The potential for transcription and translation to regulate each other was recognized by the team of Marshall Nirenberg, who discovered that the processes are physically connected through the formation of a DNA-ribosome complex. As part of the efforts of Nirenbergs group to determine the genetic code that underlies protein synthesis, they pioneered the use of cell-free in vitro protein synthesis reactions. Analysis of these reactions revealed that protein synthesis is mRNA-dependent, and that the sequence of the mRNA strictly defines the sequence of the protein product. For this work in breaking in the genetic code, Nirenberg was jointly awarded the Nobel Prize in Physiology or Medicine in 1968. Having established that transcription and translation are linked biochemically (translation depends on the product of transcription), an outstanding question remained whether they were linked physically - whether the newly synthesized mRNA released from the DNA before it is translated, or if can translation occur concurrently with transcription. Electron micrographs of stained cell-free protein synthesis reactions revealed branched assemblies in which strings of ribosomes are linked to a central DNA fibre. DNA isolated from bacterial cells co-sediment with ribosomes, further supporting the conclusion that transcription and translation occur together. Direct contact between ribosomes and RNA polymerase are observable within these early micrographs. The potential for simultaneous regulation of transcription and translation at this junction was noted in Nirenbergs work as early as 1964.