Portugaliae Acta Biologica - 1949
A Cytophysiological Theory of the Gene, Gene Mutation
and Position Effect
J. A. Serra
Secção de Zoologia-Anthropologia, Faculdade de Ciências. Universidade, Coimbra
In this paper are presented a detailed discussion of the interpretations hitherto proposed for the genetic units and a new theory of the gene, gene mutation and position effect. Conclusions about the main questions treated were exposed at the end of each section. The chief points discussed may be summarized as follows:
1. The bases of the modern gene concept are the physiological units which correspond to the mendelian factors, and the cytological units more or less correspondent to the chromomeres. The true roots of the gene concept are physiological, the unitary character of the gene results primarily from an individual difference during ontogenesis. The weak side of the gene concept is the cytological; only in certain cases and in certain stages does a direct correspondence between unitary ontogenetic differences and chromomeres of the chromosomes exist. This is especially the case of the pachytene, when crossing-over takes place, while in salivary chromosomes each physiological gene unit may correspond to several, in mean 4-5 salivary bands (Fig. 2, page 424). The question of pleiotropism is discussed and it is concluded that, excepting special cases, of certain chromonema segments in rapid evolutionary change, each genic unit is well delimited from its neighbors by it producing a unitary primary action. The cytological counterpart of the physiological gene unit is a postulated "elementary chromomere" which only in favourable cases is possible to observe cytologically as a discrete chromomere.
2. Position effect is considered to be a reality and not merely another name for gene mutation, as is supposed by the architectural theories of the gene. However, a distinction between gene mutations and position effects or, more generally, rearrangements effects, is only valid for the extreme cases. In what concerns the phenotype produced, there are two types of rearrangement effects: mimic, which give a phenotype like a gene mutant located near the points of breakage; and pure, which give new phenotypes, without correspondence to known gene mutants near the breaks. Cases of pure position effects are Bar, and very probably also Hairy wing, Abruptex and Confluens, Hairless, Moiré, Henna, Lobe, Star-asteroid, etc. in Drosophila melanogaster. The first type, the mimic effects, includes euchromatin and heterochromatin effects, while the pure effects are typically euchromatic. In what respects the nature of the chromonema regions, there are eu-eu (euchromatin-euchromatin) and eu-het or het-eu (heterochromatin-euchromatin) rearrangements. While the eu-eu effects are short range ones, extending only over a few salivary bands, the eu-het are typically long range effects which may extend over 50 salivary bands, corresponding in a mitotic chromosome of Drosophila to about 2,500 A if account is taken of spiralisation. Kinetic hypotheses, which involve short range forces, can not explain position effects at such distances. Of the structural hypotheses, the recent one of the pairing-deformation can not be valid on account of it implying heterozygoty and somatic pairing, which are not invariably associated with position effect. Van der Waals forces and dipole moments are also unable to explain long range position effects. The explanations proposed for mimic effects are: For eu-het effects an extension of the matricial nucleoproteins over the chromonema piece relocated to near heterochromatin, that is, heterochromatization of the adjacently relocated piece Fig. 3, page 447. The reverse phenomenon, euchromatization, may occur when relatively small heterochromatic pieces are inserted in euchromatic zones. Intercalary heterochromatin, in small pieces, may give position effects more or less intermediary between eu-eu and eu-het ones. The extension of the matricial nucleoproteins causes inactivation and a mimic variable effect, according to the degree of extension in each tissue area. Variegation of the typical eu-het effects is thus accounted for. The strictly eu-eu mimic effects are supposed to be due to the breakage of the genes and the relocation of their constituting pieces (a gene is usually a composite of several sub-units) at different points, which causes inactivation. These strict eu-eu effects are similar to gene mutation. Pure position effects are generally repeats, but may also result from other types of rearrangements. They are «genes in the making», assemblies of gene materials which give rise to an incipient group of chromonema blocks capable of working together. Due to the mild unbalance they cause, small repeats are more apt to cause this type of effects. The nature of heterochromatin and the phenomenon of heterochromatization are also discussed in detail.
3. Isomorphic and antimorphic gene action are discussed. Ordinary mutation is isomorphic, of the hyper-hypomorphic type, and results from partial reactivation or inactivation of the gene. For the question of the functioning of the gene it is indifferent whether inactivation takes place by gene mutation or by mimic position effect. Reasons are given for believing that gene activity generally is exerted through the production of a relatively simple but strongly specific compound, the haptogene, which serves as a kind of haptene group for the building of apoferments by the nucleoplasm-cytoplasm. The enzymes are built upon these apoferments Fig. 4, page 464 by interaction of the nucleus-cytoplasm system. Gene reduplication is supposed to be subsidiary to haptogene production only in the measure as this latter serves for the synthesis of more complicated polypeptides, and possibly also nucleic acids, necessary, for gene reduplication; gene reduplication and genic action are relatively independent, as is demonstrated by the case of heterochromatic inert regions which, nevertheless, may reduplicate at a normal rate. The functioning of the gene takes place according to definite steps, the activity of each gene has a certain number of levels, the quanta levels Q. There are Q+1 levels of activity for each gene, including the o or ground level of total inactivation (page 467). Generally Q is of the order of 3-10 and corresponds roughly to the valency of the gene. Above the D level the gene brings about the maximum of phenotype expression.
