SEWAGE DISPOSAL

Perhaps no factor is more useful in the control of disease than the science of sewage disposal. It safeguards a community's water supply by removing water-carried wastes including microscopic dissolved material, solid matter such as human waste, and harmful chemicals and bacteria.

Sewage is generally divided into two classes: domestic, or sanitary, sewage and industrial waste. Domestic waste water includes the used water of businesses and homes; industrial waste water is that discharged during industrial operations. Both the strength and volume of waste water may be markedly influenced by industrial wastes, which constitute about 80 percent of the sewage in the United States.

Sewage Treatment

Sewage systems collect waste water and treat it before discharging it back into the environment. These systems consist of intricate networks of underground conduits, or sewers, that convey the sewage through the treatment process to the point of disposal.

Sewage systems also handle the flow of rainwater, either separately or as part of a single system. Separate systems are generally preferable because, in single systems, heavy rainfall can overload treatment plants, with the result that untreated overflow can become a source of pollution. In separate systems, rainwater is often allowed to flow into streams untreated because it is assumed to be relatively clean.

Sewage is processed in three major steps, called primary, secondary, and tertiary treatments. Most areas do not use all three, and different areas use the treatments in different ways.

Primary treatment. The initial, and sometimes the only, method of cleaning waste water is primary treatment, which consists of removing floating chunks and fine particles of solid waste. The simplest form of primary treatment is a cesspool, now found primarily in rural areas. A cesspool is a big tank with a porous bottom and sides that lets the liquid waste water filter into the ground while holding the solid waste. Periodically the tank must be cleaned; the solid matter, called sludge, is sometimes used for fertilizer or landfill. Septic systems are somewhat similar, though the tank is connected to a drainage field so that more waste can be dispersed over a wider area.

In larger communities, sewer water passes first through a screen, which filters out the larger debris. It then runs through a grit chamber, a long, shallow trough with a dip in the bottom that acts like a trap. As water moves through the trough, small, hard materials in the water drift down to the bottom and fall into the trap. Grease floats to the surface and is skimmed off. The trap, like a cesspool, is periodically scraped clean.

After going through the screen and grit chamber, the sewage still contains small suspended solids about 1 ton per million gallons (3,790,000 litres) of waste water To remove some of these, the sewage is trickled into a sedimentation tank, or settling basin. The water enters through a pipe, then circulates slowly while the suspended particles settle to the floor. The top layer of water continually runs out through exit holes.

The sludge from sedimentation tanks may be sent through a tank called a digester, where bacteria digest it, producing carbon dioxide and methane gas and other by-products. Any combustible gases may then be collected and used to heat the digestion tanks and buildings and to fuel gas engines in the plant. The sludge may also be buried or dumped as landfill, burned, or dried in sludge drying beds for use as fertilizer.

Primary treatment removes about half of the suspended solids and bacteria in sewage, and about 30 percent of the organic wastes. Sometimes chlorine gas is added to the effluent (the liquid remaining after sedimentation) to kill most of the remaining bacteria. Some cities use chemicals that coagulate some of the solids into particles of a size and weight that will settle, so that they can be separated in a settling tank. The use of chemicals makes it possible to remove 80 to 90 percent of the suspended solids.

Secondary treatment. Today, large cities are usually required to put their wastes through both primary and secondary treatment because primary treatment alone removes so little organic material. Secondary treatment uses aerobic, or oxygen-breathing, bacteria to decompose organic wastes. The main object is to put the waste water in contact with as many bacteria as possible while keeping it aerated so that the bacteria have an adequate supply of dissolved oxygen.

One of the most common secondary treatments of this type is the activated-sludge method, so called because it uses sludge that is activated, or teeming with micro-organisms After going through primary treatment, the sewage is put into the activated-sludge tank, where it is aerated by pumps or blasts of compressed air. The compounds produced by the bacteria remain mostly suspended in the water and flow out with it into a secondary sedimentation tank.

The sludge from the bottom of the tank is handled in much the same way as the sludge from the primary sedimentation tank, except that about a quarter of it is recirculated back into the activated-sludge tank. This recirculation serves to seed the activated-sludge tank with fresh bacteria. The activated-sludge method permits almost any desired degree of treatment by varying the period of aeration. It removes about 95 percent of bacteria and more than 90 percent of suspended solids and organic matter.

Another method of secondary treatment is the trickling-filter method. Generally, rotating arms slowly spray the sewage over a shallow circular tank containing a layer of gravel or crushed rock. The rocks are covered with a slimy coating of micro-organisms that break down the organic wastes in the sewage. After this process, as in the activated-sludge method, the water that has been filtered is passed into a secondary sedimentation tank for removal of organic matter that has sloughed off from the stones of the filter. Trickling filters, together with primary treatment and final sedimentation, will remove most suspended solids.

Tertiary treatment. Waste water that has received primary and secondary treatment still contains dissolved materials that make it unsuitable for almost all uses except irrigation.

Tertiary treatments, which depend largely on artificial chemical processes, are designed to remove these materials in order to make the effluent safer to discharge into waterways and safer for industry to use. A number of methods may be used, including radiation treatment, discharging the effluent into lagoons, and chlorination.

