So, right now we're going to explore a problem that you might not have even had occurred to you as being a problem. And that is, we just looked at all of those antibodies, and each one of them has a recognition site that very specifically interacts with a very specific part of a pathogen, an antigenic epitope. And since there is this huge, huge range of possibilities for antigenic epitopes you might wonder how we could possibly have the resources to code for enough different antibodies to recognize all of these possible threats. And of course, that was a really big question from the first. Now, initially, people began to look at this as, well, when you have an antibody you have a heavy chain and a light chain, so let me just do that. So maybe, if I have genes for the light chain and genes for the heavy chain, if I say, had 1,000 heavy chain genes and 1,000 light chain genes, then between the two of them, I could mix and match them in different ways, and get a million different possible combinations. And that didn't seem too unreasonable initially, when we first began looking into the human genome we thought there might be as many as 100,000 genes. And also we didn't realize how many different possibilities there were for making antibodies. It turns out that okay, I could say it's somewhere between ten to the eighth and ten to the eleventh, and those numbers really are quite meaningless. So Daniell Dennett had a way of describing very large numbers that were not technically infinite by using the word vast. And that's the word that I like to use for the number of potential possibilities of different antibodies and with the different T cell receptors. There's a vast number of possibilities. Now if I had to code for them all in the DNA, the next thing you know, I have the cell, it was so huge, it could not slog its way up the female reproductive tract. So the question then is how do we solve the problem of making so many different forms of antibodies from a reasonable number of genes or, shall we say, an amount of DNA that doesn't seem excessive, given the overall complexity of the genome. Now, indeed, we can still mix and match heavy and light chains, but that's only part of it. The real thing comes from mixing and matching parts of the instructions for the chains and this is done at the level of the DNA. So if I look, say, at a light chain gene, I can see that I really only have one set of instructions for the constant region, but I have a huge number of different possibilities for mixing and matching elements to put in to make the variable domain. And here's some more of them. I actually did all of this, dear, it's fallen apart. Sometimes you get mutations, all right. So this first suggestion, that you could actually rearrange the genes, was made in the 1960s by Dreyer and Bennet. And a guy named Tanagawa who actually did the work that proved that the genes for both the light and the heavy chains of the antibodies will change, will be mutated, will be picked up, cut apart, rearranged, put together in different ways in the course of development in a developing B cell. Now, this is a very, very big deal. The ability to make specific changes in the DNA as part of a developmental event is a possibility that was proposed for a long time in the 1800s, but no examples of it ever turned up. For example, we do know that we can make extra copies of the gene. Okay, that's called gene amplification. We will sometimes use this for genes, for ribosomal RNAs, during development of O sites, but they're all the same copy. We just make copy, copy, copy, copy, copy. When we're fooling around with this gene, we're actually changing it. Another thing that you can do is get rid of parts of the DNA. So in the 1800s, they thought well, as a cell, say, became a red blood cell, before it got rid of its nucleus entirely, little by little maybe it got rid of the genes it needed if it wanted to become a brain cell or a muscle cell. Or some other kind of cell and then just kept the ones that it used. Well, we have never seen any evidence of that type of thing in ordinary development. We have seen cases, it's called chromosome diminution, where a germ cell, that is the cell that's going to make an egg or sperm, keeps all of it's DNA, but the cells that are going to become somatic cells, those cells will all throw away part of the DNA. But they all throw away exactly the same part of the DNA so it's not specific. Then came Barbara McClintock, and she won a Nobel Prize for showing that genes can move around. And when they move around, they can often insert themselves into other genes and turn them on and off. And she thought this might be a developmental mechanism. Well it turns out that, in most cases at least, it's not. Because when the genes move around, they move around selfishly. They are not part of the developmental process. Now, in recent years, one exception to this has been the novis, and that is when your nerve cells develop, they actually unleash some of the mobile elements that McClintock discovered, and those elements move around in your nerve cells. So you toss the dice a wee bit. And, well, it does mean that two identical twins are not going to be identical, because let's face it, different parts of their nerve cells will have different changes in the insertion pattern. But it's still not a very controlled event. What we have here in this gene, however, is a specific set of rearrangements that's going to give us one piece of the gene to code for the constant region of the light chain say. And then two more regions in here, those are the yellow ones, that will code for we'll see the joining region. And then we have this huge number of pink regions that we'll code for the rest of it, and we're going to put the pink and yellow ones together, at random. To do that we're going to need specific signals, which I have done in other colors, and I'm going to use them in a later part of this lecture in order to show you how you can cut and break and put things forward. But I want you to know, before we get into this, that this is a very big deal. It is the only system that we have that has a controlled developmental event that involves the specific cutting out and throwing away of various pieces of DNA. And the reason that this gives us the opportunity for making these huge, vast possibilities, number of possibilities is that we're going to have a huge variety of we'll see pink variable regions. A number of J joining regions, and we can put them together in a random way and start out by producing a huge variety of different possibilities in different cells from this same linear piece of DNA. And the way that this works and the mechanism that's actually used to produce it, is the subject of the rest of this lecture.
