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Protein synthesis in the cellular factory

RNA may have been the origin of life on Earth. Go on a whirlwind tour of RNA’s evolving role through billions of years of evolutionary history. Created by NOVA.

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Video transcript

If you could step inside one of your cells, you'd see something that looks a lot like a factory building thousands of different molecular machines. It would have tiny assembly lines starting in the cell's nucleus and stretching out to structures called ribosomes. These assembly lines start with DNA and build RNAs and Proteins----in a very real sense, you. DNA has all the blueprints for everything the cell will ever need to build. RNAs are assorted molecular machines that do all kinds of work in the cell. They carry components from place to place, ratchet up the speed of operations, switch other machines on and off, and relay coded instructions from the nucleus out to the factory floor. The other product, proteins, are strong molecular machines that do things like hold the cell together and send signals to other cells. There are two parts to each assembly line: the first builds RNAs, and the second builds proteins. The process that builds RNAs is called transcription. It happens in the cell's central office: the nucleus, where DNA blueprints are stored. Let's say that a cell needs to make a transfer RNA, or tRNA, a triangle shaped RNA, which transports a building material called an amino acid. It starts at a tiny, specific region of DNA with the code that makes the tRNA we need. This region is called a gene. A protein machine inside the nucleus pries apart the weak bonds that hold the two strands of DNA together. RNA building blocks swarm in and form a conga line complementary to the DNA. This RNA strand now needs to fold up. In the world of tiny cellular machines, shape determines function. RNA's four bases bond to each other and give the tRNA its 3D shape. And voila! Our transfer RNA is ready for action. It floats out of the nucleus and picks up an amino acid. We'll come back to our tRNA in just a moment. Now let's look at the second half of the factory's assembly line----making proteins from RNA----a process called translation. Remember the scrape? This cell needs to make Thrombin, a protein machine which helps blood clot and make a scab. Thrombin is just one of the tens of thousands of proteins your cells can make. Before translation begins, the cell transcribes a special type of RNA called mRNA, or messenger RNA. This mRNA carries the code for Thrombin out of the nucleus and onto the cellular factory floor. Our mRNA drifts until it runs into a ribosome, a protein-making machine. The ribosome clamps down around the mRNA. tRNAs, just like the one we made earlier, drift in. The messenger RNA carries a coded message, which the tRNAs translate into amino acids----the language of proteins. This process, translation, proceeds down the mRNA, creating a chain of bonded amino acids. Just as with RNA, molecular shape determines molecular function, so these amino acids fold up into the protein's 3 dimensional form. Finally, the ribosome comes to a bit of code that says "ok, we're done here" and releases its completed protein into the cell. So there you have it. DNA encodes genes to make RNA machines, and the ribosome translates special mRNA, which in turn makes protein machines. Unlike most factories, your cell uses just two processes to make tens of thousands of different molecular machines. It's like one factory that can build toasters, phones, cars, and even repair itself.