Sewage may also be passed through filters made of activated carbon, which consists of finely ground charcoal grains with rough, pitted surfaces that trap impurities. Alternatively, sewage may be strained through a screen made of tiny seashells called diatomaceous earth. The effluent may also be treated with chemicals that transform the dissolved organic material. Some chemical compounds, for example, combine with the nitrates in sewage to produce various salts. Such treatments are expensive, however, and are difficult to perform routinely.

History

The use of specially constructed sewers dates to the time of Babylon and ancient Greece, but only during the 19th and 20th centuries was the water-carriage sewage system adopted in the Western world. In these early systems, streams often served the dual purpose of sewage disposal and water supply, and hence there were frequent, disastrous epidemics of cholera, typhoid fever, and other water-borne diseases. The most effective methods of sewage treatment were not developed until the second quarter of the 20th century. Today, because of the greater amount of sewage from growing populations and industrial activity, there is an unprecedented quantity of legislation designed to control water pollution. As a result, scientists and engineers continually search for methods to further increase the levels of sewage treatment.

GENETICS (Part 1 of 3)

DEFINITION: 1 the branch of biology that deals with heredity and variation in similar or related animals and plants 2 the genetic features or constitution of an individual, group, or kind.

April 28, 1994: Biological clock found in mice. Evidence for a so-called biological clock in mice was announced by scientists at Northwestern University in Illinois. It was the first time that a gene governing the daily cycle of waking and sleeping, called the circadian rhythm, had been found in mammals. Previously, genes governing circadian rhythms had been found only in fruit flies and bread mould The biological clock gene in mice was found on mouse chromosome number 5. The chromosomes of all living things hold the DNA, which determines the genetic make-up of each individual. Scientists hoped that this research would someday help them find a similar gene governing the biological clock in humans.

Why do human children resemble their parents? Why do the offspring of any species resemble their parents? Biologists have shown that the factors which cause such resemblances are passed on relatively unaltered from generation to generation by a process called heredity. Resemblances, they say, are transmitted by genes, cell units too tiny to be seen even with a microscope. The branch of biology that deals with genes is called genetics.

Through the ages men have speculated about heredity. In ancient Greece, for example, it was thought that the blood was in some way responsible for the transmission of hereditary traits, and the word "blood" is still often used to mean ancestry. Since the beginning of the 20th century, however, genes have been known to be the carriers of traits, though until the 1940s very little was known about them. Scientists recognized that genes were directly responsible for the characteristics of an organism and that genes were transmitted from parents to offspring. However, they had little idea of the gene structure and composition that made these actions possible.

By the 1950s scientists had learned a great deal about the chemistry of genes. Genes were found to be segments of certain complex molecules located in the cell nucleus. The molecules have the unique ability to duplicate themselves and, in so doing, to pass on body-building instructions to the next generation of a species.

THE ORIGINS OF MODERN GENETICS

Even before the beginnings of written history people were aware of some of the ways in which heredity takes place. The domesticated animals and plants of today are proof of this. Today's domesticated horses, cattle, dogs, corn, wheat, and cotton differ greatly from their primitive, "wild" ancestors. They are products of the ancient breeders' art, an art that included the proper selection of parents, well-controlled matings, and the careful choice of the best offspring to further improve a breed.

Early Theories of Heredity

Over the centuries more and more became known about the control of heredity for practical purposes. However, scientists remained baffled about the actual processes of trait transmission. All sorts of what proved to be erroneous explanations were advanced.

In the 17th century, for example, a group of biologists called the ovists held that the ovaries of females contained the hereditary material and that the male sperm merely triggered embryonic development. Other scientists were of the opinion that tiny but fully formed creatures were present in the sperm.

Early in the 19th century the French biologist Jean Baptiste Lamarck suggested that traits and abilities acquired during the lifetime of an organism could be transmitted to future generations. This theory was termed "the inheritance of acquired characteristics." Long before Lamarck, notions of this kind had led expectant mothers to practice the piano, gaze at beautiful pictures, or think "kind" thoughts in the hope that this would affect the character of their unborn children. For similar reasons, many breeders exposed plants and animals to the environmental conditions their breeding programs were intended to combat.

Genetic discoveries in the mid-1800s proved Lamarck's view to be mistaken.

1859: Darwin's theory of evolution. A heated debate that continues to this day was sparked in 1859 with the publication of Charles Darwin's 'On the Origin of Species by Means of Natural Selection'. This work was immediately recognized by the scientific community as a landmark treatise on biology and evolution, but some Christians saw it as a threat to their theology.

Charles Darwin began his observations in December 1831 when, at age 22, he left England for South America aboard the exploratory ship HMS Beagle. During this five-year voyage Darwin observed many species of animals and birds and collected many fossils. His observations on the differences and similarities of species, both living and extinct, led him to ask many questions: Why did some species survive and others die out? Why did certain species live in certain places and not in others? These questions preoccupied him when he returned to England in 1836.

