Hey everyone and welcome to this updated video for protein synthesis AQA topic 4. And just to make you aware, I've got a brand new freebie. If you head to missestra.co.uk/youtube workbooks, I now have for all of my entire topic YouTube videos a workbook that you can complete alongside the video. So for example, here's a little sneak peek at the topic 4 one. So you could use that to fill in some of the information that we're about to go through linked to protein synthesis. So let's get into protein synthesis. Then proteins are synthesized on the ribosomes. And the production of proteins from the DNA is a two-stage process. We start in the nucleus where transcription occurs. And this is where one of the genes on the DNA is copied into mRNA. Step two is translation and this happens on the ribosomes in the cytoplasm or on ribosomes on the rough endopplasmic reticulum and this is where the mRNA joins to a ribosome and corresponding tRNA molecules will bring specific amino acids matching the antic-codon to the codon. That's a brief overview. Let's have a look at the exact mark scheme answers needed for AQA. Starting with transcription. Now this is still just a bit of information before we get to that mark scheme list. So transcription is the creation of a complimentary mRNA copy of one gene. And this is needed because mRNA is much shorter than DNA cuz it's only the copy of one gene rather than all of the genes in DNA. And it's so important that we do have this shorter molecule because that then means it's able to fit through the nuclear pores and then get to the cytoplasm to the ribosomes. Whereas DNA is too big to fit through those nuclear pores and we don't want it to fit out because it could then get broken down and digested by enzymes in the cell. So it is a massive advantage. Let's then get to the exact mark scheme answer you'd need to give if you were asked to describe transcription. Number one, hydrogen bonds between the DNA bases break. This is referring to the hydrogen bonds between the complimentary base pairs opposite each other in that double strand making the double helix. And because we've broken those hydrogen bonds, that causes the DNA helix to unwind and the two strands to separate. And both of those strands are now separate. But only one of them is going to act as a template. And that's our second point. Only one DNA strand is going to act as a template for the formation of mRNA. Next step is within the nucleus there are three RNA nucleotides and those will align opposite their complimentary base pairs on that DNA template strand. Sometimes there's an extra mark that I've put in here as bullet point 4 for pointing out the fact that this idea of complimentary base pairing is slightly different because RNA has uricil whereas DNA has thymine. So wherever there is an adinine base on the DNA the complimementary mRNA nucleotide would be uricil rather than thymine. Mark number five, there's always going to be a mark for this and that's knowing the role of the enzyme RNA polymerase. And this is catalyzing the condensation reaction between adjacent RNA nucleotides. So it joins together those RNA nucleotides that creates phosphodestester bonds between those nucleotides and it creates that new mRNA strand. And once the mRNA is copied, it then gets modified before it can leave the nucleus through those nuclear pores. So let's have a look at those modifications. When that RNA is first made in ukareotic organisms, it's called premrna because it's before the modifications have happened. And the modification that you need to know is the introns are removed. They're removed by a process called splicing. And that then just leaves the exxons behind. So we can see here our DNA molecule and that would be one gene. We've made an RNA copy but that still has the introns within it which are those sequences of bases that do not code for sequences of amino acids. So we need to splice out meaning cut out the introns. So we're just left with the exxons which are the sequences of bases that do code for sequences of amino acids. Now it's worth noting that transcription in proarotic organisms does not have that stage. When transcription occurs in proarotes such as bacteria, they will create the final finished mRNA molecule straight away. And that's because proarotic DNA doesn't contain introns. So they don't need this extra step of splicing. So now we've got our mRNA molecule. The next step is translation and this is the stage when the polyeptide chain is created and it uses two types of RNA. M which is our messenger RNA which we just made in transcription and TR RNA which stands for transfer which brings the specific amino acids. So let's have a look at the mark scheme answer for this process. Number one, once that modified mRNA has left the nucleus, it attaches to a ribosome. Second mark is the ribosome will read the sequence of codons on the mRNA one codon at a time. Not always a mark for that one, but it's worth remembering and putting it in. There's always going to be a mark for number three. the concept that the tRNA molecules will have an antic-codon which is three bases and they are complementaryary to three bases known as the codon on mRNA. So there's that mark to do with the idea of complimentary base pairing between the antic-codon and the codon on mRNA. There's always going to be a mark for saying that each tRNA molecule brings a specific amino acid. There's going to be a mark for pointing out that those amino acids are joined together by peptide bonds. And the formation of that peptide bond requires energy from ATP. ATP is also actually required for the attachment of those specific amino acids onto the tRNA molecule too. Now once those peptide bonds have formed the tRNA molecule is released and then the ribosome is going to move along the mRNA to the next codon and the process repeats with new tRNA molecules bringing new amino acids and this will continue until a stop code on is reached. Now in terms of your key marking points those are the bits I've put in bold. The rest of it is just to make sure that the sentence makes sense as a whole. So that is how we create our polyeptide chain that is now ready to enter the GGI apparatus or GGI body where it's going to be folded and modified to completely finish it ready for use. So that takes us to the end of protein synthesis. Don't forget if you do want mark scheme specific notes which have examiner tips, recall questions to test you throughout essay links and so much more then you can get those linked in my description. or if you want my flash cards, those cover all of the key marking points for the theory. Again, mark scheme specific linked below. That is it though for this video. Hopefully, you found it helpful and I'll see you in a video very soon.