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Biology: Cell Biology

Biology: Cell Biology

Introduction

The discipline of cell biology is devoted to the study of cells, which are the basic structural units of all living things. Cell biology involves the study of biochemical mechanisms in animal and plant cells that are involved in cellular reproduction, communication, respiration and cellular architecture; the process of cellular differentiation into specialized cells that comprise plant or animal tissues; classification of cells by type and level of evolutionary development; and the study of genetics and heredity.

Historical Background and Scientific Foundations

Early Cell Biology

Before the microscope was invented in the early seventeenth century, the concept of the cell as the basic unit of living things was unknown, and natural philosophers (“scientists”—though that term was not invented until the nineteenth century) could only speculate about the basic structure of life.

A thriving center of trade and culture, the seventeenth-century Netherlands was a particularly rich place of scientific innovation and keen empirical observation. In the early 1600s, Dutch linen merchant Antoni van Leeuwenhoek (1632–1723) invented a microscope that allowed accurate observation of human and animal tissue. Van Leeuwenhoek saw and described cells for the first time.

Van Leeuwenhoek's first single-lens instrument was approximately 4 inches (10 cm) high and made of two brass plates riveted together; a small hole was drilled in the plates between which was placed a spherical high-powered lens. His best microscope could magnify images 270 times, which allowed him to see cells, which he called animalcules (tiny animals), in semen, blood, and pond water.

After his microscopic observations, however, van Leeuwenhoek asserted that “it is exclusively the male semen that forms the foetus,” and the eggs had “no other purpose than to serve as food for the semen.” His conclusions were a direct challenge to William Harvey (1578–1657), the English anatomist who discovered the circulation of the blood. In his 1651 book called Exercitationes de Generatione Animalium (Disputations touching the generation of animals), Harvey argued that all animals were the product of an egg or primordium, with the role of sperm being more uncertain.

Though neither van Leeuwenhoek nor Harvey were correct, their explanation for reproduction from cells made the idea of spontaneous generation (direct generation of mice, maggots, and other animals from nonliving matter) more questionable. The work of Italian physician Francesco Redi (1626–1697) and French chemist Louis Pasteur (1822–1895) eventually confirmed that spontaneous generation was impossible.

Van Leeuwenhoek reported his discoveries to the Royal Society in England. The society's “Curator of Experiments,” Robert Hooke (1635–1703), conducted experiments of his own with a compound microscope that he devised. He published his observations in 1665. Besides doing the first microscopic engravings of bird feathers, sponges, and small insects (a flea and a tick), he described a thinly sliced sample of cork as being divided up like a honeycomb. Hooke coined the term “cell” to describe the cavities he saw in cork, which were the walls of the dead plant cells. Hooke did not realize, however, that the “cells” he was seeing were basic units of life. In fact, he believed that in the living state these “cells” were channels through which fluid moved.

Improvement of the Microscope and Discovery of Organelles

After a burst of experimental activity in cytology (study of cell biology) in the seventeenth century, the optical limits of compound microscopes hindered more detailed cell observation for two centuries. Some biologists, such as Caspar Friedrich Wolff (1733–1794), believed the basic subunit of all tissues was an air-filled vesicle, much like Hooke's cork “cell,” Others, such as Swiss anatomist Albrecht von Haller (1708–1777), postulated that the basic elements of bodily composition were fibers, which provided a framework for tissues. Italian naturalist and physiologist Felice Fontana (1730–1805), the first to describe nerve fibers accurately, believed that all animal tissues were made of twisted cylinders. His idea was reinforced by his microscopic observation of fibrous tissues in nerves, muscles, and tendons.

Not until the early nineteenth century did microscopic observation improve enough to allow more accurate cell observations. In 1833, Scottish botanist Robert Brown (1753–1858) discovered the cell nucleus in the epidermal cells of an orchid. He was convinced that this organelle (“small organ”) had an important purpose, but did not know what it was.

