Lecture 2 - DNA Mutations & Transcription

追加: 2026-05-15

[00:00:01]hi everyone welcome back this week we're going to be moving on from looking at how we can create identical copies of our dna through dna replication to then looking at some of the mutations that can take place while we're replicating and then also how we produce that working copy of messenger rna because remember our dna houses our genetic code all of those genes are located in our dna in order to produce proteins we need to be able to take working copies of that dna have them leave the nucleus so that we can synthesize a protein from that so this week we're going to be looking at how do we make that working copy through the process of transcription now last week we had talked about how long our dna would be if we were to stretch out even just say a single chromosome right it was about two inches for a chromosome and we worked that out to then if every cell in our body that would be about 92 inches total and then if all of the cells in our body we were to stretch all of those out we would have roughly 100 trillion meters of genetic material so when you think about every single time one of your cells divides your dna polymerase enzymes are responsible for creating all of those identical copies and so because so much dna is being replicated every cell cycle there's a huge potential for errors to occur now in our whole genetic code we have about 130 genes that themselves code for dna repair enzymes in addition dna polymerase has the capacity to be able to proofread as it's going along laying down those new nucleotides it is capable of being able to check for errors along the way because remember if you have an adenine in the parent strand you need to pair that with a thymine and guanine paired with cytosine now even though proofreading exists with our dna repair enzymes being able to repair mistakes as well as dna polymerase being able to catch that it's not perfect and so mutations where you have an incorrect base or where a base has been omitted in the sequence can occur now one tumor suppressor gene that we talked about was p53 remember p53's function when it's operating properly is to signal those dna repair enzymes to fix mistakes but if the p53 gene has a mutation in it itself that's where things can go rogue now if those mutations still existed remember we had two main options either we would signal for programmed cell death which is apoptosis or our own natural killer cells or our immune cells specifically the t cells they would then take care of these mutated cells to prevent them from being able to continue to divide now when we miss these mutations and they have impacts that cause them to replicate rapidly and ultimately can become cancerous that's where cancer and tumors can develop from now recall we have two main cell lines we have the germ cell line and we have somatic cell line so the germ cells those are the ones that are ultimately in males going to be producing the spermatozoa and in females producing the ovum right so they produce the gametes the somatic cells remember they produce all of the other cells in our body so if we have a somatic cell mutation that's a mutation that's taken place in the organism itself and will only impact that organism okay so we're looking at humans here so that individual ultimately that will not be passed on to future generations when we were looking at genetic inheritance last semester it was the germ cell mutations that would then be passed on to future generations and if they were advantageous and if they were advantageous they would then be the fittest for that environment now when a mutation occurs within the germ cell that's where that can be passed on to future generations and this is how new alleles are formed within populations primarily so it's important to understand not all mutations are necessarily bad some of them don't even impact protein synthesis which are known as silent mutations but many dna mutations that do occur can be detrimental now there's two general classifications of dna mutations we have point mutations and recombination mutations now point mutations occur at a specific point hence their name these errors typically occur during dna replication okay so dna polymerase is typically the culprit in these cases and you can either have a substitution where you have an incorrect base pair that's added in you can have additions where an additional base pair in the sequence is added or a deletion where one of the base pairs that should have been there is omitted in both of these cases with additions and deletions you can end up with what's known as a frame shift okay and we'll get into that when we look at translation a little bit later then we also have recombination mutations and there are specific genes that are known as transposons and these transposons are capable of being able to be removed from a sequence of your dna and moved to another part of the gene and so you can appreciate if you have a genetic code that is coding for something and you move a gene into the middle of that code well that's that could have a huge impact on how that protein is going to be synthesized so we're going to take a look here at some examples of some point mutations so here at the top we're looking at a normal dna sequence so you can see you have your two parent strands the top strand is running from five prime to three prime left to right and the bottom strand is running three prime to five prime left to right well whenever we are going to be transcribing that is taking that copy we are going to be working with the strand that's running from three prime to five prime and so you can see here we've used this as our template strand and then we are creating that mrna copy from that template now remember during transcription we're