Darwin's observations led him to doubt the commonly held belief that all the species had been created at once and had remained unchanged through time. The problem was to find out what forces made organisms change. Darwin's answer was his theory of natural selection: certain members of a species have traits that make them better adapted to their environment. These animals are more successful and therefore have more surviving offspring that inherit these traits. Animals that are not well adapted do not have as many offspring and eventually die out. In this way, species change and certain groups become extinct.

Although Darwin devoted much of his time to his theory of natural selection, he did not publish it for more than 20 years. He knew that his explanation of the species would anger many people, since it did not agree with the dominant Christian theology of the time. Despite early scientific and religious opposition, Darwin's theory of natural selection is now accepted as the explanation of evolution, at least within the scientific community. However, arguments continue between evolutionists and creationists (those who believe all species were created by God in their present form). Darwin's theories have indeed changed the way most people view the world, from the evolution of humans to the philosophical bases of science itself.

1865: The birth of genetics. It was unfortunate for the biological sciences that Gregor Mendel was an obscure Austrian monk. His pioneering work in the field of genetics was being done at the time that Charles Darwin's publications on evolution were beginning to create worldwide controversy, but Mendel's work would remain unknown for years.

Mendel became an Augustinian monk in 1843, but his abilities in mathematics and the sciences were evident. His experiments on the principles of heredity were begun in about 1856 in what is now Czechoslovakia. By crossing various strains of peas with one another, Mendel found that traits were passed on from generation to generation in what he called "discrete hereditary elements" in sex cells, or gametes.

Mendel reported the results of his experiments to a local society for the study of natural science in 1865 and published his findings in the society's journal. They were as good as buried there for the next 35 years. Although the journal found its way to libraries in Europe and North America, few paid any attention to his writings. When other botanists obtained results similar to Mendel's, they began searching through earlier writings on the subject. Only then was Mendel's 1865 research revealed.

His "discrete hereditary elements" are now called genes, and the new science once called Mendelism is known as genetics.

Two Pioneers of Genetics

In 1859 the English biologist Charles Darwin published his epic 'The Origin of Species', an attempt to demonstrate that all living things are related through the common bond of evolution. Darwin assumed that all species produce more offspring than reach maturity.

Those offspring that survive and reproduce, he reasoned, do so because they are better suited to the existing environment. Because environment changes with time, he argued, species must either adapt to the new conditions or become extinct. Darwin did not know just what mechanisms made it possible for such changes in species to take place. He recognized, however, that if his theory were correct, changeable or mutable units of heredity must exist and that variations in species must arise as a result of an accumulation of small changes in these units of heredity.

In 1865 Gregor Mendel, a monk in an Austrian Roman Catholic monastery, wrote a paper that laid the foundation for modern genetics. Mendel was the first to demonstrate experimentally the manner in which specific traits are passed on from one generation to the next. He concluded that "discrete hereditary elements" (not called genes until the 1900s) in the sex cells are responsible for the transmission of traits. Mendel was ahead of his time, however. The significance of his work was not realized until 1900.

Mendel's Contributions to Genetics

Pea, a climbing pod-bearing plant (Pisum sativum), or its seed.

In the monastery garden where he conducted his experiments, Mendel observed the inheritance of traits in the easily available garden pea, Pisum sativum. The plant is an ideal genetic working material because a number of progeny can be produced in a short time and because its reproductive parts are so constructed that accidental fertilization is nearly impossible.

Mendel began by tracing the inheritance of one or two contrasting traits at a time. Thus, he crossed tall peas with short peas or red-flowered peas with white-flowered peas. Then he recorded how many of the progeny developed each of the contrasting traits. He used the progeny in subsequent matings to follow the progress of the traits under study through a number of generations.

Somatic cells (or body cells), cells of the body that compose the tissues, organs, and parts of that individual other than the germ cells.

Gamete (or germ cell), sex cell that fuses with a cell of the opposite sex to form new life.

From the evidence obtained in this way, Mendel reasoned that contrasting traits are governed by units of inheritance existing in pairs in somatic, or body, cells but singly in gametes, or sex cells. If the genotype R stands for red and the genotype r for white, then homozygous red-flowered peas have RR somatic cells and R gametes. The somatic cells and gametes of homozygous white-flowered peas are, by contrast, rr and r, respectively.

Allele, in genetics; an alternate form of gene located on a specific site on a chromosome.

The separation of alleles (R from r, for example) in gamete formation is called the principle of segregation. Mendel correctly assumed that chance determines which gene of a pair finds its way into a given gamete. A red-flowered pea may be a heterozygous, or hybrid, Rr. That is, in some way the allele for red flowers (R) "dominates" the allele for white flowers (r). However, the R and r alleles of the hybrid segregate during sex-cell division to produce an equal number of R and r gametes. This is proved by test crossing the hybrid with a homozygous white (rr) plant. Since the homozygous white produces only r gametes and the hybrid produces both R and r gametes, the ratio of red plants to white plants is one to one.

