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With the basic patterns of genetic inheritance established, many biologists turned to investigations of the physical nature of the gene. In the 1940s and early 1950s, experiments pointed to DNA as the portion of chromosomes (and perhaps other nucleoproteins) that held genes. A focus on new model organisms such as viruses and bacteria, along with the discovery of the double helical structure of DNA in 1953, marked the transition to the era of molecular genetics.
In the following years, chemists developed techniques for sequencing both nucleic acids and proteins, while many others worked out the relationship between these two forms of biological molecules and discovered the genetic code. The regulation of gene expression became a central issue in the 1960s; by the 1970s gene expression could be controlled and manipulated through genetic engineering. In the last decades of the 20th century, many biologists focused on large-scale genetics projects, such as sequencing entire genomes.
Pre-Mendelian ideas on heredity
Aristotle's model of transmission of movements from parents to child, and of form from the father. The model is not fully symmetric.
The most influential early theories of heredity were that of Hippocrates and Aristotle. Hippocrates' theory (possibly based on the teachings of Anaxagoras) was similar to Darwin's later ideas on pangenesis, involving heredity material that collects from throughout the body. Aristotle suggested instead that the (nonphysical) form-giving principle of an organism was transmitted through semen (which he considered to be a purified form of blood) and the mother's menstrual blood, which interacted in the womb to direct an organism's early development. For both Hippocrates and Aristotle--and nearly all Western scholars through to the late 19th century--the inheritance of acquired characters was a supposedly well-established fact that any adequate theory of heredity had to explain. At the same time, individual species were taken to have a fixed essence; such inherited changes were merely superficial. The Athenian philosopher Epicurus observed families and proposed the contribution of both males and females of hereditary characters ("sperm atoms"), noticed dominant and recessive types of inheritance and described segregation and independent assortment of "sperm atoms".
In the Charaka Samhita of 300CE, ancient Indian medical writers saw the characteristics of the child as determined by four factors: 1) those from the mother's reproductive material, (2) those from the father's sperm, (3) those from the diet of the pregnant mother and (4) those accompanying the soul which enters into the fetus. Each of these four factors had four parts creating sixteen factors of which the karma of the parents and the soul determined which attributes predominated and thereby gave the child its characteristics.
The preformation theory is a developmental biological theory, which was represented in antiquity by the Greek philosopher Anaxagoras. It reappeared in modern times in the 17th century and then prevailed until the 19th century. Another common term at that time was the theory of evolution, although "evolution" (in the sense of development as a pure growth process) had a completely different meaning than today. The preformists assumed that the entire organism was preformed in the sperm (animalkulism) or in the egg (ovism or ovulism) and only had to unfold and grow. This was contrasted by the theory of epigenesis, according to which the structures and organs of an organism only develop in the course of individual development (Ontogeny). Epigenesis had been the dominant opinion since antiquity and into the 17th century, but was then replaced by preformist ideas. Since the 19th century epigenesis was again able to establish itself as a view valid to this day.
Plant breeders were also developing an array of stable varieties in many important plant species. In the early 19th century, Augustin Sageret established the concept of dominance, recognizing that when some plant varieties are crossed, certain characteristics (present in one parent) usually appear in the offspring; he also found that some ancestral characteristics found in neither parent may appear in offspring. However, plant breeders made little attempt to establish a theoretical foundation for their work or to share their knowledge with current work of physiology, although Gartons Agricultural Plant Breeders in England explained their system.
Between 1856 and 1865, Gregor Mendel conducted breeding experiments using the pea plant Pisum sativum and traced the inheritance patterns of certain traits. Through these experiments, Mendel saw that the genotypes and phenotypes of the progeny were predictable and that some traits were dominant over others. These patterns of Mendelian inheritance demonstrated the usefulness of applying statistics to inheritance. They also contradicted 19th-century theories of blending inheritance, showing, rather, that genes remain discrete through multiple generations of hybridization.
