This is a short educational video transcript explaining protein synthesis in cells, focusing on the two-stage process: transcription (copying a single gene from DNA into messenger RNA) and translation (reading mRNA at ribosomes to assemble amino acids into a protein). The transcript describes the roles of RNA polymerase, complementary base-pairing (including DNA A→mRNA U), codons/anticodons, tRNA delivering amino acids, and ribosomal movement and peptide-chain formation.
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In today's video, we're going to take a closer look at protein synthesis, which is the process of making proteins. We can think of this in terms of two transcription and translation. In a nutshell, transcription is the process of taking a single gene of DNA and copying it into a structure called mRNA. Then translation is the process of taking this MRNA strand and using it to produce a protein. Before we go through the details, though, we need to look at why we actually need these two steps. Inside almost every cell is a nucleus that contains all the genetic material of that cell in the form of DNA. The reason we say DNA is so essential to life and controls what the cells do is because it contains thousands of genes, which are each small section of the DNA that have a specific sequence of bases and so are able to code for a specific sequence of amino acids, which, when combined, will form a particular protein. In order to actually make a protein, though, the specific sequence of bases has to be read by one of these structures called ribosomes, which, importantly, are outside of the nucleus. However, because the DNA is so big, it can't leave the nucleus itself. And so if we want to use a gene to make a protein, we're going to have to make a copy of that gene first. And because we're only copying a single gene, rather than the entire DNA strand, the copy will be small enough to leave the nucleus and so it can make its way to the ribosome. Now, this copy we're talking about is mRNA, which stands for messenger RNA and is just a copy of a single gene. The structure of MRNA is mostly similar to DNA, but it does have a few important differences that you need to know about. For one, it's much shorter than DNA because it's only a single gene long. It's also only a single strand, rather than a double strand like DNA. And finally, instead of containing the base thymine, it contains uracil. Now that we've got these basics covered, let's take a closer look at transcription and see how this MRNA is actually formed in the first place. In this image here, we can see a small section of DNA that contains two strands coiled into a helix. This is how it's normally found in the nucleus. But to make it easier to understand transcription, let's uncoil the two strands and show it using this simplified diagram instead. To be clear, though, this is the same piece of DNA as above. We're just showing the two strands side by side rather than wrapped around each other. And we're showing the bases as their letters. So a for adenine, T for thymine, and so on. For the sake of our example, let's say that this region here between these two lines is the gene that we want to copy, even though in real life, genes are normally much bigger than this. The process starts with an enzyme called RNA polymerase, which binds to the DNA just before where the gene starts. Then, just ahead of the RNA polymerase, the two strands of DNA separate apart so that all of their bases are exposed. Then the RNA polymerase is basically going to move along the DNA strand and read the bases one by one and use them to make an MRNA strand. For this to make sense, though, you need to remember that the MRNA bases will always be complementary to the DNA bases. So a C on DNA will always pair with a G on mRNA, G will pair with C, T with A. And then the odd one out is that if you have an A on the DNA strand, it's going to pair with a U on the MRNA strand, because MRNA doesn't have T. Like we said a minute ago, all of the thymines, which are the T's, have been replaced with uracils, which are used in mRNA. So an A on DNA will be complementary to a U on mRNA. So in our example here, the RNA polymerase will start with this C base. So it will start the MRNA strand with a complementary G base. Next it will read the T base, so it adds a complementary A to the mRNA, Then it will move on to this A base and add a complementary U, then go to the G and add the complementary C, and then so on. So the RNA polymerase is going to carry on doing this, building up the MRNA base by base as it moves along the entire gene. One thing to notice here is that the DNA strand keeps on separating just ahead of the RNA polymerase and closing just behind it, so that only a small section of the DNA is ever exposed. Once it's moved along the entire gene and finished making the MRNA strand, the RNA polymerase detaches from the DNA and the DNA strands can close back up. This means we're left with an MRNA that's then free to leave the nucleus and head off to the ribosome. One last thing to mention, though, is that this strand of DNA, which the RNA polymerase moved along, is called the template strand. So it's the template strand which is used to make the mRNA. So, going back to our whole cell for a minute, we've now got an MRNA copy of the gene here inside the nucleus, which is free to leave the nucleus and make its way to the ribosome, where it can undergo translation to produce a protein. To help understand this part, it's important to remember that for both DNA and mRNA, each group of three bases, which is called a triplet or codon, codes for a specific amino acid. To make proteins, our cells use 20 different amino acids and each one has a different three base codon. For example, this triplet AGU codes for the amino acid serine, whilst CCA here codes for proline. You don't have to remember these examples, we're just using them to illustrate the point. So let's now zoom in to a single ribosome and go through the process of translation. To start the process off, our MRNA strand and the ribosome both bind together, and we're now ready for the ribosome to start building the protein by adding one amino acid at a time. The amino acids themselves are brought to the ribosome by molecules called tRNA, which stands for transfer rna. TRNA molecules have the amino acid at the top and an anticodon at the bottom. The anticodon is this sequence of three bases which are complementary to these three bases on the mRNA, and it's these three bases on the MRNA that code for the amino acid that the TRNA is carrying. So because each type of TRNA molecule is specific to a particular triplet on the mRNA, it can ensure that it always brings down the correct amino acid. For example, because the first triplet of our MRNA sequence here is agu, it will attract this TRNA molecule with the anticodon uca, because UCA is complementary to agu, and a TRNA brings with it the amino acid serine, because AGU is the code for serine. Meanwhile, this second triplet CCA will attract this TRNA molecule with the complementary anticodon of ggu, and that will be carrying the amino acid proline, because CCA codes for proline. Now, the whole point of this seemingly complex process is that the TRNAs have now brought the correct amino acids down to the ribosome in the correct order. So the ribosome is able to join them together and start building up a chain of amino acids. Once it's joined these first two amino acids together, the ribosome moves along the MRNA slightly, and so another TRNA molecule will come down and bind to its complementary codon on the mRNA, bringing with it the next amino acid. This allows the first TRNA molecule to detach and repeat the process. But importantly, it leaves the amino acid behind. This same process then repeats all the way along the chain until the ribosome reaches the very end and has joined together a complete chain of amino acids. At this point the amino acid chain will detach from the ribosome and then finally the chain can fold up on itself to form a protein. Anyways, that is finally the end of this video, so I really do hope you found that useful. 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