Messenger RNA

As soon as DNA was identified as the source of genetic information, it became clear that DNA could not directly encode proteins. In eukaryotic cells, DNA resides in the nucleus, yet most protein synthesis takes place in the cytoplasm. Geneticists recognized that an additional molecule must take part in the transfer of genetic information.

The results of studies of bacteriophage infection conducted in the late 1950s and early 1960s pointed to RNA as a likely candidate for this transport function. Bacterio-phages inject their DNA into bacterial cells, where the DNA is replicated, and large amounts of phage protein are produced on the bacterial ribosomes. As early as 1953, Alfred Hershey discovered a type of RNA that was synthesized rapidly after bacteriophage infection. Findings from later studies showed that the bacteriophage T2 produced shortlived RNA having a nucleotide composition similar to that of phage DNA but quite different from that of the bacterial RNA. These observations were consistent with the idea that RNA was copied from DNA and that this RNA then directed the synthesis of proteins.

At the time, ribosomes were known to be somehow implicated in protein synthesis, and much of the RNA in a cell was known to be in the form of ribosomes. Each gene was thought to direct the synthesis of a special type of ribo-some in the nucleus, which then moved to the cytoplasm and produced a specific protein. Using equilibrium density-gradient centrifugation (see Figure 10.2), Sydney Brenner, François Jacob, and Matthew Meselson demonstrated in 1961 that new ribosomes are not produced during the burst of protein synthesis that accompanies phage infection

(I Figure 14.4). The genetic information needed to produce new phage proteins was not carried by the ribosomes.

In a related experiment, François Gros and his colleagues infected E. coli cells with bacteriophages while radioactively

Question: Do ribosomes carry genetic information?

— Medium with 15N and 13C E. coli culture ffi E. coli were grown in medium containing heavy isotopes through several generations so that the heavy isotopes would become incorporated into all E. coli ribosomes.

— Medium with 15N and 13C E. coli culture

Move to new medium

Bacteriophage added

Move to new medium

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