In this video, we're going to introduce another type of eukaryotic chromatin modification, which is DNA methylation. In addition to histone modifications like histone acetylation and histone deacetylation, the actual DNA sequence, not just the histones, can also be chemically modified to regulate transcription. The most common DNA modification is methylation, and methylation is really just the process of adding a methyl group or a CH3 group, to another substance. Typically, when it comes to DNA methylation, it turns out that the nucleotides, cytosine, are the ones that are most susceptible to methylation. The cytosine residues are the ones that are going to be methylated. DNA methylation is a way to prevent transcription by blocking RNA polymerase's access to the promoter. DNA methylation will turn off a gene or turn off transcription. In our image down below, we can see in our example that methylation of cytosine nucleotides is going to block transcription and turn off a gene. Notice that we have acetylated histone modifications on the left-hand side. The stars, the green stars from our last lesson video, represent acetylation. Acetylation we know is going to turn on genes. Genes are going to be turned on by acetylation because the DNA is going to take a euchromatin state, and that is going to promote transcription. The gene's turned on by acetylation by promoting transcription. The RNA polymerase will be able to bind to this open and available DNA and will be able to transcribe genes in this DNA that is available here. This is a way of turning on the gene. However, methylation of specific cytosine residues, cytosine nucleotides. Methylation of cytosine, which methylation is adding a CH3 group to, the molecule. It's going to be represented using one of these red star shapes. Notice that the DNA itself can actually be methylated. This modification occurs to the DNA sequence directly, not to the histones. DNA can be methylated and the DNA methylation is a way to turn the gene off. The genes are going to be turned off. Notice that the methylation is blocking RNA polymerase from binding to the DNA. The RNA polymerase will not be able to transcribe the DNA, and the gene is being turned off through methylation of cytosine nucleotides. Through acetylation and methylation, eukaryotic organisms have a very complex way to be able to regulate their genes, turn the genes off and turn the genes on depending on the type of modification that's made. This here concludes our introduction to DNA methylation, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
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16. Regulation of Expression
Eukaryotic Chromatin Modifications
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