producing messenger rna so if it's rna you know number one it's going to be single stranded but you also know there's a difference in bases right instead of thymine we're going to have uracil and so if you follow along here you can see this is our mrna copies so guanine pairs with cytosine thymine pairs with adenine adenine adenine the adenine here is going to pair with uracil and so on and when we read this mrna we're going to read it in a sequence of three base pairs known as a codon and each codon codes for a specific amino acid so aug codes for methionine acc codes for threonine ggu codes for glycine uca for serine ugc for cysteine and uua for leucine alright so we have a sequence here now of amino acids this is our polypeptide that we have formed so this is under normal conditions this is what should be happening okay we have our dna this is where the genetic code is stored we create a working copy of that and then ultimately we're going to translate that working copy by the ribosomes and that will create our polypeptide our protein now if we have a point mutation that occurs anywhere along this sequence that's going to then be transferred to our mrna working copy and that's also likely going to change what we end up with with our final protein okay so let's take a look at some examples some of the point mutations that we can have are substitutions as mentioned insertions and deletions so with substitutions we're looking at we have changed a base the base that should be there is not there and we've put in another base now three different types of substitutions can take place first we can have missense substitution mutations and missense mutations are those that are going to change the amino acid that is being coded for with nonsense mutations these actually cause a change in the way that we're going to read the mrna causing translation to stop at that point okay known as a stop codon and then with silent mutations these are ones in which you will not see any changes okay as we're going to see a little bit later when we look at translation there are a number of different codons that all code for the same amino acid and for many of these the third base can be substituted for any of the nucleotides and it would still code for the same amino acid okay that doesn't apply for all of them but that tends to be a trend for a lot of them now we can have insertions where a base is added into the sequence or we can have a deletion where a base is removed from the mrna sequence and so we're going to talk about the consequences of those deletions and insertions because what's going to happen is the whole frame through which your ribosome is going to read that mrna that's all going to shift okay so here's our normal dna and this is the polypeptide that we are going to be forming from it now if a point mutation occurs specifically a missense mutation okay so here we had t and a the mutation that's taken place is now we have cytosine pairing with guanine so now when we read that and take our mrna copy from the template that's going to change this base here so instead of having uracil we now have a cytosine and now this codon cca is going to code for a completely different amino acid before we had serine being coded for and now we have proline now when you have a completely different amino acid in this sequence that is going to change the overall structure of the protein as well as its function okay so with a simple substitution here where we've had a point mutation that's taken place we have a missense mutation because it's changing the codon and ultimately the amino acid that's being coded for now we're going to take a look at this sequence so originally we had cca and so our mrna would be ggu and ggu codes for the amino acid glycine now we're going to look at a frame shift mutation here specifically in insertion and so here what's happened is during dna replication we have had an insertion of a base pair here that was not there in the original dna sequence so what happens then is that it shifts all of the other bases in terms of this reading frame because remember we have a start codon and that signals where the ribosome is going to start to read these nucleotides in three base pair sequences so with an insertion that ends up shifting the way in which our ribosome is going to read the mrna so you can see here that the insertion of thymine in the template strand is going to be copied over as an adenine in our mrna and so now this codon is going to read g a g well that's going to code for a completely different amino acid what's going to happen next completely different amino acids so you can see here that insertions as well as deletions they cause frame shifts and those typically have a very detrimental impact on the overall protein and its function as a whole the whole frame in which we are going to be reading these triplet codons is going to be shifted and now you're going to have completely different amino acids being coded for so here are some examples of point mutations just using simple sentences all right so we have our initial sentence the cat ate the rat all right and so we've used three letter words to symbolize our triplet codons that we have within our mrna so if we have a substitution here we substitute the c for letter b well that's going to change the sentence but does it have a huge overall impact well in terms of context it does so likely this would be something like a missense mutation so ultimately looking at mrna that would change the amino acid coded for but it's only the one that's being impacted here with an insertion okay here what we've done is we've inserted an additional letter here and as soon as you do that and you shift all the other letters down well that completely makes the cca tht f era it's a completely different sentence and similar