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small image of Francois Jacob's lac operon drawing. So by comparing the system of the viruses, the lysogenic bacteria on the one hand and the induced synthesis of galactosidase, it was clear that it was two very similar system, and by comparing this system we came to a model which I am trying to describe now, the idea what you had a gene controlling the structure of proteins, which we called structural gene, and we can call that SG-1, SG-2, cloned to each other, for instance the Z gene of galactosidase and the Y gene permease. And somewhere else, which could be either far or close to the other, a regulatory gene, that is a gene, which control the expression of these two structural genes. So the idea were that from this you had a messenger, that the gene which is, it's DNA, a messenger which was RNA, and from this messenger proteins were done. So that goes like that. Now the regulation was by, what we call this negative system, that is a regulatory gene made a product, which at first we thought was RNA but finally was protein, so it made a protein which we called a repressor. And this repressor was acting on a small sequence of DNA adjacent to the structure, which we called the operator. So there was here, so the regulatory gene produced a repressor, a protein, which binds to a specific sequence, which is the operator, which control the initiation of messenger production. And the lactose that is the inducer, was supposed to interact with the repressor and prevent him from attaching to the operator. So the repressor was a double-sided structure, one which recognize the operator and the other which recognize the small molecule, the lactose. And this we call the negative system because it blocked, so that was the model, which we call operon, 'ron', with 'on'.
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