hello and welcome to learn a level biology for free with miss estrich in this video i'm going to be going through protein synthesis if you are new here just click subscribe so you don't miss any of the latest videos so first of all just an overview of protein synthesis this is the process where proteins are made so the polypeptide chain or that primary sequence of a protein and primarily that is happening on the ribosome it happens in two stages the first stage is actually inside of the nucleus and this is called transcription and this is where one of the genes on the entire dna molecule is copied into mrna and if you aren't familiar with rna i'll link my video here so you can learn about rna as a biological molecule the second step is called translation and this is where the mrna then attaches to a ribosome within the cytoplasm which we can see here and then corresponding t rna molecules bring specific amino acids that match the codons on the mrna so let's have a look at this in a bit more detail so transcription the first step and as i said this is happening inside of the nucleus and step one is we have to create a copy of the gene of interest on the dna the reason for that is dna is too big to leave the nucleus so it cannot leave and attach to a ribosome secondly we don't want the dna to leave the nucleus because in the cytoplasm there are enzymes which could damage the dna so instead we make a copy of the gene of interest and that copy is made in the form of messenger rna or mrna now because this is only a copy of one of the genes on dna and humans have approximately 23 000 genes it is a much shorter molecule than dna and that is why is actually small enough to fit through the nuclear pores in the nuclear envelope and therefore it can move into the cytoplasm and attach to ribosomes so the actual process then and i've put all of the key marking points for this in bold so you know what are the key terms you would have to include in a long answer question about transcription so step one we have to get the dna double helix to unwind and expose the two strands and that is because one of those dna chains will act as a template to make mrna and it is only one of the dna strands this time compared to dna replication that acts as a strand because mrna is single stranded now to do this it is very similar to dna replication in order to unwind that double helix and break the hydrogen bonds between the bases of these two chains of dna the enzyme dna helicase is used so that breaks the hydrogen bonds so we now have one of those strands exposed ready to be the template you'll then have within the nucleus there are three floating rna nucleotides and we can see the four options of those down here now if they happen to align opposite their complementary base pair on this exposed strand they'll be able to stay in that position and then rna polymerase will join all of those nucleotides together by phosphodiester bonds and this is all through the idea of complementary base pairing again so the dna bases the rna bases which are complementary to those will align opposite so you've then got your entire copy of mrna and at this stage we actually call it pre mrna and that's because it has to be modified before it leaves the nucleus so those modifications the main modification and the one that you need to learn about for aqa a level is what we call splicing and that is when we remove the introns so our dna is approximately 98 introns and introns are the sequences of bases in your dna which do not code for amino acids and therefore we have to remove them from the mrna before it goes to the ribosome and these introns are removed by a spliceosome which is a protein which can attach to the dna cut and break those phosphodiester bonds and therefore it cuts out the introns however because this is done by a spliceosome we actually describe that as splicing rather than cutting so the introns have been spliced out and you would have to use that terminology rather than saying they've been cut out so they get spliced out and removed so that then means your finished mrna is just made up of exons which are just the coding regions of the dna now i did actually talk about introns and exons in my genetic code video which i'll link here so you can watch you might be wondering why do we have 98 of our dna as introns if it's described as junk dna it doesn't code for anything and it gets cut out the answer to that isn't actually on the aqa specification so i'm not going to go through it however if you are interested research alternative splicing so the next step and the final step is translation so this is the stage where the polypeptide chain gets created which is that primary sequence of a protein the primary structure the sequence of amino acids and this is created using mrna and trna and this is within the cytoplasm with the mrna attached to the ribosome so that's the first step once that modified mrna leaves the nucleus it then attaches to a ribosome at the start codon and the start codon is the first three dna or in this case mrna bases at the start of the sequence the trna molecules and these are trna molecules that are just floating around within the cytoplasm each trna molecule has an anticodon which is three bases at its bottom and an amino acid attachment site at the top and therefore each trna molecule will bring a specific amino acid determined by the anticodon and therefore the codon on mrna so the trna molecule with the complementary anticodon to the start codon which is our first codon on the mrna will align opposite mrna and the ribosome holds them in place so that's the role of the ribosome it holds the mrna and trna in place the ribosome will then move along and the mrna codon which matches to a different potentially complementary trna anticodon will then align and the ribosome can hold about two trna molecules at a time so we now can see we've got two trna molecules here they are aligned opposite the complementary codon to their anticodon and while the ribosome holds them in place what then happens is the two amino acids at the top of the trna molecule join together and this is where the peptide bond is created now to create that peptide bond there has to be an enzyme and it also requires atp so this is why protein synthesis requires energy it's because to make that peptide bond between amino acids you do need atp now this will continue to happen the ribosome will now move along and we can see the next trna molecule with a complementary anticodon to the codon on mrna will then align the previous trna will now detach and it can go and be reused and this will happen over and over and over again until the ribosome reaches the stop codon at the end of the mrna molecule and stop codon is the last three bases on the mrna and these three bases do not code for an amino acid and therefore there is not a corresponding anticodon on a trna molecule and as a result it causes the ribosome to detach from the mrna and that is what ends translation so the stop codon is causing translation to stop and therefore you have your finished polypeptide chain now that is just the primary sequence or primary structure of a protein to get your final 3d shape this then links back to your knowledge in topic two in cells knowing what all the different organelles in a cell do so that polypeptide chain will then go to the golgi body for folding modifications to make the final finished product so that's it for protein synthesis i hope you found it helpful if you have please give the video a thumbs up [Music] you