This is the second video for chapter 4. We are going to go over protein synthesis. Our previous definition of a gene, was a segment of DNA that carries the code for a particular protein. Alright there's a problem with this. The body has millions of proteins, but we only have 20,000 genes. So now we have changed our definition. A gene is an information containing segment of DNA. It codes for production of a molecule of RNA that plays a role in synthesizing one or more proteins. Our amino acid sequence in a protein, is determined by the nucleotide sequence in the DNA. So we are looking at the order of our four letters T, A G, and C. The body can make millions of different proteins from only 20 amino acids. And then the information for these 20 amino acids, are encoded in genes, made up of four nucleotides A, T, C, and G. The genetic code is a system that where we use these four nucleotides to code the amino acid sequence of all of our proteins. And what we have found, that every three nucleotides is an amino acid. So every three nucleotides in DNA, this is called a base triplet. This is the information for one amino acid. When we transcript this code into messenger RNA, that code is called a codon. So it's a three base sequence in messenger RNA. What we find, is that although there are 64 possible codons, we actually only have 61 of them code for amino acids. Three of these codons are stop. And so these stop codons, are like the period at the end of the sentence; this is the end of the message. We have one of these codons that is always the start. AUG has an amino acid on it, methionine. This always begins every protein. Our process of making protein is a two-step process. We have to first of all, get the code from DNA, and this is where messenger RNA comes in. And then from that messenger RNA, we get the correct order of amino acid for the protein. So in transcription, this is where we get a messenger RNA from DNA. So this occurs in the nucleus. The DNA is not going to be able to leave the nucleus. In translation, that messenger RNA is the code for the protein, and that usually occurs in the cytoplasm. So this two-step process, we're going to look at these separately. Transcription, this is copying that genetic instruction from DNA to a messenger RNA. You're going to need an enzyme; it's RNA polymerase. That is going to bind to the DNA, and that's going to help make our messenger RNA. So that polymerase, that RNA polymerase, opens up that DNA helix and unwinds it. And it reads those bases on one side of your DNA, and so it makes that messenger RNA. So this is a very simplified view. I'm not showing all of my DNA, I'm only going to show one part of my DNA and it's been unwound, okay. So you don't see a spiral staircase. So the first thing that I'm going to do is, I'm going to open it up. And one side is going to be used as my template. What we find, is that every three of these nucleotides, is called a triplet, and this is going to be the code for a particular amino acid. Now inside my nucleus, I have nucleotides for making RNA. And I'm going to make RNA next to one half of my DNA strand. This is called transcription. Every three nucleotides on messenger RNA is called a codon, and as it happens the start codon is always the same, it's AUG. Now what's going to happen, is that this messenger RNA leaves that nucleus, and then my DNA closes up. So remember, what holds the two halves of my DNA, is going to be just hydrogen bonds. So now I'm done with transcription. When we look at making proteins, remember, that we said that a gene can make more than one protein. We have left off a little piece of this. So looking at the gene and the DNA, it gives you a pre-messenger RNA, and you'll notice where it looks light and dark, light and dark, that it has introns and exons. And so those introns are going to be cut out, and when I make my messenger RNA they could be in different sequences. So this first messenger RNA uses A, C, and D. That gives me a particular protein. Here's a different messenger RNA, uses B, D, and E. A different protein is made, and then this last messenger RNA has A, E, and F; there's a third protein. So this is how one gene can make multiple proteins. Now Let's look at the second half of this, translation. This is happening in the cytoplasm. This is where I'm going to put together the correct order of amino acids to make a protein. I have three main participants. Messenger RNA is carrying the code from the nucleus to the cytoplasm. Transfer RNA is bringing in a single amino acid. So it brings it in to the ribosome, and it's going to be added to my protein chain. On that transfer RNA is an anticodon that has to match the codon on the messenger RNA. The last participant is the ribosome. So we find it free in the cytosol. It could be on rough ER; it can actually be on the nuclear envelope. So that ribosome is made out of enzymes and it has ribosomal RNA in it. When we look at the transfer RNA, this is a relatively small one. It's coiled up on itself; kind of an L shape. On one end of it is my anticodon. On the other end of it, is going to be where my amino acid is going to be bound to. So my transfer RNA picks up a free amino acid in the cytosol. Each transfer RNA will pick up only a specific amino acid. The cost of binding to that amino acid is going to be one ATP. We have three steps to this. We have initiation, that's the beginning of it. We have elongation, so you're making the protein longer and longer, and then you have to stop. That's your termination. So initiation, you always have a transfer RNA bringing in, it always has the same amino acids, and that's going to pair with your start codon. And that start codon is always going to be the same; AUG on my messenger RNA, and that's where you're going to start. So that messenger RNA is going to be pulled through the ribosomes; kind of pulls it in like a ribbon, and as soon as it finds AUG, it starts. All proteins start with methionine. Then you're going to elongate. You're going to bring in the next transfer RNA with its amino acid, and then that anticodon has to pair with the next codon of the messenger RNA. You're going to make a peptide bond between your amino acids, and that keeps on going. So the first transfer RNA that brought the methionine is now empty, and it leaves. It goes back to the cytoplasm and looks for another methionine. And that just keeps on, you elongate your protein, and you keep on making these peptide bonds between your amino acids. Termination is when you reach a stop codon, and a release factor then binds to it; then the protein breaks away from the ribosome, and everything finishes. So this is a video that's on your APR, Anatomy and Physiology Revealed in Connect. So you can go back to this and look at this again. What happens is, in your cytoplasm, you have a messenger RNA, and on that messenger RNA you're going to always start with the start codon. And you'll notice here, AUG is going to be your start codon. So you have your ribosome come in, and then your first transfer RNA, and it has an anticodon. It has to match the codon and so when it matches, here comes the larger part of the ribosomal subunit, and this kind of starts everything off. There's your first amino acid. What happens next, is that you're going to bring in your next transfer RNA. Your second codon is CCG in this case, that has to match up with the anticodon on the transfer RNA, GGC. So only this one will be able to come in. And so when it comes in, what happens between the two amino acids, you get a peptide bond. Now that first transfer RNA is empty. He's going to leave, and he's going to go to the cytoplasm and pick up another methionine. Now comes in the next one, again, the codon has to match the anticodon. You're going to make a peptide bond, and again the empty transfer RNA leaves. Comes the next, and you elongate; this is the elongation of my protein chain. Now what stops it? At the very end, there should be a stop codon. You're going to have a release factor come in, and then once that release factor comes in, everything comes apart. You have set that protein free. When we look at actual messenger RNA, so this red ribbon is a messenger RNA. These blue dots are different ribosomes, and they're reading along this messenger RNA. The green coming off are the proteins. So I can make proteins very, very fast. So remember, how we can process our proteins. On the rough ER, you have these ribosomes. The protein goes inside of my rough ER, pinches off in these vesicles. These are sent to my Golgi. The proteins can be further modified, then it's repackaged in another vesicle and it may be released, or it may be stored inside of the cytoplasm, or it may be a lysosome. So as a review on how we make proteins. The DNA is your double helix. This is where you have the codes. When we look at the sequence of your DNA, so this is DNA, because you know it has thymine in there, T in there. Every 3 is a triplet; every 3 tells you which amino acid you need. Then you're going to do transcription. You're going to make a messenger RNA. AUG always starts, and there'll be a stop codon at the end. Your transfer RNAs have the anticodons, and the anticodon has to match the codon. They are bringing in the amino acids. When they bring in the amino acids, the amino acids then form peptide bonds. Now you have a peptide chain. This is the end of the second video. The last video is going to be looking at cell division.