Cell Division
Cell division is the process by which an organism grows or replaces damaged tissue. The growth of a fertilized embryo is accomplished through the division and differentiation of cells, and while some cells, such as skin cells, divide almost continuously after birth, other highly specialized cells, such as some neurons, do not and cannot be replaced after disease or injury. The two forms of cell division, mitosis and meiosis, are the biological mechanism by which the principles of heredity and evolutionary theory are realized.
German botanist Wilhelm Hofmeister (1824-1877) first examined the process of cell division in 1847 and showed that the nucleus did not truly disappear. He appeared to have been very close to discovering chromosomes, but that honor was left to Walther Flemming some 30 years later.
Prior to Flemming's work, little headway had been made in the field of cytology since Hofmeister's discovery, due in large part to the lack of effective cell staining techniques and poor microscopes. Advances in synthetic dyes allowed scientists to better study cells. By using the new dyes, Flemming was able to observe and correctly identify the stages of cell division.
Flemming found material within the cell nucleus that readily absorbed dye. He named the material chromatin, and by observing it at different phases, he could trace the action of cell division. As the process began, the chromatin arranged itself into short, thread-like objects, which Wilhelm von Waldeyer-Hartz later termed chromosomes. The chromosomes then doubled and pulled apart, each half migrating to opposite ends of the cell. In the final stage, the cell divided, leaving two daughter cells with equal amounts of chromatin.
By the end of 1879, Flemming had investigated all the stages of mitosis—a name which he derived from the Greek word for "thread"--and identified them in a variety of tissues. His terms for each stage (wreath, star, equatorial plate, and nuclear barrel) have since been replaced by today's familiar technical phrases (prophase, metaphase, early anaphase, and telophase).
Unfamiliar with the work of Gregor Mendel, Flemming was unaware of the genetic significance of his findings. However, with the rediscovery of Mendel by Hugo de Vries at the turn of the century, Flemming's work has since provided the physical basis of Mendel's theories of inheritance and his Zellsubstanz, Kern, und Zeltlteilung (1882; Cytoplasm, Nucleus, and Cell Division) is considered a classic text by cytologists.
In 1887, Flemming began concentrating on cell division in the spermatozoa. Although he detected that differences existed as to how sex cells divided, Flemming failed to identify the process later termed meiosis by Edouard van Beneden. Beneden, a Belgian cytologist, began experimenting with bivalves in 1887. He observed that when sex cells are formed before fertilization, the egg and sperm cell possess only half the usual count of chromosomes. From this he surmised that in the creation of sex cells, chromosomes do not double as they do in mitosis. Instead, through the process of meiosis, the chromosomal pairs split, leaving sperm and egg cells with half the genetic material of the parent cell. Fertilization, therefore, result in an embryo with one set of chromosomes each from the mother and the father.
Like Flemming's works, Beneden's findings were consistent with Mendel's inheritance theories--every genetic factor existed in duplicate, one coming from the male and one from the female. Meiosis also provided the mechanism by which vast combinations of traits could be achieved within a species and upon which natural selection could work.
Scientists have since learned that, when cells divide, one copy of each chromosome is pulled to each daughter cell along protein guide wires called microtubules. In cancer, this process may goes awry, with daughter cells receiving more or less chromosomes than they should. Researchers suspected that the centrosome, part of the cell that helps form and organize microtubules, might be involved. In the late 1990s, Jeffrey Salisbury and his colleagues showed that cancer cells contained excess amounts of a centrosome protein, suggesting the presence of extra centrosomes that could recruit more protein guide wires. This remains an active area of research.
Another topic of current interest is the role of telomeres—pieces of DNA that protect the ends of chromosomes, but which gradually wear away as the cell divides repeatedly. There does seem to be a causal link between the wearing away of telomeres and the decline in a cell's ability to divide. However, scientists are still debating whether this may have significant implications for aging of the body as a whole.
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