From his statistical analysis, Mendel defined a concept that he described as a character (which in his mind holds also for "determinant of that character"). In only one sentence of his historical paper, he used the term "factors" to designate the "material creating" the character: " So far as experience goes, we find it in every case confirmed that constant progeny can only be formed when the egg cells and the fertilizing pollen are off like the character so that both are provided with the material for creating quite similar individuals, as is the case with the normal fertilization of pure species. We must, therefore, regard it as certain that exactly similar factors must be at work also in the production of the constant forms in the hybrid plants."(Mendel, 1866).
Mendelian inheritance states characteristics are discrete and are inherited by the parents. This image depicts a monohybrid cross and shows 3 generations: P1 generation (1), F1 generation (2), and F2 generation (3). Each organism inherits two alleles, one from each parent, that make up the genotype. The observed characteristic, the phenotype, is determined by the dominant allele in the genotype. In this monohybrid cross the dominant allele encodes for the color red and the recessive allele encodes for the color white.
Mendel's work was published in 1866 as "Versuche über Pflanzen-Hybriden" (Experiments on Plant Hybridization) in the Verhandlungen des Naturforschenden Vereins zu Brünn (Proceedings of the Natural History Society of Brünn), following two lectures he gave on the work in early 1865.
Diagram of Charles Darwin's pangenesis theory. Every part of the body emits tiny particles, gemmules, which migrate to the gonads and contribute to the fertilised egg and so to the next generation. The theory implied that changes to the body during an organism's life would be inherited, as proposed in Lamarckism.
Mendel's work was published in a relatively obscure scientific journal, and it was not given any attention in the scientific community. Instead, discussions about modes of heredity were galvanized by Darwin's theory of evolution by natural selection, in which mechanisms of non-Lamarckian heredity seemed to be required. Darwin's own theory of heredity, pangenesis, did not meet with any large degree of acceptance. A more mathematical version of pangenesis, one which dropped much of Darwin's Lamarckian holdovers, was developed as the "biometrical" school of heredity by Darwin's cousin, Francis Galton.
August Weismann's germ plasm theory. The hereditary material, the germ plasm, is confined to the gonads. Somatic cells (of the body) develop afresh in each generation from the germ plasm.
In 1883 August Weismann conducted experiments involving breeding mice whose tails had been surgically removed. His results -- that surgically removing a mouse's tail had no effect on the tail of its offspring -- challenged the theories of pangenesis and Lamarckism, which held that changes to an organism during its lifetime could be inherited by its descendants. Weismann proposed the germ plasm theory of inheritance, which held that hereditary information was carried only in sperm and egg cells.
Rediscovery of Mendel
Hugo de Vries wondered what the nature of germ plasm might be, and in particular he wondered whether or not germ plasm was mixed like paint or whether the information was carried in discrete packets that remained unbroken. In the 1890s he was conducting breeding experiments with a variety of plant species and in 1897 he published a paper on his results that stated that each inherited trait was governed by two discrete particles of information, one from each parent, and that these particles were passed along intact to the next generation. In 1900 he was preparing another paper on his further results when he was shown a copy of Mendel's 1866 paper by a friend who thought it might be relevant to de Vries's work. He went ahead and published his 1900 paper without mentioning Mendel's priority. Later that same year another botanist, Carl Correns, who had been conducting hybridization experiments with maize and peas, was searching the literature for related experiments prior to publishing his own results when he came across Mendel's paper, which had results similar to his own. Correns accused de Vries of appropriating terminology from Mendel's paper without crediting him or recognizing his priority. At the same time another botanist, Erich von Tschermak was experimenting with pea breeding and producing results like Mendel's. He too discovered Mendel's paper while searching the literature for relevant work. In a subsequent paper de Vries praised Mendel and acknowledged that he had only extended his earlier work.
