Volume 1

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

    64/668 (page 42)
    anneals with T2 DNA but not with other DNAs. When the RNA of bacterial cells is extracted shortly after infection with T2, newly synthesised molecules can be identified by their incorporation of a P 32 label. After mixing with heat denatured phage DNA, centrifugation to equilibrium on CsCl density gradient separates the resulting RNA-DNA double stranded hybrid. Experiments using more stringent conditions for hybridization have since been used to character ize different classes of RNA coded by phage DNA (see page 262). The isolation of DNA corresponding to particular bacterial genes—by its incorporation into an episome or phage DNA—has made possible the develop ment of more precise hybridization assays for the corresponding RNA molecules (see page 282). The critical test for any putative messenger is to characterize the protein(s) whose synthesis it directs; using the DNAs of phages carrying bacterial genes, it is possible first to transcribe the genes into RNA and then to translate the RNA into proteins. All stages of gene expression can be followed in these systems. Messenger RNAs of eucaryotic cells have been less well characterized and individual messengers have been isolated only more recently. Their synthesis involves the transcription of a large precursor molecule from which the messen ger sequence is cleaved in the nucleus; it is then transported to the cytoplasm where it is translated by the ribosomes in the same way as bacterial mRNA (see chapter 5 of volume 2). Transfer of genetic information from gene to protein through messenger RNA was until recently thought to be unidirectional. This was the implication usually ascribed to the central dogma proposed by Crick (1958); genetic information can be perpetuated in the form of nucleic acid but cannot be retrieved from the amino acid sequences of proteins. The dogma has thus often been expressed in the form: implying that genetic information can be maintained by the replication of DNA or expressed by the synthesis of RNA and then protein. Both the steps of expression, transcription and translation, have generally been regarded as irreversible; this implies that it is impossible to utilise the information of RNA sequences to synthesize DNA. But more recent research has suggested that the original form of the dogma is more accurate, for the nucleotide sequences of messenger RNA can be converted to duplex DNA by the reverse transcriptase activities (and other associated enzymes) discovered in RNA tumour viruses (for review of the dogma, see Crick, 1970). We do not know whether such mechanisms are used only by RNA tumour viruses during infection or may also operate in uninfected eucaryotic cells; and reverse transcription does not seem to take place in bacteria. The usual