4. Gene and chromonema evolution are discussed in relation to the control of mutation and of heterochromatization. Intra- and inter-group differences, in species or higher groups, are chiefly of the quantitative small-amplitude or polygenic type. Great-amplitude mutations, of the ordinary mendelian type, generally only concern qualities with little importance in evolution and in systematic differentiation (excepting the possible case of saltations). Two kinds of characters resist evolution, either basic characters common to a whole group of class, order, or higher rank, or properties. of accessory nature like pigmentation markings, small form, modifications, etc.; these accessory properties are of oligogenic, classical mendelian, type. Small amplitude quantitative differences are probably half due to chromosome rearrangements and the other half to gene mutation. The chief characteristics of the organisms are controled by a buffer system of genes with small amplitude mutations. Mutability is also controlled by a buffer system of polygenes, not only in what concerns the rate of mutation as also the qualitative aspect, the amplitude of mutation. This is valid for species in evolutionary equilibrium with our relatively quiet interglaciary ambient. Mass gene-inactivation is heterochromatinization and may be primary, controlled by genes, generally of a buffer system type, and secondary, due to position effects, especially of the centromere and nucleolar zones. The amount of heterochromatin is typical of the state of evolutionary balance of the species.
5. Production of breaks and composition of chromonemata and chromosomes are considered, previous to a discussion of the nature of the gene and gene mutation. Radiation effects upon the chromosomes are of two orders: matricial or so-called physiologic or primary, and chromonematic or structural or secondary. The two orders of effects are supposed to be relatively independent. It is accepted as a general result that chromosome breakage does not necessitate of a great amount of energy; in what concerns the matrix, the chief effect is thymonucleic acid depolymerization, while on the chromonemata radiation induces the break of the chemical bonds which unite successive chromonema blocks. Chemical production of breaks is supposed to depend more on the «penetrability» of the compound to be tested than o a special mutagenic property; it is postulated that almost every compound which may react with proteins will be mutagenic providing that it will attain the chromonemata. Gene mutation is supposed to depend also primarily on breakage within a gene unit and therefore all which respects to chromonema breakage aplies also to gene mutation. The supposed distinction between chromonema breaks and gene mutations, consisting in the former necessitating of a densely ionizing track while the second depend upon a single ion-pair, is doubted. The two processes are admitted to be similar, intra-genic links being identical or very similar to inter-genic ones. The composition of chromonemata and genes is chiefly of proteins; chromonemata in their simplest composition are simply proteic and not nucleoproteic. The genes in their simplest composition must also be protein and not nucleoprotein in nature. When working, however, either in haptogene production or in auto-reduplication, the gene may become nucleoprotein. The proteins of the chromosomes are histones or protamines in the matrix nucleoproteins, while the proteins of the chromonemata are not markedly of a basic character. The chromosomes in their condensed state have chiefly matrix histone-thymonucleic acid nucleoproteins, besides the chromonemata proteins. In the resting stage there are also--a considerable part of nucleoplasm included within the «chromosomes». A small amount of ribonucleic acid exists perhaps at certain points of the chromonema, but in general neither the genes nor the viruses have necessarily nucleic acids in their simplest composition. The physical state of the chromonema proteins is admitted to be between fibrous and globular; in the main the polypeptide chains run parallel to the chromonema long axis. In order to make a chromonema with the minimum thickness of 0.2u, at least 104-106 polypeptide chains are necessary. The polypeptide chains can not run uninterruptly from one end to the other of the chromonema: even admitting that the whole energy of the 17 ionisations which effect the breakage of a Tradescantia chromosome is employed in breaking a chromonema, this amount of energy is considerably below the 486,000 kcal/mole which would be necessary to break peptide bonds. The chromonema is composed of discrete units or minimal blocks, which may be called nemameres. The links between successive nemameres are relatively weak.
6. These conclusions on the composition of the chromonemata are the basis of the discussion on the nature of the gene and gene mutation. Theories which suppose that the gene is similar to a big molecule do not correspond to the known cyto-genetical facts and the architectural theories faiI to explain why a change in chromonema structure bring about a unitary physiological difference. The chief points of the theory advanced in this paper are the following: The gene is a composite of several minimal chromonema blocks' or nemameres. The number of these nemameres is equal to the maximum quantum number Q. Each nemamere is joined to the others by relatively weak links with an energy of the order of 102-103 kcal, resulting from Van der Waals forces plus chemical bonds, chiefly of the weak types. This relatively weak link is, however, strongly reinforced in stages other than the cross-over of the pachytene by interactions, with the nucleoplasm and with the matrix. Inter-genic links are alike to intra-genic ones; the unity of the gene in cross-over results from differential deposition of matrix nucleoproteins due to haptogene specificity of each gene. According to the mimic position effects or mutations which happened respectively near or within the gene, the nemameres may be inactivated or reactivated. The number of nemameres in activity is q, the effective quantum number. Reasons are given for believing that activation or reactivation of each nemamere is an all-or-none process. Admitting that each nemamere of a gene produces about the same amount h of haptogene (haptogene constant) the q nemameres in activity produce h.q. It is supposed that the haptogene takes part in the formation of the apoenzymes, which will be synthesized in an amount a.hq. Now the catalysts act exponentially b.eahq until an inflection point is reached; after this the action is sigmoid, according to an equation of the form c(I-ehq)k, in which a, b, c, k are constants characteristic of each gene.
7. Gene diversification takes place by pure position effect and by gene mutation. Ordinarily, however, gene mutation is only reactivation and inactivation and so only quantitative differences result. It is probable that there are also mutations implying the acquisition of new active groups, but this must be very rare. Ordinary mutation must result from a break of a contact between two nemameres of a gene, which may cause the folding (inactivation) or unfolding (reactivation) of the region of the polypeptide chains which synthesizes or serves as templet for the haptogenes Figs. 6 and 7. The links at the contacts themselves remain in the end with about the same mean strength and the stability of the gene chains is also little affected; possibly the inactivated state is somewhat more stable. The problem of directed mutation is briefly discussed.