in procaryotic cells such as bacterial cells operons are used to regulate gene expression now what about in eukaryotic organisms how do eukaryotic cells such as the cells found in our body actually regulate gene expression well because eukaryotic cells are so much more complex than procaryotic cells we see that the method by which genes are regulated in eukaryotic organisms is much more complex than that in procaryotic organisms and there are many different levels at which Gene regulation and gene expression can actually take place and in this lecture we're going to focus on the most common method of regulating genes in eukariotic cells and this is on the level of transcription itself so by actually changing the rate at which transcription takes place so in this lecture we're going to briefly discuss the components that are found along a eukariotic gene so this basically describes a diagram of a basic eukariotic Gene and we have five important components we have exons we have introns we have our transcription start site we have promoters and we have enhancers so let's begin by discussing what we mean by exons and introns so in any eukariotic Gene we we have those segments of DNA that actually do code for a polypeptide and those segments that do not code for polypeptide so in this particular diagram these are the exons they are segments of DNA that do code for a polypeptide while these orange sections are the introns they do not code for anything useful and eventually during the process of RNA splicing these sections are actually removed from that mRNA molecule and only these exons are actually left now what about this segment right over here so this green segment is known as the transcription start side and this is where transcription actually begins so a special protein complex that consists of 12 proteins known as RNA polymerase 2 binds onto the start side the transcription start side and this is where initiation actually takes place this is where we begin the process of transcription so this is our transcription start site so this is our start site this is Exon number one this is Exon number two and Exon number three this is let's say intron number one and this is intron number two so we have introns exons a start sign site now what about this segment and this segment here so together the this entire region is known as the promoter so just like operons and procaryotic cells contain promoters so do these eukaryotic genes but the promoters of eukaryotic genes are much more complex so we have different parts of the promoter so the promoter can be broken down into the core promoter which is basically shown in pink this is our core promoter and these remaining promoters are known as the Upstream promoters now notice that the core promoter this entire segment is found Upstream to the left of the start side in fact usually this is located about 40 bases to the left of our start side now this core promoter basically doesn't change from one eukaryotic Gene to another eukaryotic Gene so it remains consistent in fact the most common type of core promoter is the Tata box core promoter and the reason we call it tatab box is because the sequence of nucleotides is TA a and that doesn't change when we go from one eukaryotic Gene to another so that recurs between different or that recurs within different uh types of eukaryotic genes now what's so special about the Tata box what's so special about the core promoter in general so this is where all the different types of transcription factors and Regulatory proteins actually bind to and they form a complex that consists of over 50 different types of proteins and this complex is necessary for transcription to actually take place so we have this complex of proteins that binds onto our Tata box and it basically calls upon other proteins those other proteins bind forming a larger complex and so forth and eventually that interacts with the protein found at the start side and that initiates the process of RNA transcription so two important types of proteins you should be familiar with is transcription Factor 2D and transcription Factor 2B now transcription Factor 2D is itself a complex of proteins and one of the important proteins within the transcription Factor 2D is known as The tatab Binding protein or tbp and this is the protein that actually binds to the sequence ta a found on the core promoter now we also have the transcription Factor 2B and this is the protein that is needed for the interaction to take place between the RNA polymerase 2 protein found on the start side and the Tata binding protein so that's why we need the transcription factor to be so we see that the core promoter is where the majority of the proteins actually uh congregate when they actually bind now what about these other Upstream promoters so these are known as the Upstream promoters what's so special about the Upstream promoters well these are also segments of DNA that allow The Binding of different types of regulatory proteins so they can either be proteins that activate transcriptional proteins that repress transcription inhibit transcription from taking place now unlike the core promoter which doesn't really change when we go from one eukariotic gene to another we see that the number and the types of Upstream promoter do vary between different types of eukaryotic genes now the final type of section that we're going to focus on is known as the enhancer so this section is known as the enhancer and notice it is found much farther away from our Gene than anything else in fact the enhancer is usually found thousands of bases upstream or Downstream our Gene now what's so special about the enhancer well basically the enhancer can also bind special transcription factors and when a transcription Factor binds onto the enhancer the enhancer will essentially loop around and bind onto the protein complex found on the promoter and this interaction will stimulate transcription and increase the rate of transcription of this Gene so the enhancers are segments of DNA that are typically located far away from the gene they can be either up Upstream they can be either Downstream or they can also be in some cases inside that Gene itself now enhancers B special transcription Factor proteins that increase the rate of transcription and how that takes place is this entire complex essentially loops around so this is our enhancer this is the transcription Factor so the transcription factor that bonds onto our enhancer and then and then this entire complex so the transcription factor bounc of the enhancer binds onto the protein complex found on the promoter and once that binding takes place that stimulates the transcription of that particular Gene it increases the rate at which transcription actually takes place so this is one method by which eukariotic cells actually regulate the rate of their transcription and we'll focus on this in much more detail when we'll get into biochemistry