here with a deletion if we remove the c and then all of these shift to the left we end up with also another completely messed up sentence okay so what i'm trying to show you here is that the substitutions even if they're miss sense and they change the meaning of the amino acid it's typically only one there on that spot but if you have an insertion or you have a deletion typically that protein would become non-functional so you can appreciate if you have enough of these types of mutations that take place and get missed that's where cells can end up becoming non-functional now those were point mutations we also mentioned that we have recombination mutations now recombination mutations occur typically with specific portions of our dna sequences that are known as transposons also known as transposable elements and these transposable elements are sequences of dna that are capable of being able to be moved or transpose themselves within a cell and that transposon can either just be copied from where it exists and then paste it into a sequence into one of our genes or it can simply be cut out and then also inserted or pasted into one of our gene sequences we have portions of our genetic code that are non-coding what we have illustrated here is a sequence of your dna and this is showing you that for a given gene there are portions before and after that gene so after that sequence of dna that are known as regulatory sequences okay these tell the rna polymerase hey this is where you're going to start to copy and this is where you're going to stop copying and so what we're going to look at is what would happen if this transposable element was inserted into different portions in and around this one gene that we're looking at so in our first example here we're looking at a mutated gene where the transposon or transposable element has been inserted into the middle of the coding region okay and so remember these transposons these are also sequences of dna they're dna segments and so this is working like an insertion mutation here where we've added an entirely new sequence right into a coding region of that gene now because the regulatory regions the start and the stop are unaffected the rna polymerase is going to continue to produce the protein normally so this gene is going to be expressed but the protein is likely going to be non-functional because the amino acid sequence from this transposon is going to render itself now let's take a look at another example here this transposon now has been inserted over here before the regulatory sequence now because it's existing here before the start sequence that's going to alter the way in which this protein is expressed so for example you might have inappropriate amounts of this protein okay remember the regulatory sequences these are to regulate where rna polymerase starts and stops but also responsible in controlling how much of that specific protein is going to be produced so if we alter the regulatory sequence the protein that's produced is going to be completely functional but you might have a lot of it or you might not have enough of it now in this third sequence here we're looking at a transposon that has been inserted somewhere outside of that gene not affecting the regulatory sequences or any of the coding regions within that gene and so ultimately here you won't see this as any type of mutation that's going to be evident all right so this would be an example here of a silent mutation okay so the mutation has taken place outside of this coding region and outside of the regulatory region and so you won't actually see any physical manifestation of that all right so if the if the transposon is inserted into the coding sequence that is going to change the protein and so the protein will be non-functional if the transposon affects the regulatory sequence the protein is going to be normal but the amount of it is going to be abnormal and then if the transposon is inserted outside of the gene and the regulatory sequences well then that's a silent mutation that's taken place so it will not affect production of that protein in any way now remember last semester we took a look at a lot of different phenotypic variations that exist within our population right you know we had taken a look at in humans and flowers also within fruit flies we had taken a look at some of the variations that can exist we had taken a look at phenotypic variations that can exist within populations now this is showing you examples of mutations that are non-detrimental it's going to it's going to affect maybe the size of the wing it's going to affect the actual shape of the thorax shape of the eyes but ultimately this individual is going to be capable as will this one and this one and this one of being able to survive now some might be more fit to their environment and others less fit to their environment so ultimately mutations can occur that are non-detrimental to the organism but they can also occur where they are detrimental to the organism now when looking at this gene you can see that there are green regions and there are gray regions within our dna there are specific portions that are coding portions within a gene and those coding portions are shown here highlighted in green there's other portions within our dna sequence that are non-coding so for example if this mutation were to occur within a non-coding region well that's going to be removed anyway before we translate that mrna and so ultimately those types of mutations would also be silent the sequences within our dna and ultimately then are mrna that are going to code for the protein those are those green regions those are known as exons the gray regions that you saw that do not code for any protein synthesis those are known as introns and there's a process through which we go through and