Cell Theory and Particle Theory

In 1832, Jan Evangelista Purkyně (1787–1869), a Czech anatomist, acquired a new achromatic compound microscope, which he used to begin a systematic examination of animal tissues. He noted there were similarities between animal and plant cells. This was also perceived by Purkyně's collaborator Gabriel Valentin (1810–1883), who discovered that both plant and animal cells had nuclei and also saw the nucleolus (an oval body within the nucleus now known to contain DNA and RNA).

Purkyně and Valentin's work contributed to the formulation of cell theory, a concept fundamental to modern cell biology. It states that all organisms are made of similar organizational units called cells. The concept was first formally stated in 1839 by two German scientists, botanist Mathias Schleiden (1804–1881) and physiologist Theodor Schwann (1810–1882).

Many scientists found it difficult to accept cell theory at first, due largely to the limitations of nineteenth-century microscopes. The difficulty of viewing some particular types of living tissue led to misconceptions. For instance, it was widely believed that the nervous system was a connected mesh, rather than a system of separate cells. Even after Schwann and Schleiden proposed the cell theory, most scientists did not think their ideas applied to the brain or nerves, since neural tissue examined under the microscope looked like an interconnected web. The nerve cells' long axons and dendrites were in fact quite tangled together, and it was difficult with light microscopes to discern each cell nucleus. As a result, cell theory was not applied to neural tissue until 1900.

The public also found it difficult to accept that something so small could be the building block of even the largest masses of living tissue. Germ theory (the idea that infectious disease is caused by microorganisms) met much the same skepticism when it debuted. Germ theory, promoted primarily by French chemist Louis Pasteur (1822–1895) in the 1880s, showed that bacteria and other single-celled microorganisms caused most infectious diseases.

Cell Division

German embryologist Robert Remak's (1815–1865) work advanced cell division theory. Remak's observations of binary fission confirmed that animal cells arise from the division of preexisting cells. Remak also proved that the intracellular fluid in plant and animals cells is similar, coining the term “protoplasm” for both.

In the 1870s and 1880s, the development of stains and better microscopic lenses enabled a series of discoveries in cell division, including the identification of chromosomes (“colored bodies” found in cell nuclei). This led to the discovery of mitosis, from the Greek word for thread, which reflected the shapes of mitotic chromosomes. By the turn of the century, scientists had discovered that mitosis, the process by which a cell duplicates its genetic material, consists of four stages in all eukaryotic cells (cells with nuclei): prophase, metaphase, anaphase, and telophase. The period between mitotic divisions is called interphase, the period of time during which the DNA that composes the chromosomes is replicated.

IN CONTEXT: SPONTANEOUS GENERATION

Though William Harvey (1578–1657) and Antoni van Leeuwenhoek (1632–1723) had promising experimental results, the experiments of Francesco Redi (1626–1697), personal physician to the Duke of Tuscany, in 1668 led to the abandonment of spontaneous generation as a scientific theory. The Greek philosopher Aristotle (384–322 BC) who was known, among other things, for his detailed observations in marine biology, proposed after observing fiies arise from rotting meat that life arose spontaneously from nonliving matter. Since Aristotle's works were the main textbooks for medieval universities, spontaneous generation was an accepted scientific theory until Redi's experimental work.

Redi speculated that maggots developed from eggs laid by flies. To test this hypothesis he put meat in two sorts of flasks—some open to the air, and some sealed (the control experiment). Maggots appeared only in the open flasks, because the flies could reach the meat and lay their eggs in it. Redi's results were published in 1668 as Esperienze intorno alla generazione degl'insetti (Experiments concerning the generation of insects).

However, even Redi's brilliant experiments did not disprove spontaneous generation completely. The doctrine was revived in the early nineteenth century by German scientists and romantic philosophers who believed in a universal natural spirit, a geist or a “pure unity of being.” The whole universe was seen by these philosophers as essentially living or organic, with no distinction between living and nonliving matter. Evidence for this view was that no one had figured out how parasitic worms, like tapeworms, got into the intestines. It took the work of Louis Pasteur to definitively put the idea of spontaneous generation to rest.

