General cytogenetics / J. Sybenga.

  • Sybenga, J. (Jacob), 1926-
Date:
1972
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Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: General cytogenetics / J. Sybenga. Source: Wellcome Collection.

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    8 General cytogenetics §1-1 biochemical system of an organism. Therefore all forms of life have mechanisms that protect their DNA against a high mutation rate, and other mechanisms that can repair mutational damage. A few mutations, however, may result in a favourable mutant pheno- type (the actual appearance of an individual, determined by a heri¬ table component, the genotype and environmental factors). The individual in which this mutation is expressed, may have a repro¬ ductive advantage relative to the non-mutant individuals, or it may be able to maintain itself under unusual conditions. Mutations are the most important means of living material to improve itself even when it is at the expense of a large number of failures. Therefore, for no form or life is it desirable that mutation is precluded com¬ pletely. 1.1.6. Gene, cistron, operon, factor, allele For a long time geneticists have used the term gene for the hypothetical basic unit of function, of mutation and of recombi¬ nation (see chapter 2). It has become clear that no single unit would simultaneously satisfy the criteria of these three categories. The unit of mutation may be a small group of atoms in one nucleotide. The unit of recombination, at least in lower organisms may be one nucleotide. The unit of function, however, can be quite large, consisting of a segment of DNA corresponding to at least one polypeptide chain, often to several. Since 1961 when Benzer published his studies on complementa¬ tion of different mutations of a single function in bacteriophage the term cistron has been in use as a unit of function. The term refers to specific differences in complementation when two muta¬ tions occur in the same DNA double strand (eis) and when the two occur in different double strands (trans) in the same cell. The cistron perhaps corresponds with one polypeptide chain. As com¬ plementation phenomena are more complicated than originally un¬ derstood, the value of the experimental cistron is somewhat doubtful, but as a concept of functional unit it has survived. Another unit of function, or rather of regulation is the operon (Jacob and Monod): a number of DNA segments coding for differ¬ ent enzymes, usually related to a single metabolic function in