process the mrna that's been produced and we remove those introns so any mutations that take place within these gray regions these introns these are going to be removed known as splicing and so any mutations that occurred are not going to have an impact on the overall protein itself now there are a number of genetic diseases these we all looked at last semester actually that occur due to some sort of dna mutation that's taken place with cystic fibrosis one mismatched base in the gene that codes for chlorine channels has occurred and as we know it's the defect in those chloride channels that causes all of the signs and symptoms of someone with cystic fibrosis with huntington's disease remember this was an insertion of multiple cag so cytosine adenine guanine repeats in a specific gene on chromosome four okay so that's one specific type of insertion mutation that's occurred to create this disease remember with insertion mutations and having multiple insertions of cag being repeated that's going to insert a whole bunch of specific amino acids into that sequence ultimately changing the function of the protein coded for by that gene and then with sickle cell anemia it's simply one mismatch in the hemoglobin gene that causes the change in the cell shape to be more sickle cell than by concave so with cancer typically you have two or more mutations in genes that are either coding for repair enzymes so when the mistakes happen we can't repair them or those that affect the cell cycle so up regulation typically when you have a combination of both that's the perfect storm right when we up regulate that cell cycle such as those proto-oncogenes you're increasing the rate at which that mutated cell is now going to divide and since we can't repair it you're in the perfect situation now for the development of cancer so we've mentioned so far that we can acquire dna mutations through mistakes that are made by dna polymerase through replication you've seen the transposable elements so we have transposition of specific sequences those jumping genes that are capable of being able to either be copied and put into a gene or spliced out and then put into that gene those that occur within the germ cell line can be inherited approximately about five to ten percent of diseases are from inherited mutations that occur in that germ cell line remember if it's a somatic mutation that will not be passed on to a future generation only the germ cell mutations will okay and this is how certain viruses are capable of leading to cancer and then finally the most common cause environmental mutagens and carcinogens those that are capable of being able to oxidize our dna and cause those mutations to take place whether they be insertions deletions or substitutions now some of the examples of viruses specific mutagens that are capable of being able to cause cancer we talked about these last semester when we looked at viruses so human papilloma virus there's two specific strains that produce genital warts and can ultimately cause cervical cancer and remember there are a number of different strains of hpv it's only two that are known to be capable of causing cervical cancer with hiv hiv ultimately leads to aids so remember hiv is the virus itself and the virus infects our t cells so then there's a certain threshold which when our t cells reach that level um the individual is defined as having aids acquired immunodeficiency syndrome an age which can lead to composure sarcoma which is an overgrowth of blood vessels okay so if you have an overgrowth of blood vessels more blood supply perfect situation to be feeding cancerous cells in the surrounding areas hepatitis b and c can lead to liver cirrhosis but can also cause further damage leading to liver cancer and then epstein-barr virus can also in rare situations cause lymphoma which is cancer of the lymph nodes and here's a list through radiation either through uv light from the sun or even something like tanning beds or x-rays and also chemicals within our environment that are capable of being able to produce mutations that can lead to the production of cancer so now we're moving on to taking a look at chapter 9 where we look at gene regulation and expression so how are we actually going to take that specific sequence of dna transcribe it to produce that working copy of mrna which will then leave the nucleus and then be translated by the ribosome which will then form our polypeptide the protein so today's main focus is going to be here looking in the nucleus at how we take that working copy of our dna to produce our messenger rna now as a refresher remember the information contained within your dna is stored in those segments that are known as genes when we looked at the chromosomes which were the condensed dna you saw the genes as individual lines on those chromosomes right remember what that identifies is a specific sequence of that dna so remember our dna is linear and so a gene would say b from this portion here to this portion here and when we need to create the protein that that gene codes for we take a working copy of it so that it can be translated producing that protein ultimately those genes code for specific mrna sequences and that's the messenger rna that is going to leave the nucleus and then ultimately be translated by the ribosomes so that we can produce the protein so one example of a gene is the human insulin gene right this sequence is 4044 nucleotides long and exists on chromosome 1. and so when we need insulin the specific cells that produce insulin the beta cells within the pancreas to produce that hormone which is a protein ultimately we need to then have our messenger rna produced from this gene sequence on chromosome 1 in order to make the protein that we need to help us regulate blood sugar and so remember this central dogma