The Gene in Cells
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We need to know where in an organism the hereditary factors lie. To that end we need to know the fundamental building block of living organisms: we need to know that all living things are assemblages of cells.
That living tissues consist of repeating units instead of being a kind of chemical continuum was the fundamental discovery necessary to an understanding of heredity. After Hans Lippershey (1570 XXX YY – 1619 Sep YY) invented the microscope, around 1610, biologists had to wait until the 1660's before microscopes had been improved enough for them to see the faintest inklings of the cell structures of living organisms. It took nearly two centuries after that for microscopy to advance enough for zoologist Theodor Schwann (1810 Dec 07 – 1882 Jan 11) and botanist Matthias Schleiden (1804 Apr 05 – 1881 Jan 23) to convince most biologists that all living organisms, plants and animals, consist of cells and cell products. In 1839 they presented their theory that cells are the fundamental structural and functional units of living matter.
Schleiden and Schwann were not the first to propose something like the cell theory. In 1824 Henri Milne-Edwards (1800 Oct 23 – 1885 Jul 29) proposed the idea that all animal tissue consists of globules, but his insistence that the globules all have the same size raises serious questions about his observational skills. Rene Joachim Henri Dutrochet (1776 Nov 14 – 1847 Feb 04) stated that plant and animal cells are the same phenomenon and added the hypothesis that the cell was not only the structural unit of life but also the physiological unit. He also believed that new cells emerge from within old cells. François-Vincent Raspail (1794 Jan 25 – 1878 Jan 07) also set out his own version of the cell theory, exemplified by the statement "Omnis cellula e cellula" (All is cells from cells). By 1839 the idea behind the cell theory had attained a critical mass that led to rapid acceptance of Schleiden and Schwann’s theory.
To be complete the cell theory must not only assert that living things consist of cells; it must also say whence new cells come. Schleiden and Schwann believed, as did many biologists of the time, that cells precipitated or crystallized out of fluids circulating throughout the organism that they comprise. In 1832 Barthelemy Dumortier (1797 Apr 03 – 1878 Jun 09) observed the reproduction of plant cells. He saw the formation of the midline partition that splits a pre-existing cell into two new cells. Like the cell theory itself, the idea of binary fission of cells didn’t catch on right away, not surprising when you consider the difficulty of making the relevant observations with the instruments available at the time. In 1852 Robert Remak (1815 Jul 26 – 1865 Aug 29) added his own impetus to the binary fission theory by publishing the results of his observations of animal embryos. But it was Rudolf Virchow (1821 Oct 13 – 1902 Sep 02) who gained wide acceptance for the binary fission hypothesis when he presented it in his book "Cellularpathologie" in 1858.
Once biologists had determined the existence and nature of life’s basic building block, they had to find out where in that block heredity occurs. How does the cell store whatever it is that determines the traits of the organism of which the cell is part?
Again discovery required advances in microscopy, but not merely improvements in the optical engineering of the microscopes themselves. As biologists zoomed in on ever smaller structures they found that the cells that they wanted to study possessed an unfortunate degree of transparency, a fact that made discovering features in the cell difficult at best. Around 1840 biologists discovered that basophilic aniline dyes would stain cells unevenly. That fact enabled them to discern features of cells that they otherwise would have missed seeing and let them peer ever deeper into the structure of living matter.
In 1842 Karl Wilhelm von Nägeli (1817 Mar 27 – 1891 May 11) observed some heavily stained objects in the nuclei of plant cells and Edouard van Beneden (1846 Mar 05 – 1910 Apr 28) observed similar objects in the cells of ascaris worms when he was studying the worms’ infestation of horses. In 1879 Walther Flemming (1843 Apr 21 – 1905 Aug 04) discovered mitosis, the process in which a cell’s chromosomes duplicate themselves and separate into two sets prior to the cell’s fission, and he noticed that the chromosomes split longitudinally. In 1884 Rudolf Albert von Kölliker (1817 Jul 06 – 1905 Nov 02) hypothesized that the chromosomes were involved with heredity. In 1888 Heinrich Wilhelm Gottfried von Waldeyer-Hartz (1836 Oct 06 – 1921 Jan 23) suggested the name chromosome (Anglicized Greek for colored body) for these objects. Wilhelm Roux (1850 Jun 09 – 1924 Sep 15) devised the hypothesis that each chromosome somehow manifests a set of hereditable elements, that some chemical form laid out on the chromosome carries one of the traits that the organism will display. In accordance with that hypothesis Roux conceived the longitudinal splitting of the chromosomes as the means of ensuring that the same elements occur in the same order on the two new chromosomes that then go into the two new cells.
