In 1665, when Hooke used the microscope he had designed to examine a piece of cork, he saw a honeycomb pattern of rectangles that reminded him of cells, the chambers of monks in monasteries. His observations prompted scientists to speculate that living tissue as well as nonliving tissue was composed of cells. The French scientist René Dutrochet (1776–1874) performed microscopic studies and concluded in 1824 that both plant and animal tissue was composed of cells.
In 1838 the German scientist Matthias Schleiden (1804–81) presented his theory that all plants were constructed of cells. The following year, Theodor Schwann (1810–82) suggested that animals were also composed of cells. Both Schleiden and Schwann theorized that cells were all created using the same process. Although Schleiden's hypotheses about the process of cell formation were not entirely accurate, both he and Schwann are credited with developing cell theory. Describing cells as the fundamental units of life, they asserted that all living things are composed of cells, the simplest forms of life that can exist independently. Their pioneering work enabled other scientists to understand accurately how cells live, with German pathologist Rudolf Virchow (1821–1902) launching theories of biogenesis with his statement indicating that cells reproduce themselves.
Improvements in microscopy and the increasing study of cytology—the formation, structure, and function of cells—enabled scientists to identify parts of the cell. Key cell components include the nucleus, which directs all cellular activities by controlling the synthesis of proteins, and the mitochondria, which are organelles (membrane-bound cell compartments) that catalyze reactions that produce energy for the cell. Figure 1.1 is a diagram of a typical animal cell that shows its component parts, including the contents of the nucleus, where chromosomes, which contain the genes, are located.
The Germplasm Theory of Heredity
Studies of cellular components, processes, and functions produced insights that revealed the connection between cytology and inheritance. August Weissmann (1834–1914) studied medicine, biology, and zoology, and his contribution to genetics study was an evolutionary theory known as the germplasm theory of heredity. Building on Darwin's idea that specific inherited characteristics are passed from one generation to the next, Weissmann asserted that the genetic code for each organism was contained in its germ cells (the cells that create sperm and eggs). The presence of genetic information in the germ cells explained how this information was conveyed, unchanged from one generation to the next.
In a series of essays about heredity published from 1889 to 1892, Weissmann also observed that the amount of genetic material did not double when cells replicated, suggesting that there was some form of biological control of the chromosomes that occurred during the formation of the gametes (sperm and egg). His theory was essentially correct. Normal body growth is attributable to cell division, called mitosis, which produces cells that are genetically identical to the parent cells. The way to avoid giving offspring a double dose of heredity information is through a cell division that reduces the amount of the genetic material in the gametes by one-half. Weissmann called this process reduction division; today it is known as meiosis.
Weissmann was also the first scientist to successfully refute the members of the scientific community who believed that physical characteristics acquired through environmental exposure were passed from generation to generation. He conducted experiments in which he cut the tails off five consecutive generations of mice and observed that none of their offspring were born tailless.
A typical animal cell
SOURCE: "A Typical Animal Cell," adapted from Robinson, in Genetics, vol. 1, A–D, Macmillan Reference USA, Gale Group, 2002