Roger David Kornberg (1947 - present) was awarded the Nobel Prize in Chemistry in 2006 for his studies of the molecular basis of eukaryotic transcription. While working as a postdoctoral fellow in the 1970s, Kornberg proposed that the basal unit of chromatin, called the nucleosome, is made of 200 base pairs of DNA and eight histones (proteins that act as spools around which DNA winds). Kornberg later developed an in vitro transcription system using the yeast Saccharomyces cerevisiae and discovered a multiprotein complex called Mediator composed of about 200 different proteins that transfers signals from transcription factors to RNA polymerase II and other transcription factors. Kornberg also developed methods to visualize the atomic structure of RNA polymerase. He then extended these studies to obtain structural images of RNA polymerase associated with other proteins. Through these studies, Kornberg was able to create a picture of how transcription works at a molecular level.