Har Gobind Khorana (1922- present) was awarded the 1968 Nobel Prize in Medicine together with Robert W. Holley and Marshall Warren Nirenberg for work on how the genetic code works to make proteins. In the 1960s, Khorana confirmed Nirenberg's findings that the way the four different types of nucleotides are arranged on the spiral “staircase” of the DNA molecule determines the chemical composition and function of a cell. Khorana also helped determine which combinations of nucleotides form which specific amino acids. He proved that the nucleotide code is always transmitted to a cell in groups of three, called codons and determined that some of the codons prompt the cell to start or stop making proteins. Also, in 1970, he synthesized the first artificial copy of a yeast gene.