François Jacob

François Jacob (17 June 1920 – 19 April 2013) was a French biologist. He shared the 1965 Nobel Prize in Physiology or Medicine with Jacques Monod and André Lwoff.
Jacob and Monod discovered a fundamental control system in cells. Enzyme levels in all cells is regulated by feedback on transcription.
Like his colleague Monod, Jacob was an almost lifelong atheist. He was also of Jewish descent.
Research.
In 1961 Jacob and Monod explored the idea that the control of enzyme expression levels in cells is a result of feedback on the transcription of DNA sequences.
For many years it had been known that bacterial and other cells could respond to external conditions by regulating levels of their key metabolic enzymes, and/or the activity of these enzymes.
For instance, if a bacterium finds itself in a broth containing lactose, rather than the simpler sugar glucose, it must adapt itself to: 
It was known that cells increase the enzymes for these steps when exposed to lactose.
With work on DNA, it became clear that all proteins were being produced from their genetic code. Jacob and Monod demonstrated that, in the bacterium "E. coli"), there are specific proteins that repress the transcription of the DNA to its product (RNA. This reduces the production of those particular enzymes.
The lac repressor.
The lac repressor is a DNA-binding protein which inhibits the expression of genes which code for lactase enzymes. The repressor is active in the absence of lactose.
This repressor (the lac repressor) is made in all cells. It binds directly to DNA at the genes it controls, and physically prevents transcription. When lactose becomes available, it is converted into allolactose, which inhibits the lac repressor's DNA binding ability.
In this way, a robust feedback loop is constructed that allows the set of lactose-digesting enzymes to be made only when they are needed.
The regulation of gene activity has developed into a very large sub-discipline of molecular biology. There are many mechanisms and many levels of complexity. Current researchers find regulatory events at every level of the processes that express genetic information. In the relatively simple genome of baker's yeast, ("Saccharomyces cerevisiae"), 405 of its 6,419 protein-encoding genes are directly involved in transcriptional control, compared to 1,938 that are enzymes.