With the rediscovery of Mendel’s genetics in 1900, biologists had a picture to guide their testing of the chromosome hypothesis of heredity. In 1902 Theodor Boveri and Walter Sutton took the first steps in that direction. Theodor Heinrich Boveri (1862 Oct 12 – 1915 Oct 15) experimented on sea urchins and discovered that unless all of the chromosomes characteristic of a sea urchin were present in the fertilized egg the embryo would not develop properly, if it developed at all. Walter Stanborough Sutton (1877 Apr 05 – 1916 Nov 10) also worked with marine organisms and studied the process of reduction division (which we now call meiosis), in which cells about to become sperm or eggs undergo division without duplicating their chromosomes. Meiosis provides a ready explanation of Mendel’s law of segregation of traits and ensures that when sperm and egg unite, the resulting cell has the correct number of chromosomes. As Sutton put it, "the association of paternal and maternal chromosomes in pairs and their subsequent separation during the reduction division ... may constitute the physical basis of the Mendelian law of heredity."
That was good evidence, but not compelling. In law it would be regarded as probable cause to investigate the chromosome hypothesis further, but not sufficient to gain a conviction of that hypothesis being true to Reality. However, more evidence was on the way.
In 1905, two years after receiving her Ph.D., Nettie Maria Stevens (1861 Jul 07 – 1912 May 04) discovered that in insects, especially the mealworm Tenebrio, the sexes have different chromosomes. Thus she was the first person to show that a difference in chromosomes correlates with some difference in the physical traits of the organism (in this case sex). She identified the Y-chromosome as the determinant of sex. She also made another, indirect, contribution to the chromosome theory by introducing the fruit fly, Drosophila melanogaster, into the laboratory run by Thomas Hunt Morgan (1866 Sep 25 – 1945 Dec 04).
Morgan led the team that made the final breakthrough and thereby transformed the chromosome hypothesis into the chromosome theory of heredity. Beginning around 1908, he and his collaborators at Columbia University in New York set up a laboratory devoted to the study of fruit flies. Using chemicals, radiation, and other means, they tried to induce mutations in the flies’ heredity in experiments analogous to what Mendel had done half a century before. In 1910 Morgan discovered a male fly that had white eyes instead of the usual red eyes: when they bred that fly and tracked its progeny, Morgan and his team found that the trait of having white eyes followed the Mendelian pattern of a recessive trait. Morgan’s team found other mutations and tracked them as well, correlating them with each other and with other traits. In 1913 Alfred Henry Sturtevant (1891 Nov 21 – 1970 Apr 05) drew up the first crude map of genes on chromosomes. And in 1915 Morgan, Sturtevant, Calvin Bridges, and H. J. Muller wrote "The Mechanism of Mendelian Heredity", a book in which they described their experiments and their results. That book had such an impact on biologists that some compared Morgan to Galileo or Isaac Newton.
A brief look at another side of Morgan’s career becomes appropriate here. We often compare science to a living thing. Like a living organism, science, especially biology, can get sick, producing hideous cancer-like excrescences like the vile doctrine of Social Darwinism. At the beginning of the Twentieth Century as genetics advanced it misbegat Eugenics, a thing so foul that Morgan devoted considerable effort to opposing it. We can only speculate here, but perhaps if scientists had followed Morgan’s example and been more effective in opposing the morally repellant doctrines of Social Darwinism and Eugenics for the intellectual frauds that they are, their scientific descendants would not now be fending off the equally fraudulent doctrine of Creationism.
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