now that we've set up what transcription and translation are we are going to turn our attention to how those processes actually work and um like at the molecular level what is happening to cause DNA to turn into RNA and then RNA into or how to use RNA to make a protein just like DNA replication transcription has three steps translation also has the same three steps there is initiation elongation and termination during initiation RNA polymerase is going to bind our DNA uh during elongation RNA polymerase builds the RNA um adding nucleid to the three prime end of the growing mRNA um chain and then during termination the RNA polymerase is going to detach from the DNA once it reaches a specific sequence that kind of tells it to hey this Gene is done let's stop transcribing in order for RNA polymerase to bind DNA it needs to find a certain uh DNA sequence called a promoter these promoter sequences are often full of A's and T's um because if you remember adenine and thyine they only have two hydrogen bonds that hold them together whereas cytosine and guanine have three uh so because of that these spots of DNA are kind of easier to pull apart separate which is necessary for transcription to happen so RNA polymerase is going to find a promoter with the help of some proteins and procaryotes the proteins are called Sigma factors but in UK carots they are transcription factors these transcription factors will bind DNA um both directly on and around the promoter region and by binding these transcription factors will recruit RNA pimas to this promoter sequence where it will begin um uh the process of transcription for UK carots uh each gene has its own promoter whereas in procaryotes uh multiple functionally related genes will have the same promoter and we'll look more closely at those sort of um uh Gene Gene sequences in our next module when we get into regulation of gene expression um but for now just know that the ukar have have a promoter for each gene whereas procaryotes have multiple genes controlled by a single promoter so when an RNA polymerase binds a promoter in a UK carote it is typically there just to transcribe a single Gene whereas in procaryotes when an RNA polymerase binds a promoter it is going to transcribe multiple genes elongation is pretty straightforward once RNA polymerase binds DNA it is going to uh uh recruit uh complimentary nucleotides to uh the growing mRNA chain and uh catalyze the formation of a phosphodiester bond between between the nucleotide on the three prime end and the new nucleotide so these nucleotides are kind of floating around the nucleus and will only bind to a growing nucleic acid uh if RNA polymerase is there and it is complementary to the template strand that the MRNA is being uh built off of uh and again the RNA polymer polymerases can't proof read super well uh and so they are are fairly prone to mistakes but as we discussed in the last recording it's not as important for RNA to proof read because these mrnas are more transient um we don't need to use them over and over again whereas DNA those templates are used for transcription for pretty much the rest of the cell's life um unless the cell divides uh so so if the DNA is wrong we're going to make tons and tons of wrong RNA whereas if the RNA is wrong we've just made one bad RNA and we can always make another one for termination there are multiple uh proteins involved but I'm not going to get into those um really all I need to know is that termination uh there is a sequence at the end of the gene that RNA polymerase recognizes and when RNA polymerase gets to that sequence it knows to to unattach from the DNA uh and then with it the MRNA will also detach and then we have our mRNA transcript and before that mRNA transcript is sent out to the nucleus for translation in new carots we need to do some processing this slide uh reiterates what I was just talking about uh the difference between promotors and UK carots and procaryotes and it also brings up that for UK carot there are lots of noncoding uh sections of genes uh these are stretches of DNA that are not used to make a protein and these stretches of DNA are interspersed within genes they are called indons and that's labeled here uh in light blue whereas the exons are the stretches of DNA that are used for uh that are are going to help be used to to help build a protein um after so this is our DNA after transcription we have what is called pre-mrna so this is mRNA that includes our intron sequences before we leave the nucleus this premrna is going to lose those introns and also uh get the addition of a couple structures a five Prime cap and a poly a tail so let's talk about how that processing Works RNA splicing is going to remove the introns uh and then splice together or combine together the exons that are left behind there is a short sequence uh that sort of highlights for the cell where the beginning and end of an intron is and that's where structure called the sply is going to bind uh the sply is made up of uh small rnas and proteins and essentially what it does is cut the DNA at that border between the Exxon and intron and then join the two exons together um and so it does this for each of the exons sorry for each of the introns in a given Gene so what we're left with is no more introns and just continuous stretch of exons uh after splicing occurs um we have our mRNA but that mRNA is really vulnerable to being broken down it's not a very stable structure like DNA is uh and so uh this is you know one reason why it it exists transiently and we have to make them over and over again um but we want it to last in the cytoplasm long enough for translation to occur so we give it some protections the first protection is the five Prime cap this is on the five Prime end of the MRNA the addition of a methylated guanine uh nitrogenous base uh the methylated guanine is a a modified version of a regular guanine um that you don't need to understand why but is just more stable than most other rnas and it won't be targeted by ribonucleases which are enzymes that break down RNA on the three prime end we add something called a polyat tail which is just a long stretch of Adin that will be targeted by the ribonucleases but they act as kind of a buffer for the MRNA kind of similar to the function of telr at the ends of our chromosomes the polyat tail is there to get destroyed but maintain the you know in quotes important part of the uh mRNA the part of the MRNA that is used to make a protein this polyal is Expendable and therefore just protection of the coding region of the MRNA transcript um so once we have this mature mRNA we exit the nucleus and then that's where transl is going to occur and that's what we'll talk about on the next two recordings