Mendel also demonstrated that non allelic genes (for tall or short and red or white phenotypes, for example) segregate independently of one another into the gametes. This phenomenon is called the principle of independent assortment. For example, a cross between pure strains of tall plants with red flowers (TTRR) and short plants with white flowers (ttrr) produces hybrid progeny that are all tall with red flowers (TtRr). A test cross between these tall, red hybrids (TtRr) and short, white pure strains (ttrr) results in four equally distributed types of progeny 25 percent tall, red TtRr, 25 percent short, red ttRr, 25 percent tall, white Ttrr, and 25 percent short, white ttrr. Modern geneticists have learned, however, that independent assortment does not always hold true because non alleles located side by side on the same chromosome tend to be inherited as a package.

1953: Discovery of DNA structure. The full name of DNA is deoxyribonucleic acid. It carries the codes of genetic information that transmit inherited characteristics to successive generations of living things.

DNA was discovered in 1869 by Friedrich Miescher. In 1943 its role in inheritance was demonstrated. In 1953 its structure was determined by an American biochemist, James D. Watson, and an English physicist, Francis H.C. Crick. Watson and Crick showed the structure to be two strands of a phosphoryl-deoxyribose polymer arranged as a double helix. Watson and Crick were awarded the Nobel prize in physiology or medicine in 1962.

1973: Biotechnology. Two American biochemists, Stanley H. Cohen and Herbert W. Boyer, inaugurated the science of genetic engineering and its associated field of biotechnology in 1973. They showed that it was possible to break down DNA into fragments and combine them into new genes, which could in turn be placed in living cells. There they would reproduce each time a cell divided into two parts.

Genetic engineering makes it possible to modify existing organisms or create organisms that already exist in the human body but that are difficult to isolate. For example, one early product was a genetically engineered form of insulin, used in the treatment of diabetes. Other genetically engineered products include interferons, which are used in the treatment of viral infections and showed promise in the treatment of various forms of cancer. Scientists hope that genetically engineered products will someday prevent or cure such genetic disorders as muscular dystrophy and cystic fibrosis.

Genetic engineering also opens the possibility of creating entirely new organisms. In 1980 the United States Supreme Court ruled that newly developed organisms could be patented, thus giving ownership rights to the companies that made them.

GENETICS (Part 2 of 3)

Genetic Research After Mendel

Chromosome, microscopic, threadlike part of the cell that carries hereditary information in the form of genes; among simple organisms, such as bacteria and algae, chromosomes consist entirely of DNA and are not enclosed within a membrane; among all other organisms chromosomes are contained in a membrane-bound cell nucleus and consist of both DNA and RNA; arrangement of components in the DNA molecules determines the genetic information; every species has a characteristic number of chromosomes, called the chromosome number; in species that reproduce asexually the chromosome number is the same in all the cells of the organism; among sexually reproducing organisms, each cell except the sex cell contains a pair of each chromosome.

Weismann, August (1834-1914), German biologist; advanced theory that changes in the characteristics of a species are due to changes in germ plasm.

Sutton, Walter S. (1876-1916), U.S. geneticist and physician; noted for studies of chromosomes.

Boveri, Theodor Heinrich (1862-1915), German scientist whose work with roundworm eggs proved that chromosomes are separate, continuous entities within the nucleus of a cell.

Chromosomes, structures in the cell nucleus that carry genes, were discovered after Mendel's work was published. However, accurate accounts of their behaviour were not generally available until about 1885. Earlier the German biologist August Weismann had suggested that heredity depends on a special material called germ plasma that is transmitted unaltered from one generation to another. In the 1880s Weismann and other scientists advanced the idea that the germ plasm was located in the chromosomes. In 1902 Walter S. Sutton of the United States and Theodor Boveri of Germany independently recognized the connection between the segregation of alleles as described by Mendel and the segregation of homologous pairs of chromosomes in the division of sex cells.

Morgan, Thomas Hunt (1866-1945), U.S. zoologist, born in Lexington, Ky.; professor Columbia University 1904-28; director of biological laboratories, California Institute of Technology; received 1933 Nobel prize for work on role of chromosomes in heredity; wrote books on embryology, evolution, and heredity.

In 1910 the American geneticist Thomas H. Morgan and his associates discovered that genes occur on chromosomes and that those genes lying close together on the same chromosome form linkage groups that tend to be inherited together. They also showed that linkage groups often break apart naturally as a result of a phenomenon called crossing over.

Beadle, George Wells (1903-89), U.S. biologist, born near Wahoo, Neb.; professor and chairman of biology division California Institute of Technology 1946-60, acting dean of faculty 1960-61; president University of Chicago 1961-68; director Institute of Biomedical Research, AMA, 1968-70; received 1958 Nobel prize for work in biochemical and microbial genetics.

Tatum, Edward Lawrie (1909-75), U.S. biochemist, born in Boulder, Colo.; professor Yale University 1946-48, Stanford University 1948-57, and Rockefeller University 1957-75; received 1958 Nobel prize for discovery that genes act by controlling specific chemical processes.

Avery, Oswald Theodore (1877-1955), U.S. bacteriologist who determined that deoxyribonucleic acid (DNA) is the basic genetic material of the cell.