ultimately our genetic code is stored up in our dna we produce messenger rna as a working copy of parts of it those genes and once we have the working copy of the gene that working copy the messenger rna can then be read and translated into a functional protein by the ribosomes now once that initial polypeptide is formed remember we then go through the process of folding that protein into its specific tertiary shape and a lot of that modification takes place then in the golgi apparatus so that we can produce that functional protein finally now so far we've just looked at the fact that we have specific sequences of dna that are genes those genes are transcribed and we produce mrna there is a process here known as mrna processing that has to take place first because remember in our gene we have regions that are coding and in this case they're red and other regions that are non-coding remember those non-coding regions are known as introns and the coding regions are known as exons and it's the introns that will need to be removed through a process known as splicing so that we can then put together a complete copy of that processed mrna so it is ready to leave the nucleus and ultimately have the ribosome create the protein so we're going to start here looking at this initial process of transcription before we take a look at how that mrna is going to be further processed so this might look somewhat similar to what we looked at last week when we were looking at dna replication remember we had our dna polymerase molecule and that dna polymerase 3 would then lay down the new nucleotides in a given sequence while we were replicating our dna remember there were a number of enzymes involved here and remember we were replicating our entire dna sequence so we could divide the cell what we're looking at this week is throughout all the other parts of the cell cycle when we need to produce proteins needed for the cell to function we are going to be transcribing specific sequences specific genes of our dna by a specific enzyme known as rna polymerase all right so it's a completely different enzyme from what we looked at last week and we're going to be looking at how that rna polymerase is capable of producing the mrna now as we know rna is single stranded and dna is double stranded rna contains ribose sugars whereas dna contains deoxyribose sugars and lastly we have uracil base here where we have a thymine base and dna molecule now within our cell we have three different kinds of rna to this point we've just looked at it as ribonucleic acid but we actually have three messenger rna we've already talked about okay this is that working copy that we're going to be producing from our dna from that gene so that we can produce the protein also have ribosomal rna and remember where is that stored up where do we store up our ribosomal rna well it's stored up within the nucleolus remember that dark region within the center of the nucleus that's where all that ribosomal rna is stored up and when we need to produce functional ribosomes that our ribosomal rna leaves the nucleus and then we can assemble the large and the small subunit of the ribosome out in the cytoplasm now the third type of rna we're going to talk about this next week in more detail is transfer rna and transfer rna is also known as trna and it's the trna that binds specific amino acids within our cytoplasm and then brings in those amino acids to help the ribosome form that polypeptide okay so that's the process of translation and we're going to get into looking at both ribosomal rna as well as trna in more detail next week so ultimately mrna is the message that is being transcribed from the gene the rrna is the ribosomal rna stored within the nucleolus that's making up the ribosomes and then the trna that's the rna that is responsible for transferring amino acids to the ribosome in order to allow for that polypeptide to be formed now remember prokaryotes they have a nucleoid region they do not have a nucleus so transcription which in eukaryotes is going to take place within the nucleus for prokaryotes it takes place within their cytoplasm because they don't have a defined nucleus and so for prokaryotes the process by which they make their working copies and then produce proteins it occurs all at the same time and they also don't have those non-translated regions that are known as introns remember the introns are the non-coding regions we have to remove those through splicing before that mrna can be translated to produce that protein and so because prokaryotes don't have those they're able to transcribe and translate very rapidly and this is part of the reason why they're capable of replicating so quickly because they can produce the proteins needed in order to then have themselves divide into two separate cells with enough proteins to be able to produce the functions that that cell will require now transcription in both prokaryotes and eukaryotes has three main stages so we have an initiation stage where the process is started we have an elongation stage where the mrna is going to be produced and then we have the termination stage where the process of producing that mrna is going to be signaled to stop so now we're going to walk through the process of transcription remember transcription is taking place to produce a working copy of a specific segment of our dna which is known as a gene and it's a new enzyme that's going to be doing this whole process and that enzyme is known as rna polymerase so here's our molecule of dna and here is the rna polymerase enzyme that is going to be transcribing and producing that messenger rna now if you recall within a gene we have specific regulatory sequences and those regulatory sequences are to signal for initiation to take place and so initiation takes place at a specific