In 1972, Walter Fiers and his team at the University of Ghent were the first to determine the sequence of a gene: the gene for bacteriophage MS2 coat protein.Richard J. Roberts and Phillip Sharp discovered in 1977 that genes can be split into segments. This led to the idea that one gene can make several proteins. The successful sequencing of many organisms' genomes has complicated the molecular definition of the gene. In particular, genes do not always sit side by side on DNA like discrete beads. Instead, regions of the DNA producing distinct proteins may overlap, so that the idea emerges that "genes are one long continuum". It was first hypothesized in 1986 by Walter Gilbert that neither DNA nor protein would be required in such a primitive system as that of a very early stage of the earth if RNA could serve both as a catalyst and as genetic information storage processor.
1869: Friedrich Miescher discovers a weak acid in the nuclei of white blood cells that today we call DNA. In 1871 he isolated cell nuclei, separated the nucleic cells from bandages and then treated them with pepsin (an enzyme which breaks down proteins). From this, he recovered an acidic substance which he called "nuclein".
1889: Richard Altmann purified protein free DNA. However, the nucleic acid was not as pure as he had assumed. It was determined later to contain a large amount of protein.
1889: Hugo de Vries postulates that "inheritance of specific traits in organisms comes in particles", naming such particles "(pan)genes".
1902: Archibald Garrod discovered inborn errors of metabolism. An explanation for epistasis is an important manifestation of Garrod's research, albeit indirectly. When Garrod studied alkaptonuria, a disorder that makes urine quickly turn black due to the presence of gentisate, he noticed that it was prevalent among populations whose parents were closely related.
1903: Walter Sutton and Theodor Boveri independently hypothesizes that chromosomes, which segregate in a Mendelian fashion, are hereditary units; see the chromosome theory. Boveri was studying sea urchins when he found that all the chromosomes in the sea urchins had to be present for proper embryonic development to take place. Sutton's work with grasshoppers showed that chromosomes occur in matched pairs of maternal and paternal chromosomes which separate during meiosis. He concluded that this could be "the physical basis of the Mendelian law of heredity."
1908: G.H. Hardy and Wilhelm Weinberg proposed the Hardy-Weinberg equilibrium model which describes the frequencies of alleles in the gene pool of a population, which are under certain specific conditions, as constant and at a state of equilibrium from generation to generation unless specific disturbing influences are introduced.
1910: Thomas Hunt Morgan shows that genes reside on chromosomes while determining the nature of sex-linked traits by studying Drosophila melanogaster. He determined that the white-eyed mutant was sex-linked based on Mendelian's principles of segregation and independent assortment.
1911: Alfred Sturtevant, one of Morgan's collaborators, invented the procedure of linkage mapping which is based on the frequency of crossing-over.
1913: Alfred Sturtevant makes the first genetic map, showing that chromosomes contain linearly arranged genes.
1930s-1950s: Joachim Hämmerling conducted experiments with Acetabularia in which he began to distinguish the contributions of the nucleus and the cytoplasm substances (later discovered to be DNA and mRNA, respectively) to cell morphogenesis and development.
1931: Crossing over is identified as the cause of recombination; the first cytological demonstration of this crossing over was performed by Barbara McClintock and Harriet Creighton.
1933: Jean Brachet, while studying virgin sea urchin eggs, suggested that DNA is found in cell nucleus and that RNA is present exclusively in the cytoplasm. At the time, "yeast nucleic acid" (RNA) was thought to occur only in plants, while "thymus nucleic acid" (DNA) only in animals. The latter was thought to be a tetramer, with the function of buffering cellular pH.
1955: Joe Hin Tjio, while working in Albert Levan's lab, determined the number of chromosomes in humans to be of 46. Tjio was attempting to refine an established technique to separate chromosomes onto glass slides by conducting a study of human embryonic lung tissue, when he saw that there were 46 chromosomes rather than 48. This revolutionized the world of cytogenetics.
1960: Jacob and collaborators discover the operon, a group of genes whose expression is coordinated by an operator.
