The concept of the gene during the time IPM 12-13 Aban 1395 Ali - PowerPoint PPT Presentation

The concept of the gene during the time IPM 12-13 Aban 1395 Ali Mohammad Banaei-Moghaddam Department of Biochemistry am_banaei@ut.ac.ir

The Gene: an evolving concept  “the idea of ‘the gene’ has been the central organizing theme of twentieth century biology” (Moss 2003, xiii; cf. Keller 2000, 9)  More than a hundred years of genetic research have rather resulted in the proliferation of a variety of gene concepts, which sometimes complement, sometimes contradict each other.  reducing this variety of gene concepts: either “vertically” to a fundamental unit, or “horizontally” by subsuming them under a general term. Others have opted for more pluralist stances 2

Milestones 3

Prehistory of the Gene; 1860s-1900s  It was only in the nineteenth century that heredity became a major problem to be dealt with in biology  With the rise of heredity as a biological research area the question of its material basis and of its mechanism took shape.  In the second half of the nineteenth century, two alternative frameworks were proposed to deal with this question. I. heredity as a force whose strength was accumulated over the generations, and which, as a measurable magnitude, could be subjected to statistical analysis. This concept was particularly widespread among nineteenth-century breeders 1998) and (Gayon and Zallen influenced Francis Galton and the so-called “biometrical school” (Gayon 1998, 105-146) . Francis Galton (1822-1911) 4

Prehistory of the Gene; 1860s-1900s II. heredity as residing in matter that was transmitted from one generation to the next. Two major trends are to be differentiated here: 1) hereditary matter as particulate and amenable to breeding analysis. Charles Darwin, The Variation of Animals and Plants under Domestication (1868) , Pangenesis (a prefix meaning "whole", "encompassing") and genesis ("birth") or genos ("origin"). : Charles Robert Darwin inheritance of tiny heredity particles he called gemmules that could be transmitted 1809-1882 from parent to offspring.  The hypothesis was eventually replaced by Mendel's laws of inheritance. nineteenth-century authors: o non-association of these particles with a particular hereditary substance. o they consisted of the very same stuff that the rest of the organism was made 5

Prehistory of the Gene; 1860s-1900s II. heredity as residing in matter that was transmitted from one generation to the next. 2) Germ Plasm theory (1892): • the body substance, the “ trophoplasm ” or “soma”, • a specific hereditary substance, the “ idioplasm ” or “germ plasm ” August Friedrich Leopold Weismann (1834 – 1914)  Weismann: (in a multicellular organism) inheritance only takes place by means of the germ cells (the gametes such as egg cells and sperm cells) . Other cells of the body-somatic cells do not function as agents of heredity.  Naegeli: it extended even from cell to cell and throughout the whole body, a capillary hereditary system analogous to the nervous system (Robinson 1979; Churchill 1987, Rheinberger 2008) . Carl Wilhelm von Nägeli (1817 – 1891) 6

Prehistory of the Gene; 1860s-1900s  Mendel: interpreting heredity not as a measurable magnitude, but as “a certain level of organization,” a “structure in a given generation to be expressed in the context of specific crosses.” (1822-1884)  alternative and “constant” (i.e., heritable) traits: Mendel believed that these traits were related by a “constant law of development” to certain “elements” or “factors” in the reproductive cells from which organisms developed. An analysis of the distribution of alternative traits in the progeny of hybrids could therefore reveal something about the relationship that the underlying “factors” entered when united in the hybrid parent organism (Müller-Wille and Orel 2007) . 7

Prehistory of the Gene; 1860s-1900s  The concept of gene as a unit of hereditary information: in an 1866 paper entitled “experiments in plant hybridization”; cited about three times over the next thirty -five years.  “ Formbildungelementen ”: purely mathematical entities Blending versus Particulate inheritance variations in traits were caused by variations in inheritable factors (or, in today’s terminology, phenotype is caused by genotype) 8