Watson, James Dewey (born 1928), U.S. biochemist, born in Chicago, Ill.; on staff Harvard University 1955-68, professor 1961-68; director Cold Spring Harbor Laboratory from 1968; received 1962 Nobel prize for discovery of molecular structure of DNA.

Crick, Francis Harry Compton (born 1916), British biochemist; on staff Cavendish Laboratory, Cambridge University 1949-77; professor Salk Institute for Biological Studies from 1977; received 1962 Nobel prize for discovery of molecular structure of DNA; elected to U.S. National Academy of Sciences 1969.

Jacob, Francois (born 1920), French biologist, born in Nancy; with Pasteur Institute from 1950, College de France from 1964; received 1965 Nobel prize for work in genetics.

Monod, Jacques (1910-76), French biologist, born in Paris; with Pasteur Institute from 1945, director from 1971; received 1965 Nobel prize for work in genetics; researched protein metabolism and RNA.

In the 1940s George W. Beadle and Edward L. Tatum of the United States began to investigate the role played by genes in the production of enzymes. By 1944 Oswald T. Avery had discovered that deoxyribonucleic acid (DNA) was the basic genetic material of the cell. The precise molecular structure of DNA was determined in 1953 by James D. Watson of the United States and Francis H.C. Crick of England. By 1961 the French geneticists Francois Jacob and Jacques Monod had developed a model for the process by which DNA directs the synthesis of proteins, thereby deciphering, in principle, the genetic code of the DNA molecule. In 1988 an international team of scientists began a project to devise a map of the human genome, all the genes that determine the make-up of a human being.

Recombinant DNA, genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments; usually uses DNA from more than one species of organism.

Clone, process of biologically purifying a gene from one species by inserting it into the DNA of another species where it is replicated along with the host DNA; used to manufacture insulin.

Since the 1970s the techniques of recombinant DNA have allowed researchers to biologically purify, or clone, a gene from one species by inserting it into the DNA of another species, where it is replicated along with the host DNA. In this manner human hormones, such as insulin and growth hormone, have been manufactured economically by colonies of bacteria.

CHROMOSOMES AND CELL DIVISION

Chromosomes are mainly aggregates of deoxyribonucleic acid (DNA) and protein. All but the simplest kinds of plants and animals inherit two sets of chromosomes (the diploid number), one set (the haploid number) from each parent. In humans, each somatic cell has a haploid set of 23 chromosomes from each parent, for a total of 46.

The chromosomes within each set vary in appearance. However, each has a homologous partner in the other set, which resembles it in both appearance and genetic characteristics.

A given gene is found on only a particular chromosome in each set. Its allele is on that chromosome's homologue in the other set. The alleles are passed on to new cells during mitosis, the division of somatic cells.

Mitosis takes place as soon as a sperm fertilizes an egg. It continues throughout the life of the organism. Prior to mitosis, the cell chromosomes make exact copies of themselves. At this point, twice the diploid number of chromosomes exist in the cell. As mitosis proceeds, one set of the doubled chromosomes goes into each of the two daughter cells. Each thus acquires a full diploid set of chromosomes. This process is repeated again and again as cells divide and the body grows. Sex cells, however, divide in a different way.

Sex cells in the adult reproductive organs produce gametes by meiosis. This process consists of two divisions. As the first division proceeds, the homologous chromosomes in the nucleus of the sex cell seek each other out and join, or synapse. They are called bivalents at this point.

Then the bivalents duplicate themselves to form a bundle, or tetrad, or four intertwined chromatids. The tetrads then thicken and separate, and a pair of homologous chromatids pass into each of two daughter cells.

Meiosis does not stop at this stage, however. The two daughter cells, still with a diploid number of chromosomes, undergo a second division, the reduction division. In this division, the homologous chromatids do not duplicate themselves but merely separate and pass randomly into two additional cells, where they thicken into chromosomes. In meiosis, each sex cell produces four gametes, each with a haploid number of chromosomes (only one allele is in each gamete). When a male gamete fertilizes an egg, the diploid number of chromosomes is restored.

Chromosomes are fully visible under a microscope during the four stages of cell division prophase, metaphase, anaphase, and telophase. However, between the telophase and the next prophase a lengthy period called the interphase occurs, during which the chromosomes are too thin and strung out to be seen. Important chemical activities take place during the interphase. Ribonucleic acid (RNA), chemically related to DNA, and proteins are synthesized during the lengthy interphase as well as during the relatively short period of cell division.

Late in the interphase, DNA is synthesized and daughter chromosomes are created. First, DNA is made. Soon afterwards, in a burst of activity, chromosomal DNA, RNA, and protein are fitted together, the chromosomes begin to take shape, and cell division begins.

During sex cell division, however, an important gene exchange between homologous chromosomes takes place.

Linked Non alleles and Crossing Over

As meiosis takes place, homologous chromosomes exchange some of their genes. This phenomenon is known as crossing over. Although the process is not well understood, it is thought that a reciprocal breakage and rejoining of homologous chromatids occurs while the tetrads are intertwined during early meiosis.