Swiss biologist Friedrich Miescher (1844–1895), a professor of pathology at the University of Basel, extracted cell nuclei from white blood cells (lymphocytes). Because these were difficult to get in large quantities, he extracted them from used hospital bandages (lymphocytes, which multiply to fight infection, are prevalent in pus). While analyzing lymphocyte nuclei, Miescher realized that they contained an unknown substance.

This strange material did not react with the digestive enzyme pepsin, which proved it was not a protein. Miescher also discovered that this substance, which he termed nuclein (renamed nucleic acid in 1889 by German pathologist Richard Altmann [1852–1900]) was found in a large variety of cells, and that it contained phosphorus, as well as carbon, hydrogen, nitrogen, and oxygen. Though Miescher speculated that nuclein could be involved in fertilization in some way, he did not take his hypotheses any further. It was not until decades later that the substance was characterized as DNA (deoxyribonucleic acid), now known to be the cell's genetic material. The structure and function of DNA was not fully understood until the 1950s, when it was revealed by the work of American geneticist James Watson (1928–) and British biophysicist Francis Crick (1916–2004).

In 1878, German physiologist and physician Walther Flemming (1843–1905) used a mixture of acetic acids known as Flemming's solution to stain salamander cells, chosen for their large size and prominent nuclei. When material inside the nucleus bound to the dye, Flemming called it “chromatin” or “colored,” which became the root of the word chromosome. Flemming also noted chromosomal splitting, with each half going to a daughter cell during division.

Chromosomes, Meiosis, and Germ-Plasm Theory

Flemming's research on chromatin opened the door for others to hypothesize about the continuity of chromosomes from parents to offspring. In 1884, Eduard Strasburger (1844–1912) concluded that half an organism's chromosomes came from the mother and half from the father. By studying sex cells in nematodes, Belgian cytologist Edouard van Beneden (1846–1910) supported this conclusion when he discovered in 1887 that both egg and sperm had half the genes of a somatic cell and that only a fertilized egg (or zygote) had a full complement of genes.

German biologist August Weismannn (1834–1914) formulated the germ-plasm theory, i.e., that the body contains both germ (sex) cells—egg or sperm—that can transmit hereditary information, and somatic or body cells, which cannot. He also believed that heredity was transmitted through a chemical substance.

Rediscovery of Gregor Mendel and the Laws of Heritability

In 1856, an Austrian monk named Gregor Mendel (1822–1884) began to experiment with the hybridization of pea plants. Like Weismannn, Mendel was testing the acquired characteristics theory of French biologist Jean-Baptiste Lamarck (1744–1829). Finding an atypical variety of ornamental plant in the monastery gardens, Mendel planted it adjacent to the typical variety, growing them side by side to see if proximity alone would change the offspring. The plants, however, were uninfluenced by the others' nearness, the next generation looking exactly like its parent.

Mendel then began to study inheritance in pea plants, which he chose because they germinate rapidly and have easily identifiable phenotypes (external characteristics): flower color, height, and seed texture. His careful observation and statistical analysis of crossbreeding led him to formulate the laws of heritability. Mendel was able to link an organism's phenotype (its external appearance) to its genotype (genetic inheritance). He also proved the effects of dominant and recessive genes in specific inherited traits, which he called “factors.” These, he showed, were inherited in mathematical ratios.

Because Mendel was outside the scientific mainstream, his pioneering work remained unknown until 1900, when German botanist Carl Correns (1864–1933), Dutch botanist Hugo de Vries (1848–1935), and Austrian agronomist Erich von Tschermak-Seysenegg (1871–1962) independently rediscovered his achievements.

While crediting de Vries, Correns combined Weismann's previous discoveries in meiosis with Mendel's laws of inheritance to formulate the chromosome theory of heredity. Mendel believed that the segregation of genetic factors occurred in the production of sex cells in meiosis, leading Correns to conclude that “each germ cell must receive one or other of the factors that Mendel held responsible for dominant or recessive traits.”

