LECTURE 15. RECOMBINANT DNA 1
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7.012 Introduction to Biology, Fall 2004 Transcript - Lecture 15
Good morning. So, I see we have a lot of parents here. How many parents have we got here? Welcome to the parents. How many of the parents have done the reading for today? [LAUGHTER] Good, because we'll call on the parents too, right? We'll see what happens.
All right, so, where are we? We've talked about this diagram that I keep coming back to. If you want to study biological function the two traditional ways to do that were to look at genetics or to look at biochemistry: genetics, the study of an organism with one broken component, those components being genes; biochemistry: the study of the purification of individual components from an organism away from the organism, particularly the most important such components being proteins.
What do they have to do to each other? The unification in molecular biology that occurred in the middle of the century from the 1950s into the '60s and really up to 1970 or so, we came to a conceptual understanding that genes encode proteins, and therefore these two different ways of looking at the organism: organism minus a component, components minus an organism were complementary points of view, and in theory, you could go from a gene sequence to a sequence, a protein sequence back to a gene sequence, to go from a gene sequence to its function, its function to a protein, except for one to a tiny detail.
This was all just conceptual. Conceptually we understood by about 1970 that the DNA made the RNA made the protein. The carried out the function but as of then, you couldn't individually work with or purify the DNA corresponding to any particular gene.
All of the inferences had been indirect inferences: indirect inferences from bacterial genetics, bacterial regulation or Meselson-Stahl experiments, and all sorts of interesting indirect ways working out the genetic code, but it didn't let you read anything.
This was a problem. Some people in the late 1960s said, great, molecular biology is over. We understand in principle how life works. Now let's go understand how the brain works. And there was an exodus of some people from molecular biology into neurobiology to now go nail the brain, figured that would be worth another ten years or so.
But in fact, remarkably, people began to focus on how you could get to work with individual specific genes. Now, what's so hard about that? I mean, it's not very hard to crack open a red blood cell and purify different proteins.
You can purify hemoglobin. You can purify different enzymes. Biochemistry allows you to purify different components from each other. I want to purify an enzyme: let's crack open a yeast cell, separate the proteins over some column that separates them based on their size or their charge, and I'll get purer and purer fractions.
I'll assay each fraction to see which one has the enzymatic activity. But basically I use the physical chemical properties of the proteins to separate them into different buckets. Why not do that with, say, the human DNA and purify out the gene for beta-globin, that encodes the beta component of hemoglobin? What would be the problem of just using physical chemical purification to purify one human gene from another? Well, I mean, it's one very big molecule.
Well, I could shear it up. Maybe I'll just break it up. Now, let's purify the beta globin-containing part. It all looks the same. It's just DNA. It's one chemical polymer with pretty boring properties, and they're not very different.
Any particular DNA sequence in any other DNA sequence basically about the same molecular , same charges, there's nothing to separate them by. How are you going to purify beta-globin? That was the problem.