Geneticists began to investigate crossing over when they noted that the traits actually inherited did not always adhere to the principle of independent assortment. Test crosses between AaBb and aabb parents A, a, B, and b representing the dominant and recessive genes of non alleles did not always produce equal numbers of AaBb, aaBb, Aabb, and aabb progeny but a greater number of the parental types AaBb and aabb and a smaller number of the recombinant types Aabb and aaBb. Geneticists concluded that the dominant non alleles A and B were linked together on one homologous chromosome and that the recessive non alleles a and b were linked together on the other. If this linkage were unbreakable, in meiosis the hybrid AaBb would form only AB and ab gametes. In fact, however, Ab and aB gametes were also formed the frequency varying for different linked non alleles It was therefore surmised that an exchange, or crossing over, took place.

GENETICS (Part 3 of 3)

Sex Linkage

Linked genes occur on the sex chromosomes as well as on the non sex chromosomes, or autosomes. In humans, a woman carries two X chromosomes and 44 autosomes in each body cell and one X chromosome and 22 autosomes in each egg. A man carries one X and one Y chromosome and 44 autosomes in each body cell and either an X or a Y chromosome and 22 autosomes in each sperm cell.

Only sons inherit traits carried by genes located on the Y chromosome, because a boy (XY) develops whenever a Y sperm fertilizes an egg. Traits carried on genes located on an X chromosome of the father are transmitted only to daughters (XX).

GENES AND THE GENETIC CODE

Genes, the arbiters of body form and organ function, work with precision. They transmit to each cell a genetic code that determines the cell's purpose.

Nucleic Acids The Key to Heredity

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

The structure of DNA makes gene transmission possible. Since genes are segments of DNA, DNA must be able to make exact copies of itself to enable the next generation of cells to receive the same genes.

Adenine, a purine base that codes hereditary information in the genetic code in DNA and RNA.

Cytosine, pyrimidine base that codes genetic information in DNA or RNA.

The DNA molecule looks like a twisted ladder. Each "side" is a chain of alternating phosphate and deoxyribose sugar molecules. The "steps" are formed by bonded pairs of purine-pyrimidine bases. DNA contains four such bases the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) and thymine (T).

The RNA molecule, markedly similar to DNA, usually consists of a single chain. The RNA chain contains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymine of DNA.

DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of a phosphate, a deoxyribose sugar, and either A, T, G, or C. RNA nucleotides consist of a phosphate, a ribose sugar, and either A, U, G, or C.

Nucleotide chains in DNA wind around one another to form a complete twist, or gyre, every ten nucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A to T and C to G A always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired bases are the foundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read: A-T C-G G-C T-A G-C C-G A-T. When "unzipped," the left strand would read: ACGTGCA; the right strand: TGCACGT.

DNA is the "master molecule" of the cell. It directs the synthesis of RNA. When RNA is being transcribed, or copied, from an unzipped segment of DNA, RNA nucleotides temporarily pair their bases with those of the DNA strand. In the preceding example, the left hand portion of DNA would transcribe a strand of RNA with the base sequence: UGCACGU.

Genes and Protein Synthesis

A genetic code guides the assembly of proteins. The code ensures that each protein is built from the proper sequence of amino acids.

Genes transmit their protein-building instructions by transcribing a special type of RNA called messenger RNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm called ribosomes, where protein synthesis takes place.

Cell biologists believe that DNA also builds a type of RNA called transfer RNA (tRNA), which floats freely through the cell cytoplasm. Each tRNA molecule links with a specific amino acid. When needed for protein synthesis, the amino acids are borne by tRNA to a ribosome.

For years biologists wondered how amino acids were guided to fit together in the exact sequences needed to produce the thousands of kinds of proteins required to sustain life.

The answer seems to lie in the way the four genetic "code letters" A, T, C, and G are arranged along the DNA molecule.

The Genetic Code

Experimental evidence indicates that the genetic code is a "triplet" code; that is, each series of three nucleotides along the DNA molecule orders where a particular amino acid should be placed in a growing protein molecule. Three-nucleotide units on an mRNA strand for example UUU, UUG, and GUU are called codons. The codons, transcribed from DNA, are strung out in a sequence to form mRNA.

According to the triplet theory, tRNA contains anti codons, nucleotide triplets that pair their bases with mRNA codons. Thus, AAA is the anti codon for UUU. When a codon specifies a particular amino acid during protein synthesis, the tRNA molecule with the anti codon delivers the needed amino acid to the bonding site on the ribosome.

The genetic code consists of 64 codons. However, since these codons order only some 20 amino acids, most, if not all, of the amino acids can be ordered by more than one of them. For example, the mRNA codons UGU and UGC both order cysteine. Because mRNA is a reverse copy of DNA the genetic code for cysteine is ACA or ACG. Some codons may act only to signal a halt to protein synthesis.

To illustrate the operation of the genetic code, assume that one protein is responsible for the development of brown hair and that this protein is composed of three amino acid molecules arranged in linear sequence for example, cysteine-cysteine-cysteine. (This is a much simplified example, since proteins actually incorporate from 100 to 300 amino acid molecules.) The gene (DNA segment) specifying formation of this protein reads: ACAACAACA. It produces the mRNA segment UGUUGUUGU. This segment then drifts to a ribosome. Three tRNA molecules, each with the cysteine-bearing anti codon ACA, line up in order on the ribosome and deposit their cysteine to make the brown-hair protein.