Discovery of Organelles: Golgi Apparatus and Mitochondria

The last significant discoveries in cytology before 1940 were organelles: the Golgi apparatus and mitochondria. Before their discovery, many biologists considered the cell only as protoplasm-filled bag of enzymes that lacked any internal structure.

The Golgi apparatus, a series of membrane-bound sacs in the cytoplasm, prepares different proteins to be secreted from the cell, packaging them into vesicles. This organelle was first observed in 1898 by Camillo Golgi (1843–1926), who showed that it existed in many different cell types. However, because the Golgi apparatus looked slightly different in tissue specimens, during the first half of the twentieth century many cell biologists claimed it was an artifact of tissue preparation.

Mitochondria are organelles in which aerobic respiration produces adenosine triphosphate (ATP), the fuel for most cellular chemical processes. Mitochondria probably arose far back in evolutionary history as a result of a symbiotic or mutually beneficial relationship between prokaryotic and eukaryotic cells. Mitochondria have their own DNA and a system to make proteins that is similar to that found in bacteria.

ROBERT BROWN (1753–1858)

Robert Brown (1753–1858) was a Scotsman who studied medicine at the University of Edinburgh. In 1795, he became an army officer and surgeon's mate in a regiment that was posted to Ireland. During his army service, Brown used most of his spare time to study botany. In 1798, the young officer was introduced to the botanist Sir Joseph Banks (1743–1820) as “a Scotchman, fit to pursue an object with constance and [a] cold mind.”

Brown was recruited as botanist on Bank's voyage to Australia, which brought back nearly 4,000 plant species. Between 1806 and 1822, Brown was “clerk, librarian and housekeeper” for the Linnean Society of London; he inherited Banks's home and collections in 1820, a bequest that ultimately gave the collections to the new British Museum after Brown's death.

Brown negotiated the transfer of the specimens to the museum in 1827, however, it was on the condition that they become a permanent part of its collections and that he remain their curator for life. This established one of the greatest botanical collections in history, one that allowed significant advances in taxonomy. Brown's macroscopic botanical knowledge proved useful in his microscopic observations, including the discovery of Brownian motion, the constant agitation of minute particles in suspension that was later shown by Albert Einstein to be the result of evaporation currents induced by thermodynamic turbulence.

Albert Kölliker (1817–1905) is credited with discovering mitochondria in muscle tissue in 1850, but it was not until 1890 that Richard Altmann discovered a means to stain them with fuchsin dye, illustrating their presence in most cellular types. Altmann, however, did not understand their purpose, postulating instead that they were living particles similar to prokaryotic bacteria. Just as with the Golgi apparatus, some investigators were skeptical of the granules' existence, claiming they were artifacts created by tissue fixation or staining.

It was not until 1898 that German physician Carl Benda (1857–1933) used a crystal violet stain to reveal these structures in living cells, proving that they were not the result of slide preparation. Benda named them mitochondria, from the Greek mito, “thread,” and khondrion, “little granule,” because they appeared as threadlike granules under the light microscope.

In 1899, Leonor Michaelis (1875–1949), a German biochemist and physician, showed that Janus green dye, colorless when reduced but blue-green when oxidized, would turn mitochondria blue-green in living cells. Mitochondria's oxidizing activity indicated that they had a likely role in cellular respiration, but another 50 years would pass before their involvement in aerobic respiration was confirmed.

Twentieth-Century Advances

The first half of the twentieth century saw many more advances in the realm of biochemistry, with the discovery of glycolysis, the Krebs (or tricarboxylic acid) cycle, and the oxidative phosphorylation. All are steps in cellular chemistry that produce energy in the form of adenosine triphosphate (ATP), the cell's energy carrier. ATP was identified in 1929 by German chemist Karl Lohmann; ten years later, German-born American biochemist Fritz Lipmann (1899–1986) showed that ATP is the universal carrier of chemical energy in the cell. Present in all living organisms, ATP captures energy released by the combustion of nutrients; removing the outermost phosphate group forms adenosine diphosphate (ADP), releasing energy that can be used for other reactions. With the addition of energy, an inorganic phosphate group can be bound to ADP and form ATP.

