[MUSIC] Hello and welcome back to our Coursera class. To recap from our last discussion, the central dogma of biology is that DNA is used as a template to transcribe RNA. And that RNA is used to synthesize proteins, and those proteins give you your unique phenotype of physical characteristics. And, different cells turn on and turn off different DNA sequences by a process called differentiation. What we're going to about today is how gene expression is controlled. Epigenetics is one of the ways gene expression can be controlled. Epigenetics is altering gene expression without actually changing the DNA sequence. There are several ways that can be done. One way is called Metyhlation. Methylatoin is a process where which methyl groups are added on to your DNA sequence and they can affect how the genes are transcribed. The chromosomes that you inherited from your mother are actually methylated differently than if you had inherited them from your father. This is called genomic imprinting. This means that the DNA received from your father will be expressed differently than the DNA received and inherited from your mother. So methylation again, will affect the DNA structure and heavy methylation is associated with DNA that is not expressed or used to make RNA, which we would say is transcriptionally inactive DNA. That means that DNA can't be read. So, imagine this cookbook. If we had heavy methylation what would we see is methyl groups would be added on, on certain DNA sequences. And what that's going to do is affect our ability to read pages of the recipe. So when I try to actually open and read the book these methylation patterns are preventing me from doing it. As I said earlier, one way methylation can happen is through imprinting. If your chromosomes came from your mother or father, we'd see different methylation patterns. Or your lifestyle can alter your methylation patterns, and some of those ways include smoking, stress, and starvation. So, especially during early development. If you were deprived of nutrients this can affect your methylation pattern. In response to starvation you have thrifty genes that can get activated and other genes are methylated. This'll change your DNA, DNA expression and that expression, those changes can affect any children in the future you may have. There have been some interesting studies about people who grew up under extreme conditions of starvation during the Great Depression in the United States. And now we know that the methylation patterns are not just those people, but their children and even their grandchildren have been affected. So even though their children or grandchildren may have had the same genes, the different methylation patterns can alter the expression of those genes, and this can have a huge impact on a person's phenotype. So even though they have a specific gene, it's not expressed. Normally genes come from protein and one of the things that we see is that the DNA sequence determines the amino sequence in a point to point manner. We talked about that during our last discussion with the sickle cell trait. When DNA is used to make an RNA template and this happens in a linear fashion and then that RNA is read in three letter words called codons. Before we discuss codons, let's talk a little bit about the nature of DNA. Only a small percentage of your DNA codes for proteins. There are a lot of regulatory regions on the DNA. Some of those regulatory regions are called promoters, and promote, these promoters promote transcription in specific regions of DNA called genes. We also have enhancer sequences, and you can guess by the name, they enhance transcription. And we have activators which bind the region such as enhancers or promoters to again, increase transcription. All of this leads to gene being highly expressed. This gene expression can be controlled on many different levels. We talked about methylation, which decreases gene expression. But we have a strong promoter and activators that can be present, and they can increase gene transcription. To review a concept I introduced during our last lecture, form dictates function, and that doesn't just apply to proteins. It actually applies to everything, from the type of chair, the shape of the chair, how a door is designed to open and close. This means that the form or shape of anything. DNA in this of course, is related directly to its function. So methylated DNA can't be read, that's changed its shape and in turn its function. So, highly compacted DNA actually has the same kind of effect. Highly compacted DNA cannot easily be transcribed. Now, the other important player we've talked about, RNA. More specifically, we'll be talking about messenger RNA or mRNA. That plays a major role in translation, mRNA also has special regions, it has a start region and a stop region for translation. And throughout the mRNA, there are regions called exons and introns. And contrary to what the name implies, Introns are actaually the regions that are taken out from the mRNA. So to think about the cookbook analogy. If you have a recipe and there are some ingredients you may not use simply because they're not available or you don't like them, those ingredients would be the introns. You remove them and they're not included in the final meal that you make. The ingredients that you keep for the sections of mRNA are going to be used to make the protein and we call those again, exons. mRNA processing can be a player in the final protein that is synthesized. By selectively splicing different exons together we can create new recipes from that cookbook. That means when you're using your cookbook to make a meal, by adding or omitting certain ingredients in the recipe you can create slightly different versions of your dinner tonight. All of these processes we discussed in lecture work together to control gene expression. So, to summarize. Physically changing the structure of DNA through compacting it or altering methylation patterns reduces gene expression. But we can also increase gene expression, translation, by having strong motors, enhancer sequences, or the presence of activator proteins. Our cells can even splice together different exons using the same mRNA template to create an assortment of different proteins from the same initial DNA sequence. We'll discuss later this week how environment can play a role in altering gene expression by initiating the release of neurotransmitters or hormones. When they bind to a specific receptor, they activate transcription of certain DNA sequences. [BLANK_AUDIO] [NOISE]