Since code transmission from DNA to mRNA is extremely precise, any error in the code affects protein synthesis. If the error is serious enough, it eventually affects some body trait or feature.

Mutations

Down's syndrome (or mongolism), a congenital condition with moderate to severe mental retardation; characteristic features include: broad flat faces, slanted eyes, small ears and noses; heart defects and other abnormalities.

Certain chemicals and types of radiation can cause mutations changes in the structure of genes or chromosomes. The simplest type of mutation is a change in the DNA or RNA nucleotide sequence. Mutations may also involve the number of chromosomes or the gain, loss, or rearrangement of chromosome segments. If a mutation occurs in parental sex cells, the change is passed on to the offspring. In humans, an extra chromosome in body cells (47 instead of 46) has been implicated in Down's syndrome, a serious mental abnormality.

Most mutations are considered harmful and are, therefore, eventually eliminated. Some, however, enable an organism to adapt to a changing environment. Biologists believe that mutations have caused the many genetic changes involved in the evolution of species.

Assisted by Val W. Woodward

Genetic Terms

allele. One of the members of a gene pair, each of which is found on chromosomes; the pair of alleles determines a specific trait.

chromosome. A structure in the cell nucleus containing genes.

dominance. The expression of one member of an allelic pair at the expense of the other in the phenotypes of heterozygotes.

gene. One of the chromosomal units that transmit specific hereditary traits; a segment of the self-reproducing molecule, deoxyribonucleic acid.

genotype. The genetic make-up of an organism, which may include genes for the traits that do not show up in the phenotype.

heterozygous. Containing dissimilar alleles.

homozygous. Containing a pair of identical alleles.

phenotype. The visible characteristics of an organism (for example, height and colouration).

recessiveness. The masking of one member of an allelic pair by the other in the phenotypes of heterozygotes.

PASSOVER

 DEFINITION: a Jewish holiday (Pesach ) celebrated for eight (or seven) days beginning on the 14th of Nisan and commemorating the deliverance of the ancient Hebrews from slavery in Egypt.

One of the major festivals in Judaism is Passover. It is a holiday of rejoicing when Jews all over the world recall their deliverance from slavery in Egypt. The word Passover comes from the idea that God passed over the houses of the Israelites, who had marked their door posts to signify that they were children of God. This way the first born sons of the Jews were spared when God smote the first born sons of the Egyptian taskmasters on the eve of the Exodus.

Passover is celebrated each spring for eight days beginning on Nisan 15 of the Hebrew calendar. Families gather at the beginning of Passover for the Seder meal. The meal is preceded by prayers and songs from the Haggadah, the narration of the events surrounding the Exodus from Egypt. All of the foods eaten are symbolic. These include bitter herbs, reminiscent of the pain of bondage; a roasted lamb bone to recall offerings that the Israelites made to God; unleavened bread called matzo, which is eaten all week instead of leavened bread because the Israelites lacked time even for dough to rise in their haste to escape from Egypt; and a tasty mixture of nuts, apples, honey, and wine to symbolize the mortar the Jewish slaves were forced to use to build Egyptian temples.

During the Seder it is traditional for the youngest child to ask four questions about the uniqueness of Passover, which the leader answers. Children are encouraged to participate and to think of their history as if they themselves had been delivered from slavery. They are also taught in the Haggadah that, because the Israelites were strangers in Egypt, Jews must remember to welcome strangers in their midst.

DRAGONFLIES

Definition: any of an order (Odonata) or suborder (Anisoptera) of large insects, harmless to people, having narrow, transparent, net-veined wings and feeding mostly on flies, mosquitoes, etc. while in flight.

Among the most beautiful and useful of all insects is the dragonfly. It has thin silvery wings. Its body may be steel blue, purple, green, or copper. The dragonfly eats mosquitoes, flies, and other insects harmful to man.

The dragonfly lives on or near the water. It is a quick-darting insect that flies swiftly from place to place. Sometimes it changes its direction so quickly in mid-flight that its sudden movement is hard to follow with the eye. It can also hover over a lake or stream as it looks for food.

The dragonfly's wings are from 2 to 5 inches long. The body is about 3 inches long. Its six legs are far forward and close together. The dragonfly can curve its legs to form a basket. It uses this basket to scoop insects from the air. Then it puts them into its jaws. These jaws have strong teeth.

Two great eyes cover most of the head. Each eye has from 20,000 to 25,000 tiny eyes joined together. With these big eyes it can see its prey easily.

Dragonflies and Damsel Flies

There are two large groups of these insects the dragonflies themselves and the damsel flies. The dragonfly darts with the speed of an express train. Some can fly 60 miles an hour. The two rear wings are larger than the front pair. They are held outspread when the insect lands. The damsel fly is more slender. It flies more slowly and lazily. The wings are the same size. When the insect rests, the wings come together over the back, like a butterfly's.