Despite these advances, the relationship of organelles to specific biochemical pathways in cellular respiration was not understood until the development of effective analytical centrifuges in the 1920s. Centrifuges rotate at high speeds, producing an artificial gravity (centrifugal) force that can separate samples into components by size, shape, density, and viscosity. Particles of different masses will settle at different rates in response to gravity, which is a means of separating differently sized organelles.

With the advent of the centrifuge, the relationship between cellular structure and function became less of a mystery. However, with only the light microscope at their disposal, cell biologists' ability to visualize cell components was extremely limited. Belgian biologist Albert Claude compared cell biologists in the first half of the twentieth century to astronomers, “who were permitted to see the objects of their interest, but not to touch them; the cell was as distant from us as the stars and galaxies…”

The solution was the development of the electron microscope, invented by German electrical engineer Ernst Ruska (1906–1988) in 1933. Electron microscopy magnifies objects up to 2 million times, about 100 times better resolution than the best light microscope. The specimen must be specially prepared by being coated with a material like gold, which will conduct electricity. The specimen is placed at the bottom of a vacuum column, and at the top of the column a “gun” shoots electrons at the sample. Inside the column, magnetic fields focus electrons on the tissue sample. Electron beams, which scatter electrons depending on the amount of matter present, are moved across the object; information from the electron scatter is used to create a picture called an electron micrograph.

Electron micrographs' improved resolution revealed a new organelle, the endoplasmic reticulum (ER), a fairly continuous system of membrane-bound cavities in the cytoplasm. Rough endoplasmic reticula (RERs) are studded with ribosomes, small membrane-less granules made up of 50 intricately bound proteins and several long RNAs that are the site of protein synthesis. (ERs without ribosomes are called smooth endoplasmic reticula.) Ribosomes were isolated from microtome fractions of liver and pancreas in 1955 and identified in electron micrographs as identical to those on the membranes of the endoplasmic reticulum. Ribosomes were found on subsequent analysis to be rich in ribonucleic acid, which indicated their role in protein synthesis.

In 1958, British chemist Peter Mitchell (1920–1992), speculating about the relationship between transport across cell membranes and cellular metabolism, studied ox-phos in plant and animal cells. He believed that metabolic enzymes acted across the cell membrane, allowing the substrate to enter on one side and the product to leave on the other, a process he called chemiosmotic. Applying this principle to the mitochondrial enzyme ATPase, Mitchell proposed that a pH gradient could drive ATP synthesis.

Others suggested that this pH gradient could be created by the respiratory chain of enzymes that were shown through biochemical analysis and electron microscopy to exist on the inner layer of the mitochondria membrane. In the 1960s and very early 1970s, when mitochondria undergoing cellular respiration were shown to eject protons, a proton gradient (with a membrane potential) had been shown to drive ATP synthesis.

The chemiosmotic hypothesis suggests that ATP synthesis is due to an electrochemical gradient across the mitochondrial membrane. Protons move back across the inner membrane through the enzyme ATP synthase. As the protons flow back into the mitochondrion, this flow provides enough energy for ADP to combine with phosphate to form ATP.

Mitchell's hypothesis eventually won him the 1978 Nobel Prize for chemistry, but the scientific community resisted it at first. Many thought the idea of a proton gradient powering metabolism was improbable. Biochemists promoting the chemical hypothesis felt that a high-energy intermediate formed by the respiratory chain was a more likely explanation. These “oxphos wars,” which raged in the 1960s and 1970s, illustrate the tension and opposition of ideas that sometimes characterize scientific discovery.

Discovery of DNA

After the chromosome theory of heredity was established in the late nineteenth century, the biochemical composition of genetic material was analyzed further. Friedrick Miescher, who discovered nucleic acids in hospital bandages 1869, did not appreciate the significance of his discovery, believing nuclein was the cell's storage place for phosphorus.

