Although the three-dimensional structure of DNA gave important clues about how DNA stores and transmits information, it left open many questions about how the genetic information in DNA is read out to control cellular processes. In 1953, when the double helix was discovered, virtually nothing was known about these processes. Within a few years, however, evidence was already accumulating that DNA carries the genetic information for proteins, and that proteins are synthesized on particles called ribosomes. But in eukaryotes, DNA is located in the nucleus and ribosomes are located in the cytoplasm. There must therefore be an intermediary molecule by which the genetic information is transferred from the DNA in the nucleus to ribosomes in the cytoplasm, and some researchers began to suspect that this intermediary was another type of nucleic acid called ribonucleic acid (RNA).

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The hypothesis of an RNA intermediary that carries genetic information from DNA to the ribosomes was supported by a clever experiment carried out in 1961 by Sydney Brenner, François Jacob, and Matthew Meselson. They used the virus T2, which infects cells of the bacterium Escherichia coli and hijacks the cellular machinery to produce viral proteins. The researchers found that while T2 DNA never associates with bacterial ribosomes, the infected cells produce a burst of RNA molecules shortly after infection and before viral proteins are made. This finding and others suggested that RNA retrieves the genetic information stored in DNA for use in protein synthesis. The transfer of genetic information from DNA to RNA constitutes the key step of transcription in the central dogma of molecular biology (see Fig. 3.2). In this section, we examine RNA and the process of transcription.