The dragonfly begins life as a water insect. Then it is called a nymph. The female lays her eggs in the water. As she flits over the pond again and again she dips under water to wash off the eggs. Some species lay their eggs in long strings on water plants. Such a string may have 100,000 eggs.

The damsel fly cuts a slit in the stems of water plants. Then she puts her eggs in the opening. She uses a part of her body called an ovipositor to do this. (The word ovipositor means "egg depositor.") Sometimes the damsel fly goes under water and walks about looking for a good cradle. The male may go with the female on this trip.

Life Cycle of the Nymphs

The nymphs hatch out of the eggs in one to four weeks. They are half an inch to nearly 2 inches long. They are flat, dark creatures and have long legs. They hide under rocks and in the mud. There they wait to jump on a careless insect or even a small fish. These nymphs are as fierce in the water as the grown flies are in the air. They have a strange way of catching their prey. The underlip has joints. It is very long, with a pair of hooks at the tip. They shoot this lip forward and catch the insect on the hooks. When they are not using the underlip they fold it over the face like a mask.

The dragonfly nymph breathes by drawing water into the back part of its intestine. Tiny air tubes take out the oxygen. The nymph then pushes out the water in quick spurts. In this way it drives itself forward, like a jet-propelled air plane The damsel-fly nymph has three leaf like gills at the end of its body. These gills take in oxygen from the water.

As the nymphs grow they shed their skins ten to 15 times. Depending on the species, dragonflies live from one to four years as nymphs. The length of their lives depends upon the species. In winter they sleep in the stream bed. The following spring the life cycle begins once again. When they reach the adult stage, dragonflies live only a few months.

Dragonflies are members of the order Odonata. The dragonflies proper belong to the suborder Anisoptera (from two Greek words meaning "unequal wings"). The damsel flies belong to the suborder Zygoptera ("yoke wings"). There are about 2,500 species scattered over the world. North America has about 300 species. One common dragonfly is known as the mosquito hawk (Anax junius). It is bright green, with clear wings about 2 inches long. The ten spot skimmer (Libellula pulchella) has three blackish-brown and two white spots on each wing. Only the male has the white spots.

Ruby spot, insect (Hetaerina americana) member of the Odonata family.

Black-wing, insect (Calopteryx maculata), member of the Odonata family.

The ruby-spot (Hetaerina americana) is a common damsel fly. Its head and upper body are coppery red. The abdomen is green. The male has a ruby-red spot at the base of the wings. In the female the wing is yellowish brown. The black-wing (Calopteryx maculata) is also common.

EASTER

DEFINITION: 1 an annual Christian festival celebrating the resurrection of Jesus, held on the first Sunday after the date of the first full moon that occurs on or after March 21 2 the Sunday of this festival.

The greatest festival of the Christian church commemorates the resurrection of Jesus Christ. It is a movable feast; that is, it is not always held on the same date. In AD 325 the church council of Nicaea decided that it should be celebrated on the first Sunday after the first full moon on or after the vernal equinox of March 21. Easter can come as early as March 22 or as late as April 25.

In many churches Easter is preceded by a season of prayer, abstinence, and fasting called Lent. This is observed in memory of the 40 days' fast of Christ in the desert. In Eastern Orthodox churches Lent is 50 days. In Western Christendom Lent is observed for six weeks and four days.

Good Friday, the Friday before Easter; observed in churches as the anniversary of the crucifixion of Christ; legal holiday in some states of the U.S.

Ash Wednesday, the first day of Lent, gets its name from the practice, mainly in the Roman Catholic church, of putting ashes on the foreheads of the faithful to remind them that "man is but dust." Palm Sunday, one week before Easter, celebrates the entry of Jesus into Jerusalem. Holy Week begins on this day. Holy Thursday, or Maundy Thursday, is in memory of the Last Supper of Christ with his disciples. Good Friday commemorates the crucifixion.

Many Easter customs come from the Old World. The white lily, the symbol of the resurrection, is the special Easter flower. Rabbits and coloured eggs have come from pagan antiquity as symbols of new life. Easter Monday egg rolling, a custom of European origin, has become a tradition on the lawn of the White House in Washington, D.C.

Lent may be preceded by a carnival season. The origin of the word carnival is probably from the Latin carne vale, meaning "flesh (meat), farewell." Elaborate pageants often close this season on Shrove Tuesday, the day before the beginning of Lent. This day is also called by its French name, Mardi Gras (Fat Tuesday).

The name Easter comes from Eostre, an ancient Anglo-Saxon goddess, originally of the dawn. In pagan times an annual spring festival was held in her honour Some Easter customs have come from this and other pre-Christian spring festivals. Others come from the Passover feast of the Jews, observed in memory of their deliverance from Egypt.

The word paschal comes from a Latin word that means "belonging to Passover or to Easter." Formerly, Easter and the Passover were closely associated. The resurrection of Jesus took place during the Passover. Christians of the Eastern church initially celebrated both holidays together. But the Passover can fall on any day of the week, and Christians of the Western church preferred to celebrate Easter on Sunday the day of the resurrection.