During the same period, German biochemist Albrecht Kossel (1853–1927) published his hypothesis that nuclein was related to the formation of new tissue and not merely a site of phosphorus storage. Kossel also isolated and characterized the bases—adenine, guanine, cytosine, and thymine—that compose the nucleic acid DNA. Kossel also discovered uracil, a base present only in RNA, in 1900. For his contributions to nuclein research, Kossel received the Nobel Prize for medicine in 1910. Later it was found that the sugars in nucleic acid can be ribose (as in RNA) or deoxyribose (as in DNA).

In 1943, Canadian-born American bacteriologist Oswald Avery (1877–1955), working at the Rockefeller Institute with his colleagues American biologist Maclyn McCarty (1911–2005) and Canadian-American geneticist Colin MacLeod (1909–1972), proved that DNA carries genetic information in bacteria.

Avery used two strains of pneumococcus, namely the R strain, which is comparatively mild, and the S strain, which causes pneumonia in mice. Avery's strategy was to remove different organic compounds from bacteria, seeing if the remaining substances could cause the R strain bacteria to transform into the S strain. If no transformation occurred, that substance could not be the carrier of genes.

First the bacteria were centrifuged to removed large cellular components. The bacteria still transformed. Next, protease, which breaks down all proteins, was added, and the bacteria transformed. Finally, the pneumococcus were treated with deoxyribonuclease, which eliminates DNA, and there was no transformation of bacteria—the mice did not develop pneumonia when injected with the bacteria. Avery's results indicated that DNA was the carrier of genetic material in cells.

Although DNA had been established as the genetic molecule, its structure had yet to be discovered.

In 1948, American chemist Linus Pauling (1901–1994) discovered that many proteins take the shape of a spring coil or helix, publishing his results in 1951. Two years later he proposed that DNA might have a triple helix structure. Biochemist Erwin Chargaff (1905–2002) discovered in 1950 that while the arrangement of adenine, guanine, cytosine, and thymine in DNA varied widely, the amount of these bases always occurred in a one-to-one ratio.

All of these discoveries, combined with the work of New Zealand—born British molecular biologist Maurice Wilkins (1916–2004) and English biophysicist Rosalind Franklin (1920–1958), based at King's College in London in 1953, provided a foundation for the discovery of DNA's double-helical structure by Watson and Crick at Cambridge University. Their discovery also allowed for the development of the central dogma of cell and molecular biology, first stated by Crick in 1958. Its basic premise is that genetic information from DNA or RNA is used to create proteins, but information flow from proteins to DNA or RNA does not occur.

Gene Sequencing and Genetic Engineering

The discovery of DNA and the assertion of the central dogma made many advances in molecular genetics possible, including gene sequencing and recombinant DNA technology, also known as genetic engineering. In the early 1970s, Paul Berg (1926–) used restriction enzymes, which are proteins made by bacteria to counter infection, to split DNA at particular points and insert new genes. He also discovered that enzymes called ligases could heal the cuts made in DNA by forming covalent bonds. He used this information to splice the genes from a bacteria virus into SV40, a simian virus frequently used in laboratory studies.

American biochemists Herbert Boyer (1936–) of the University of California at San Francisco, and Stanley Cohen (1922–) at Stanford created the first genetically modified organisms (GMOs), which carried genetic information from different species. Cohen's work involved plasmids—circular and nonchromosomal units of DNA found in and exchanged by bacteria—while Boyer studied restriction enzymes.

Using restriction enzymes, Boyer and Cohen cleaved plasmid pSC101, which is genetically resistant to tetracycline, into the intestinal bacteria E. coli. This made the bacteria resistant to tetracycline. Into the same cut they next inserted the kanamycin-resistant plasmid pSC10. Because the bacteria's genetic material had been recombined, the E. coli was now resistant to both kanamycin and tetracycline. This recombinant DNA technology led to other genetically modified organisms and biotechnology. This began a public debate about their ethical creation and use.