regulatory sequence that's known as the promoter and ultimately that promoter signals for the rna polymerase to bind to that site and there are specific regulatory proteins known as transcription factors that need to bind to that initial promoter sequence in order to allow for rna polymerase to bind to that sequence so within the promoter region we have a sequence known as the tata box and that tata box is a sequence of thymine adenine thymine adenine followed by a number of adenines and then you have your complementary sequence this tata box is where the transcription factors are going to bind to to create what's known as the transcription initiating complex all right so within the promoter region we have this specific region known as the tata box which contains the tata sequence the transcription factors will bind to the data box and the transcription initiation complex will be formed now it's important to note transcription factors they're proteins that determine which genes should be described in us as eukaryotes so when we need or when we require a specific hormone let's say or a specific protein to be produced transcription factors will bind to the tata box of the promoter sequence in order to form that initiation complex okay but the transcription factors are not bound to the data box then the transcription initiation complex will not be formed i.e rna polymerase will not bind and transcription won't take place so it's a way of sort of turning on and off transcription right when we need a specific protein such as a hormone to be produced those transcription factors will bind to the tata box and essentially initiate transcription now similar to dna polymerase rna polymerase is only going to transcribe and lay down in the five prime to three prime direction so ultimately depending on the orientation of that promoter sequence indicating the direction of transcription to take place that is going to determine whether the top or the bottom strand are going to be the template strand that rna polymerase is going to be transcribing now you can see here we have two different names referring to the two different strands so we have a coding strand and we have a template strand it's the template strand which runs three prime to five prime in the direction that rna polymerase is going to be transcribing that is the template strand it's the strand that's running five prime to three prime in the same direction as rna polymerase that's known as the sense strand or coding strand because this is the code that we want to ultimately send to the ribosome so what we do is we take the complementary strand to it and then we produce our mrna from that so you'll notice that the mrna sequence that we end up producing five prime to three prime is going to be almost identical to the coding strand with the main difference being that the coding strand would contain thymine and your mrna molecule will have uracil okay but the sense coding strand will be almost identical to the mrna molecule that is produced okay so remember it's the anti-sense strand which is complementary to that sent strand that's what we use as our template to produce our mrna from all right so you find the strand that's running 3 prime to five prime because that's where rna polymerase is going to then lay down five prime to three prime and so there will be the promoter sequence the transcription factors will bind to the data box the promoter sequence rna polymerase binds and forms that transcription initiating complex remember we don't need a primer because rna polymerase can simply just lay down and so ultimately we have this then elongation of the growing mrna transcripts remember how i said there was an initiation stage there was an elongation stage and a termination stage so initiation we looked at with the creation of that initiation complex the binding that takes place with the transcription factors then elongation takes place as the rna polymerase creates that growing mrna transcript then ultimately the rna is going to come to a terminal point of that gene where it is going to be signal to stop now this termination of that gene where we need rna polymerase to stop transcribing is going to occur when it reaches a terminator sequence typically these terminator sequences are regions that cause what are known as hairpin loops to occur where you have repeated nucleotide base pairs that cause the dna to actually loop over itself and so once rna polymerase hits that point rna polymerase comes off of the dna molecule because it can no longer properly attach to it one other thing you might notice is that hey remember in replication we had helicase unzipping the dna for dna polymerase well the rna polymerase is also capable of unwinding the dna itself so it's pretty self-sufficient and it doesn't need a primer it doesn't need an enzyme to unzip it does all of that it also has the capability of being able to proofread as it's producing this mrna transcript but you'll notice as it moves along it's also allowing for the rebinding of the dna molecule once it's finished transcribing that small portion and so as it moves along it unwinds but also rewinds the dna molecule as it's forming that transcript in summary rna polymerase is going to ultimately bind to the dna at the promoter region remember specifically the transcription factors will bind to the data box within that promoter region and when they do rna polymerase can bind and form that initiation complex rna polymerase is then going to lay down its mrna in the 5 prime to 3 prime direction and as it does it will unwind the dna creating the mrna from the template strand but also rewinding the dna as it moves along then finally once it reaches the terminator sequence those hairpin loops the rna polymerase will dissociate from the dna molecule as it has been signaled