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over-the-counter

over-the-counter, method of buying and selling securities outside the organized stock exchange. Unlike an organized stock exchange, the over-the-counter market is composed of dealers who negotiate most transactions by telephone and computer. For the most part, dealers purchase stocks for their own account and sell them to customers at a markup over wholesale prices. Over-the-counter trading represents the single largest securities market in the United States today; it includes almost all U.S. government securities and municipal and corporate bonds, as well as most commercial bank and insurance company stocks. Today, most over-the-counter dealing in the United States is done through an extensive computer network, called the National Association of Securities Dealers Automated Quotations (Nasdaq). In recent years, a number of companies that would be eligible for listing on the New York Stock Exchange have opted to remain in the over-the-counter market. In 1998, Nasdaq trading totalled $5.8 trillion, making it the second largest securities market in the world.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

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crossing over

crossing over, process in genetics by which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the first division of meiosis. At that stage each chromosome has replicated into two strands called sister chromatids. The two homologous chromosomes of a pair synapse, or come together. While the chromosomes are synapsed, breaks occur at corresponding points in two of the non-sister chromatids, i.e., in one chromatid of each chromosome. Since the chromosomes are homologous, breaks at corresponding points mean that the segments that are broken off contain corresponding genes, i.e., alleles. The broken sections are then exchanged between the chromosomes to form complete new units, and each new recombined chromosome of the pair can go to a different daughter sex cell. Crossing over results in recombination of genes found on the same chromosome, called linked genes, that would otherwise always be transmitted together. Because the frequency of crossing over between any two linked genes is proportional to the chromosomal distance between them, crossing over frequencies are used to construct genetic, or linkage, maps of genes on chromosomes. Mutations, temperature changes, and radiation all affect crossing over frequency. Under the microscope, a crossover has the appearance of an X and is called a chiasma.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

over-the-counter market

Trading in stocks and bonds that does not take place on stock exchanges. Such trading occurs most often in the U.S., where requirements for listing stocks on the exchanges are strict. Schedules of fees for buying and selling securities are not fixed in the over-the-counter market, and dealers derive their profits from the markup of their selling price over the price they paid. Many bond issues and preferred-stock issues, including U.S. government bonds, are listed on the New York Stock Exchange but have their chief market over-the-counter. Other U.S. government securities, as well as state and municipal bonds, are traded over-the-counter exclusively. Institutional investors such as mutual funds often trade over-the-counter because they are given volume discounts not offered on the exchanges. The regulation of the over-the-counter market is carried out largely by the National Association of Securities Dealers, created by Congress in 1939 to establish rules of conduct and protect members and investors from abuses. Seealso NASDAQ.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.