Rudolf Jaenisch (1942–) created the first transgenic animals in 1974, inserting leukemia DNA sequences into mouse embryos, integrating the genes into the mice and their offspring. The first knockout mice—genetically engineered mice in which certain genes have been made inoperable—were created by American molecular geneticist Mario Capecchi (1937–), British biochemist Sir Martin Evans (1941–) and American geneticist Oliver Smithies during the 1980s. These mice are created to study gene function, particularly in cases where a gene has been sequenced but its function is unknown.

By studying heritability patterns of red-green colorblindness, scientists had learned by 1911 that this trait was linked to a gene carried on the maternal X chromosome. The other 22 pairs of chromosomes, however, remained unknown territory until the early 1970s, when staining revealed dark and light bands across each chromosome, which provided mapping landmarks and made the assignment of about 1,000 genes to specific chromosomes possible, a process called karyotyping.

Recombinant DNA technology, in which genes are combined artificially and inserted into an organism's DNA, led to fluorescent in situ hybridization (FISH), a medical diagnostic tool to identify genetically linked diseases, in which radioactively- or chemically-tagged probes are used to find the suspect DNA fragment on the chromosome.

These successes led to the federally funded Human Genome Project in 1991, which assigned as many as 25,000 human genes to their locations on specific chromosomes. Completed in 2003, the project was directed by James Watson, codiscoverer of DNA's double-helical structure. The project's wealth of data will fuel studies in genetic therapy, mechanisms, and cancer research for years to come.

Gene sequencing—determining the order of base pairs in a DNA strand—was improved Walter Gilbert (1932–) and Allan Maxam at Harvard University. Basing their work on British biochemist Frederick Sanger's (1918–) findings at Cambridge University, the duo found a way to multiply, divide and fragment DNA. Each strand was radioactively labeled at one end, and reagents were applied to break it selectively along its bases, either A, G, T, or C.

Unlike Sanger, who used chain-terminating molecules to indicate the positions of each base, Maxam and Gilbert subjected their strands to gel electrophoresis, which separated them by length, revealing the position of each base via the breakage points. In the 1980s, polymerase chain reaction (PCR) technology was developed to “photocopy” DNA, significantly speeding the sequencing process.

Modern Cultural Connections

After the discovery of DNA, scientific knowledge of molecular genetics exploded. After the development of recombinant DNA technology in the early 1970s, complex proteins for commercial use could be grown in animal cells. For example, most insulin required by persons with diabetes is produced today by bacteria containing recombinant genes for the production of human-type insulin. The modification of the DNA in the body cells of people suffering from genetic diseases—gene therapy—is also an active area of medical research in the early twenty-first century, although safe, effective treatments had not yet been demonstrated as of 2008.

Twentieth-century advances in genetics have confirmed evolutionary theory and are richly informing its further development. When Charles Darwin (1809–1882) outlined his theory of natural selection in On the Origin of Species in 1859, he knew that for natural selection to work, parents had to pass their characteristics to their offspring; however, his work took place before Mendel's innovations in heredity were widely known. Darwin speculated on various mechanisms of inheritance but had no real data with which to work. In the 1930s and 1940s, Mendel's rediscovered genetics were combined with Darwin's theory of natural selection to produce the neo-Darwinian Synthesis, a powerful tool for explaining evolutionary change.

In the latter decades of the twentieth century, the neo-Darwinian synthesis was supplemented by more knowledge about the exact mechanisms of mutation and heredity. DNA sequencing technology allowed biologists to decipher the DNA of specific organisms—gene by gene at first, later genome by genome (i.e., all of a creature's DNA at once). This, in turn, has allowed biologists to trace specific genes in different species, producing an increasingly detailed picture of evolutionary relationships and of the mechanisms driving evolutionary change. Molecular biology has thus produced powerful, independent, confirmatory evidence for evolution, strengthening the scientific case against Creationism, which is the belief that a God or gods created human beings out of nothing or triggered their development by miraculous action on ancient DNA. Nevertheless, when polled, over 40% of U.S. citizens express belief in Creationism. This imbalance between scientific certitude and public doubt creates tension in the public-school system, as some anti-evolution citizens resist the teaching of evolutionary biology or seek to have their own beliefs taught as a legitimate scientific alternative to evolution. There is no debate in the community of scientists who study life's history about whether evolution has occurred: DNA techniques have added increasing clarity of detail to earlier certainty about overall process.

