Animal genetics / Frederick B. Hutt.

  • Frederick Hutt
Date:
[1964]
Catalogue details

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

Credit: Animal genetics / Frederick B. Hutt. Source: Wellcome Collection.

    474/578 page 452
    452 ANIMAL GENETICS mins, and other substances needed are synthesized to permit normal growth of the plant. By irradiation with X-rays, Beadle and Tatum induced mutations which prevented the synthesis of essential sub¬ stances, and hence prevented also normal growth. In its growing (asexual) stage, the mould is haploid, so any mutations induced would not be obscured by their normal alleles, as are single recessive mutations in diploid cells. To recognize such mutations, the irradi¬ ated stocks were first grown on complete diets, so that they did not have to synthesize any essential nutrients, and then, after their ability to thrive under those conditions had been demonstrated, conidia (spores formed asexually) were transferred to what we might call the diet of raw materials. Many of the transfers grew equally well there, indicating that they could synthesize what was needed, but some could not do so. By adding one at a time various purified amino acids, vitamins, or purines, it was possible thus to identify which one a mutant strain needed for growth, or, in other words, which one it could not synthesize. Such mutations were then labelled as arginine-less, thiamine-less, etc., as the case might be. As the result of such studies, mutations were eventually found which prevented the synthesis of almost every known amino acid and vitamin. Similar techniques have led to the identification of corresponding mutations in other filamentous fungi, in bacteria, yeasts, and other organisms. A most significant outcome of the work with Neurospora was the discovery that, in the synthesis of some one essential nutrient, the process might be blocked at different stages by entirely different mutations. Complex organic compounds are not built from simpler substances by a single big change, but by a whole series of little ones. These little ones have been compared to steps on a staircase, leading from simple beginnings at the bot¬ tom to the complex substance at the top. Expressed otherwise, anabolism, or the synthesis of complex compounds from simpler ones, is sometimes referred to as stepwise metabolism because the proc¬ ess proceeds from each stage, or step, to the next. As we shall see shortly, there are corresponding steps in catabolism, the breaking- down of complex compounds to simpler ones. When a mutant strain of Neurospora that needs some substance, D, for normal growth is found to do equally well if С is added, it is clear that the mould can synthesize D from C, hence that, in the normal process of anabolism, С is a precursor of D. Similarly, iden¬ tification of В as a precursor of C, and A of B, shows that the sub¬ stance D is normally formed by the process :
    No text description is available for this image
    No text description is available for this image
    No text description is available for this image