to stop translating so let's work through some practice here we're assuming here that we have the gene located between the promoter sequence and the termination sequence okay we have a sent strand and we have an anti-sense strand okay the sense strand remember this is going to be identical to your mrna with the exception of the thymine will be substituted for uracil and so what we do is we then take our mrna template from the anti-sense strand right because it's complementary so when we produce our mrna it's going to be from this anti-sense non-coding strand and the mrna that's produced ultimately would be five prime aug gcc uau gaa ucg with the three prime at this other end on the right okay and the way that you can do this quickly is if you look to the scent strand find your scent strand your coating strand and just replace the thymines with uracils and that will give you your mrna molecule that's sort of a shortcut to it but the way you can also do it is find your template strand create your complementary base pairs but remember it's mrna and that will give you your mrna molecule so now we have finished transcription and now we have to process that rna remember we transcribed our mrna from this gene and then we're going to be left with a number of exons that will code and introns that are non-coding regions so we have three things that we need to do here first off we need to add what's known as a five prime cap and a poly a tail on the three prime end and we also need to then remove the introns the non-coding regions and then splice together which essentially glue together these exons that you can see here to form our final mrna that will be used to create the protein so the five prime cap that's added is a number of guanine nucleotide base pairs and this is going to be utilized as an energy source to kick off translation when this mrna arrives at the ribosome then also at the three prime end we add this poly a tail which is a process known as polyadenylation where we're adding a number of adenine nucleotides here to the three prime end remember mrna is single stranded and so single stranded nucleic acid can degenerate really quickly and so similar to how our dna has the telomeres at the distal ends we want to make sure that we're protecting all of our genetic code while it's being transported from the nucleus to the ribosomes in the cytoplasm so that it can be translated so we add this poly a tail through polyadenylation in order to protect all of the genetic code that we're trying to send to produce those proteins now once we've added the five prime cap and poly a tail we then need to splice out the introns so remember introns are non-coding regions and so they need to be removed so that we can have all the coding regions sent to the ribosomes and so this processing of splicing and adding on the five prime cap and poly a tail is all going to take place in the nucleus and so specific enzymes known as spliceosomes are responsible for cutting out the introns okay they have that exonuclease activity remember an exonuclease enzyme is one that's capable of removing nucleotides from a sequence so these spliceosomes are just specific exonucleases that are capable of snipping out those introns and then bringing together adjacent exons now in our diagram here there are two introns and three exons all sort of equal length and so you can look at this thinking okay roughly about 40 percent is made up of introns in actuality it's about 90 percent of the typical human gene is made up of introns non-coding regions that are not translated and this is really to protect the coding regions right if you had all of your coding regions here and some sort of damage occurs it's more likely to affect the entire gene if it's not all spread out so by spreading out the coding regions we're reducing the likelihood that some mutation would take place and would impact the gene as a whole and to also protect the coding regions we add that five prime g cap and we add the three prime poly a tail now we have about 25 000 genes in our overall genome right so in our genetic code we have about 25 000 genes however those 25 000 genes are capable of producing over 120 000 different mrna transcripts you might be thinking well if you have a gene which is a sequence of dna how are we producing different transcripts well during this process of mrna processing where we splice together the exons we can either splice these exons together in sequence let's say this is one two and three we can splice them together in order one two and three or we could splice them together one three and two or three one two or three two one so hopefully you see here we have a number of different combinations the way in which we order these exons can create a completely different protein so the way that our body is set up is so that we have a limited number of genes but the way in which that mrna is spliced together can produce a completely different transcript that codes for some other protein and so that's how those 25 000 genes can ultimately lead to the production of over 120 000 different mrna transcripts which leads to over 120 000 different proteins that can be produced so that brings us to the end of our material for this week next week we're going to be taking a look at translation right so we're going to take this processed rna molecule so we're going to take this processed mrna molecule it's going to leave the nucleus and then it's going to bind to either a free-floating ribosome or to a ribosome that's attached to the endoplasmic reticulum remember the rough endoplasmic reticulum and then we'll look at the process by which those ribosomes are capable of producing the proteins so if you have any questions please feel free to reach out otherwise i look forward to seeing you all in tutorial take care