Prehistory of the Gene; 1860s-1900s Rediscovery of Mendel’s work (1900) Three botanists - Hugo De Vries , Carl Correns and Erich von Tschermak - independently rediscovered Mendel's work in the same year In 1889, based on a modified version of Charles Darwin's theory of Pangenesis of 1868, he postulated that different characters have different hereditary carriers. He specifically postulated that inheritance of specific traits in organisms comes in particles. He called these units pangenes, a term 20 years later to be shortened to genes by Wilhelm Johannsen. Hugo Marie de Vries (1848-1935) introducing the term "mutation", and for developing he neglected to mention Mendel's work, but after criticism by Carl a mutation theory of evolution. Correns he conceded Mendel's priority. 9

Prehistory of the Gene; 1860s-1900s Rediscovery of Mendel’s work (1900) Three botanists - Hugo De Vries , Carl Correns and Erich von Tschermak - independently rediscovered Mendel's work in the same year Correns was a student of Nägeli, a renowned botanist with whom Mendel corresponded about his work with peas but who failed to understand its significance, while, coincidentally, Tschermak's grandfather taught Mendel botany during his student days in Vienna. Erich Tschermak-Seysenegg Carl Erich Correns William Jasper Spillman (1871-1962) (1864-1933) (1863 – 1931) He also discovered cytoplasmic inheritance, an important extension of the only American to independently Mendel's theories, which demonstrated the existence of extra-chromosomal rediscover Mendel's laws of genetics. factors on phenotype. Most of Correns' work went unpublished however, His discovery was published in a and was destroyed in the Berlin bombings of 1945. November 1901. 10

Definition 1910s: Gene as a distinct locus chromosome theory of inheritance or the Sutton – Boveri theory  1902: meiotic behavior of chromosome behaved as Mendel’s element The Boveri-Sutton Chromosome Theory, as it came to be known, was discussed and debated during the first years of the twentieth century. It was embraced by some but strongly rejected by others. By 1915 Thomas Hunt Walter Sutton Theodor Boveri Morgan — initially a strong skeptic — laid the controversy to rest with studies of (1877-1916) (1862-1915) the fruit fly Drosophila melanogaster .  1902: Archibald Garrod(1857-1936) : Alkaptonurea: The first evidence that gene were necessary to make protein. 1908: genetic defects cause many inherited diseases  1904: William Bateson (1861-1926): describes gene linkage, showing that more than one gene may be required for a particular characteristic or trait William Bateson (1861-1926) 11

Paradigm shift  establishment of a categorical distinction between genetic factors on the one hand and traits or characters on the other hand  possibility of an independent assortment of discrete hereditary factors according to the laws of probability was to be seen as the very cornerstone of a new “paradigm” of inheritance  The masking effect of dominant traits over recessive ones and the subsequent reappearance of recessive traits were particularly instrumental in stabilizing this distinction.  Furthermore, it resonated with the earlier concept of two material regimes, one germinal and one bodily, already promoted by Naegeli and Weismann. 12

Genotype, phenotype and gene  Danish botanist Wilhelm Johansen made the distinction between the outward appearance of an individual (phenotype) and its genetic traits (genotype).  genotype and phenotype as abstract entities, not confining them to certain cellular spaces and remaining skeptical about the chromosome theory of inheritance throughout his life.  The word gene was coined by him in 1909 as “ a heritable factor responsible for the transmission and expression of a given biological trait”. which for him was a concept Wilhelm Johannsen “completely free of any hypothesis” regarding localization and material constitution. (1857-1927)  The proposed word traced from the Greek word genos, meaning "birth". The word spawned others, like genome. 13

Genotype as an “ ahistoric ” entity  the genotype had to be treated as independent of any life history and thus, as an “ ahistoric ” entity amenable to scientific scrutiny like the objects of physics and chemistry.  “The personal qualities of any individual organism do not at all cause the qualities of its offspring; but the qualities of both ancestor and descendant are in quite the same manner determined by the nature of the sexual substances,” Johannsen claimed (Johannsen 1911, 130).  Unlike most Mendelians, however, he remained convinced that the genotype would possess an overall architecture. He therefore had reservations with respect to its particulate nature, and especially warned that the notion of “genes for a particular character” should always be used cautiously if not altogether be omitted (Johannsen 1911, 147). 14