Post-Transcriptional Modifications of mRNA

the process of transcription involves the production of RNA molecules from DNA molecules and what this process actually involves is the passing down of the genetic information from the DNA to the RNA molecules and it's the RNA molecules that are directly involved in the process of protein synthesis that takes place in the cytoplasm of our cell now one important type of RNA molecule is known as messenger RNA or mRNA and it's the MRNA that actually serves as the template for protein synthesis that takes place in the ribosomes in the cytoplasm of our cell now before our messenger RNA actually leaves the nucleus and enters the cytoplasm where it attaches to the ribosomes and under go protein synthesis our messenger RNA has to undergo several important types of modifications several important types of post transcriptional processes now within the nucleus of our cell we produce a molecule known as the precursor mRNA so the MRNA that is formed directly following the process of transcription before any type of posttranscriptional modific ation actually took place is known as the precursor mRNA or simply as the MRNA or the pre-mrna so this is the MRNA that has not yet undergone the necessary modification processes that are needed for the molecule to exit the nucleus and enter the cytoplasm and there are three important types of posttranscriptional modifications so we have the addition of the five guanosine triphosphate cap we have the polyadenylation of the three and tail and we have the splicing of our exons and the removal of our introns so let's take a look at each one of these posttranscriptional modifications of the MRNA and see what they are and why they actually take place and let's begin with the addition of the five and guanosine tri phosphate cap So within the nucleus of the cell before our premrna actually leaves the nucleus the five end of the precursor mRNA is altered by the attachment or the addition of a guanine nucleotide via a special type of bond a special type of linkage known as the 5 to5 triphosphate linkage and following the additional the attachment of this cap the guanas nucleotide is also altered in several ways one of the ways in which the guanas nucleotide is altered is by the methylation of the seven position to form the seven methyl guanosine nucleotid so this type of five guanosine triphosphate cap is shown in the diagram so let's pretend that this is the five end of our premrna that is synthesized directly following the process of transcription so basically what we do is we add this guanas nucleotide via the following triphosphate Bond the reason it's called a triphosphate Bond or a triphosphate linkage is because we have one two three phosphorus atoms and the reason it's called a 5 to5 triphos F at linkage is because that linkage is between the fifth Carbon on this sugar and the fifth Carbon on this ribo sugar so this is the five guanosine triphosphate cap now the question is why exactly should we add this cap to our premrna molecule what is the function of this cap well basically one important function of this cap is to protect uh our mRNA from the degradation that could take place during the process of translation during the process of protein synthesis and not only that this addition of the five cap also basically gives our mRNA the ability to leave our nucleus through the nuclear pores and enter the cytoplasm of our cell so it also stabilizes the Mr RNA molecule and AIDS in transport across the nuclear membrane of the nucleus of the cell now let's move on to the second type of process second type of posttranscriptional modification that takes place in the nucleus known as the poly adenol of the three and tail so before the nucleus or before the premna actually leaves our nucleus our tail has the actually be removed so a small section of the tail of the premrna is removed and instead we add many adenosine nucleotides and this tail this three and tail is now known as the poly a tail so in this type of posttranscriptional modification the three end of the precursor mRNA is removed and a series of adeny nucleotides are added and therefore the three and tail that contains the many adenosine nucleotides is known as the polyadenosine tail or the polyadenine tail now just as the five cap adds the ability to resist different types of degradation in the cytoplasm and gives our mRNA stability the polyat tail also provides the MRNA with stability and keeps the tail from degrading in the cytoplasm of the cell and it also AIDS in the transport of our pre um of our mRNA from the nucleus to the cytoplasm of our cell so basically what happens is if this is our mRNA molecule if this is the pre-mrna molecule and this is the five n and this is the 3 end what happens is a small section of of the three end of the premrna is cleaved is removed and then we add a bunch of adenosine phosphates how many well anywhere from 200 to 250 adenosine molecules adenosine nucleotides are added to our tail to our end and that's exactly why it's known as the poly a or the polyadenosine tail now let's move on to the final type of posttranscriptional modification and this is known as the process of splicing so we remove regions uh known as the introns and we combine the regions known as the axons so not all regions of the pre RNA molecule code for our protein or proteins those regions that do code for the proteins are known as axons and those regions that do not code for the prot protein are known as introns and so uh special type of special types of enzymes that together are known as the spone basically remove our introns and splice together or combine or glue our axons and we form the final mRNA molecule so basically to see what we mean let's take a look at the following diagram so let's suppose this is our precursor mRNA that now contains the polyat tail that is shown here and it also contains the five guanosine triphosphate cap now before our premrna actually becomes the MRNA and is able to leave the nucleus the final process that has to take place is the following so the molecule contains sections known as exons and these are the sections that actually carry the genetic code that codes for our protein it also contains these introns which are sections that do not code for any type of protein so what must take place in the nucleus is a certain set of proteins known as the spone has to remove these intron and basically splice together or combine our exons so these three sections are removed and we basically splice together these four Exxon sections including the poly a tail and so we form the following molecule as shown so it also should contain our polyat tail so once all these processes actually take place once we add the five cap once we add our tail that contains the poly Iden see nucleotides and once we splice together the exons and remove the introns only then does our premrna molecule become the MRNA molecule and only then can the messenger RNA actually exit the nucleus into the cytoplasm and undergo protein synthesis and be used by the ribosomes to synthesize our proteins now the entire purpose of adding this is five cap and the polyat tail is to basically give our mRNA molecule the ability to resist degradation by different types of enzymes and different types of proteins it also gives our mRNA molecule the ability to basically travel from the nucleus and into the cytoplasm of our cell

Channel: Andrey K

Views: 176,879

Keywords: pre-mRNA, precursar mRNA, pre-messenger RNA, precursar messenger RNA, messenger RNA, post-transcriptional modification, post-transcriptional modification of mRNA, modification of mRNA, 5' guanosine triphosphate cap, guanosine triphosphate cap, Post-transcriptional Regulation, modification of messenger RNA, polyadenylation, polyA tail, exons and introns, splicing, splicesome

Id: ertPYuifrQM

Length: 10min 54sec (654 seconds)

Published: Sat Sep 06 2014