Cell-biological techniques have produced controversial choices and ethical dilemmas. For example, the possibility that we may soon be able to engineer human DNA raises the question of whether it would be morally allowable to “enhance” the offspring of those who can afford such techniques, possibly creating a generation of children privileged both economically and genetically. In the long term, such techniques might call the concepts of human equality and democracy into radical question. (Whether they can ever be rendered practical remains, however, to be seen.) In 2008, human embryos were produced from ordinary skin cells for the first time: the production of cloned human beings or the manufacture of large numbers of embryos for the harvesting of stem cells or other substances seemed imminent. Moreover, a global debate was under way about the merits of genetically modifying crop plants and livestock animals, with proponents arguing that such techniques will benefit humans and opponents arguing that they constitute reckless interference with the basis of life itself. Persons of various ethical and religious beliefs debate the ways in which such technologies might or might not be properly used.

See Also Biology: Genetics; Biology: Genetics, DNA, and the Genetic Code; Biology: Genetics: The Human Genome.

bibliography

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Bechtel, William. Discovering Cell Mechanisms: The Creation of Modern Cell Biology. Cambridge: Cambridge University Press, 2006.

Farley, John. The Spontaneous Generation Controversy from Descartes to Oparin. Baltimore and London: John Hopkins, 1977.

Harris, Henry. The Birth of the Cell. New Haven: Yale University Press, 1999.

Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. New York: Mariner Books, 2001.

Lane, Nick. Power, Sex and Suicide: Mitochondria and the Meaning of Life. Oxford: Oxford University Press, 2005.

Palladino, Michael A. Understanding the Human Genome Project. 2nd ed. New York: Benjamin Cummings, 2005.

Watson, James D. The Double Helix: A Personal Account of the Discovery of the Structure of DNA. New York: Touchstone, 2001.

Periodicals

Anderson, C. Thomas. “Robert Remak and the Multinucleated Cell: Eliminating a Barrier to the Acceptance of Cell Division.” Bulletin of the History of Medicine 60 (1986): 523–543.

Ford, Brian J. “Brownian Movement in Clarkia Pollen: A Reprise of the First Observations.” Microscope 40, 4 (1992): 235–241.

Koeffel, Peter. “Felice Fontana: Life and Works.” Bulletin of the History of Medicine 58 (1984): 430–432.

La Berge, Anna. “The History of Science and the History of Microscopy.” Perspectives on Science 7, no. 1 (1999) 111–142.

Logan, David C. “Mitochondrial Dynamics.” New Phytologist 160, no. 3 (December 2003): 463–78.

Mitchison, T.J., and E.D. Salmon. “Mitosis: A History of Division.” Nature Cell Biology 3 (January 2001).

Porter, K.R., A. Claude, and E.F. Fullam, “A Study of Tissue Culture Cells by Electron Microscopy.” Journal of Experimental Medicine 81 (1945): 235–255.

Prebble, John. “Peter Mitchell and the Ox-Phos Wars.” Trends in Biochemical Sciences 27, no. 4 (April 1, 2002): 209–212.

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Web Sites

Electronic Scholarly Publishing. “The Germ Plasm: A Theory of Heredity.” http://www.esp.org/books/weismann/germ-plasm/facsimile/title3.html (accessed December 7, 2007).

A. M. Benton

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Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

http://www.mla.org/style

The Chicago Manual of Style

http://www.chicagomanualofstyle.org/tools_citationguide.html

American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.