1961: Francis Crick and Sydney Brenner discovered frame shift mutations. In the experiment, proflavin-induced mutations of the T4 bacteriophage gene (rIIB) were isolated. Proflavin causes mutations by inserting itself between DNA bases, typically resulting in insertion or deletion of a single base pair. The mutants could not produce functional rIIB protein. These mutations were used to demonstrate that three sequential bases of the rIIB gene's DNA specify each successive amino acid of the encoded protein. Thus the genetic code is a triplet code, where each triplet (called a codon) specifies a particular amino acid.
In the late 1970s: nonisotopic methods of nucleic acid labeling were developed. The subsequent improvements in the detection of reporter molecules using immunocytochemistry and immunofluorescence, in conjunction with advances in fluorescence microscopy and image analysis, have made the technique safer, faster and reliable.
1980: Paul Berg, Walter Gilbert and Frederick Sanger developed methods of mapping the structure of DNA. In 1972, recombinant DNA molecules were produced in Paul Berg's Stanford University laboratory. Berg was awarded the 1980 Nobel Prize in Chemistry for constructing recombinant DNA molecules that contained phage lambda genes inserted into the small circular DNA mol.
1980: Stanley Norman Cohen and Herbert Boyer received first U.S. patent for gene cloning, by proving the successful outcome of cloning a plasmid and expressing a foreign gene in bacteria to produce a "protein foreign to a unicellular organism." These two scientist were able to replicate proteins such as HGH, Erythropoietin and Insulin. The patent earned about $300 million in licensing royalties for Stanford.
1983: Barbara McClintock was awarded the Nobel Prize in Physiology or Medicine for her discovery of mobile genetic elements. McClintock studied transposon-mediated mutation and chromosome breakage in maize and published her first report in 1948 on transposable elements or transposons. She found that transposons were widely observed in corn, although her ideas weren't widely granted attention until the 1960s and 1970s when the same phenomenon was discovered in bacteria and Drosophila melanogaster.
1985: Alec Jeffreys announced DNA fingerprinting method. Jeffreys was studying DNA variation and the evolution of gene families in order to understand disease causing genes. In an attempt to develop a process to isolate many mini-satellites at once using chemical probes, Jeffreys took x-ray films of the DNA for examination and noticed that mini-satellite regions differ greatly from one person to another. In a DNA fingerprinting technique, a DNA sample is digested by treatment with specific nucleases or Restriction endonuclease and then the fragments are separated by electrophoresis producing a template distinct to each individual banding pattern of the gel.
1986: Jeremy Nathans found genes for color vision and color blindness, working with David Hogness, Douglas Vollrath and Ron Davis as they were studying the complexity of the retina.
1989: Thomas Cech discovered that RNA can catalyze chemical reactions, making for one of the most important breakthroughs in molecular genetics, because it elucidates the true function of poorly understood segments of DNA.
1994: The first breast cancer gene is discovered. BRCA I was discovered by researchers at the King laboratory at UC Berkeley in 1990 but was first cloned in 1994. BRCA II, the second key gene in the manifestation of breast cancer was discovered later in 1994 by Professor Michael Stratton and Dr. Richard Wooster.
1996: Alexander Rich discovered the Z-DNA, a type of DNA which is in a transient state, that is in some cases associated with DNA transcription. The Z-DNA form is more likely to occur in regions of DNA rich in cytosine and guanine with high salt concentrations.
2007: The Barcode of Life Data System (BOLD) is set up as an international reference library for molecular species identification (www.barcodinglife.org).
2008: Houston-based Introgen developed Advexin (FDA Approval pending), the first gene therapy for cancer and Li-Fraumeni syndrome, utilizing a form of Adenovirus to carry a replacement gene coding for the p53 protein.
2009: The Consortium for the Barcode of Life Project (CBoL) Plant Working Group propose rbcL and matK as the duel barcode for land plants.
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