, and this long polypeptide tail that extends off the histone proteins can actually be chemically acetylated. Acetylation refers to the process of the addition of an acetyl group. You can see down below in our image, an acetyl group is just a specific type of functional group, the one that you see right here. In our image down below, we're going to represent acetylation and acetyl groups by using a star symbol to represent the acetylation. Now, histone acetylation actually impacts the chromatin structure because what it does is it loosens the chromatin structure and that helps the chromatin take on a euchromatin formation, making the DNA accessible to RNA polymerase and allowing that DNA to be transcribed at a high rate. In our example down below, notice that we're focusing on how histone acetylation loosens the chromatin structure forming euchromatin.

If we take a look at our image down below, again, on the left-hand side, we're showing you our miniature version of the map. You can see that chromatin modifications, which include histone acetylation, take place within the nucleus of the eukaryotic cell. Here, we're showing you the chromatin in a heterochromatin form where the nucleosomes are really tightly packed together. But notice that extending off of each of these histones are these little tails, called the histone tails. These histone tails are capable of being modified by cellular enzymes. In this formation, the heterochromatin, the DNA is basically in an off state, and it's not going to be transcribed very much. However, through acetylation, which is represented by this arrow right here, acetylation can help turn the DNA into an ON confirmation, the chromatin structure into an ON confirmation because it changes the chromatin to a euchromatin state where it is more loose and the DNA is more accessible to RNA polymerase and more accessible to transcription. You can see the little stars here on the histone tails represents the acetylation. Acetylation is going to be a way to help allow for transcription to occur.

The removal of the acetyl groups in a process called deacetylation is actually going to result in the opposite. It's going to result in tight packing of the chromatin structure, basically reverting the chromatin back to a heterochromatin state. On the left over here, we have the heterochromatin, which we talked about in our last lesson video. Over here on the right, we have the euchromatin. Notice that acetylation will help promote a euchromatin state where transcription is more active. Then deacetylation is going to promote a heterochromatin state where the DNA is not going to be transcribed as much. Histone acetylation is a chromatin modification that can occur to help regulate gene expression. We'll be able to get some practice applying this as we move forward in our course. So, I'll see you all in our next video.