Deoxyribonucleic Acid (DNA), genetic material of all cellular organisms and most viruses. DNA carriesthe information needed to direct protein synthesis and replication. Protein synthesis is the production of the proteins needed by the cell or virus for its activities and development. Replicationis the process by which DNA copies itself for each descendant cell or virus, passing on the information needed for protein synthesis. In most cellular organisms, DNA is organized on chromosomeslocated in the nucleus of the cell.
Structure of DNA A molecule of DNA consists of two chains, strands composed of a large number of chemical compounds,called nucleotides, linked together to form a chain. These chains are arranged like a ladder that hasbeen twisted into the shape of a winding staircase, called a double helix. Each nucleotide consists ofthree units: a sugar molecule called deoxyribose, a phosphate group, and one of four different nitrogen-containing compounds called bases. The four bases are adenine (A), guanine (G), thymine (T),and cytosine (C). The deoxyribose molecule occupies the center position in the nucleotide, flanked bya phosphate group on one side and a base on the other. The phosphate group of each nucleotide is also linked to the deoxyribose of the adjacent nucleotide in the chain. These linked deoxyribose-phosphatesubunits form the parallel side rails of the ladder. The bases face inward toward each other, formingthe rungs of the ladder.

Alligator

Alligator common name for two carnivorous reptiles in the crocodilian order.Alligator is the Spanish el name,"the lizard”. Alligators and their close relatives the caimans have broad, flat, and rounded snouts, as opposed to the longer, sharper snouts of other crocodilians; also unlike other crocodilians, their lower teeth cannot be seen when their mouths are closed. Alligators feed on fish, frogs, snakes, turtles, birds, mammals, and carrion. In North America they are also known to attack humans occasionally. Alligators can survive a wider range of temperatures than other crocodilians, and they are found in more temperate regions. Their breeding season is generally restricted to spring.When alligators search for a mate they bellow often, perhaps to announce their presence to females and to warn other males to stay away. Unwanted intruders are confronted with ritual gaping, lunging, and hissing, but courtship behavior is sedate. After mating, the male seeks his own territory while the female builds a nest of mud and plants nearby, above flood level. The eggs, from 30 to 60 in a clutch, are covered with mud and vegetational debris. The female stands guard as the eggs incubate in the heat from this decaying vegetation and from the sun. When the eggs are ready to hatch, in about 60 days, the young begin to croak softly within the egg. The female may then assist the young in escaping from the nest and may even carry them in her mouth to the water's edge. The female may remain near her young for a year or more. A young alligator in distress will give a series of sharp croaks that may quickly bring the female to investigate.
Only two species of alligator exist: the Chinese alligator and the American alligator. The Chinese alligator makes its home in the Yangtze River Basin of China. It is more timid and much smaller than the American alligator, seldom exceeding 2.5 m (8 ft) in length, and is considered little threat to humans. The American alligator lives mainly in freshwater swamps, lakes, and bayous in the southeastern United States, but it ranges as far west as the Río Grande in Texas. It is larger, reaching up to about 6 m (about 20 ft) in length, and is potentially dangerous to humans. Attacks occur infrequently, usually in areas where humans have recently encroached on alligator habitat or where alligators have become accustomed to the presence of humans. Hunted for generations both for sport and for its hide, populations of the American alligator dwindled until, in 1967, it was declared an endangered species. Under this protection it made a strong comeback and, little more than a decade later, hunting of the American alligator was again allowed in some states.

Raccoon

Raccoon is carnivorous mammal, found throughout the United States, southern Canada, and Central and South America, whose head is broad, tapering to a pointed muzzle, and whose ears are short and erect. The body is generally short and plump, with a long-haired coat of fur, and the tail is bushy. The legs are short; each foot has five toes, and the soles are naked. The animal walks on the soles of its feet with the heels touching the ground, similar to the practice of bears and humans. Raccoons are grayish-brown above and light gray beneath, with black cheek patches that narrow into a vertical stripe extending from the space between the eyes to the top of the head. The rest of the face is pale gray, with dark eyes and white whiskers. The tail is marked with six or seven brownish-black rings. A raccoon ranges in length from 0.62 to 1 m (2 to 3.3 ft), including its tail, which is 20 to 40 cm (8 to 16 in) long.The common species found throughout the United States usually lives in trees or near ponds and streams of forests close to civilization. During the night it hunts for poultry, mice, birds' eggs, various insects, fish, and frogs. Occasionally it varies its diet with nuts and wild fruit. It is a skillful swimmer. Northern raccoons spend the winter in a den, usually high in a hollow tree, sleeping but not hibernating, and emerging during relatively warm periods. From four to six young are born in a single litter each spring, and members of the same family live and travel together for about one year.
Another species, distributed from Costa Rica to northern South America, is the agouara, or crab-eating raccoon. This animal is longer than the other species, although the tail is slightly shorter; the teeth are thicker and stronger and the fur shorter. The crab-eating raccoon is dark gray with patches of yellow.The fur of the raccoon, especially that of the northern species, has been highly valued in North America since the 17th century. Coonskin caps, and coats and robes made of the coarse but attractive fur, are still worn today. Coon hunting is practiced extensively in the southern United States. The animals are hunted at night with dogs and are usually found near swamps or streams.

Angelfish

Angelfish is name used for several different fishes, including the true angelfishes and the butterfly fishes. Angelfishes differ from butterfly fishes in having a spine near the lower edge of the gill cover. These laterally compressed fishes are among the most beautiful of the tropical reef fishes. Most species are only a few centimeters long, but some grow to a length of 61 cm (24 in). In many species the juveniles and adults are colored differently; the young French angelfish of tropical Atlantic waters is black with bright yellow bands; the adult is predominantly black. The well-known queen angelfish is also differently colored as a juvenile. Most angelfishes feed on small invertebrates. Certain species are often called freshwater angelfish and are popular as aquarium fishes (see Cichlid). In the United States a spadefish is sometimes incorrectly called an angelfish.
Angelfishes belong to the order Perciformes. True angelfishes make up the family Pomacanthidae. Butterfly fishes constitute the family Chaetodontidae. Freshwater angelfish belong to the family Cichlidae. French angelfishes are classified as Pomacanthus paru. Queen angelfishes are classified as Holacanthus ciliaris. Spadefishes are classified as Chaetodipterus faber.

Catfish

Catfish is common name for a group of more than 2,500 species of fish, classified in about 30 families and found worldwide. Catfish are mostly nocturnal scavengers, living near the bottom in shallow waters. Two families are primarily marine, and all other families inhabit fresh water.Catfish are named for the feelers, or barbels, located around the mouth suggesting the whiskers of a cat. These feelers are used for finding food. The body is scaleless, either naked or with bony plates. The dorsal and pectoral fins are often edged with sharp spines that are used for defense. They can inflict severe wounds and are poisonous in some species.
Catfish range in size from about 32 mm (1.25 in) and 7 g (0.25 oz) to 4.5 m (15 ft) and 320 kg (700 lb). The huge sheatfish native to Europe can weigh up to 180 kg (up to 400 lb). Smaller species include stone cats and mad toms, which build nests in the mud or under stones and guard the eggs and young. In many of the marine catfish of the Ariidae family, the pea-sized eggs are carried and cared for in the mouth of the male until hatched.The blind catfish, found in caverns in eastern Pennsylvania, has atrophied eyes. The electric catfish of the Nile and tropical central Africa is capable of giving an electric shock with enough voltage to stun an animal (350 to 450 volts) (see Electric Fish). Some catfish of the family Mochokidae normally swim upside down.
The walking catfish can travel across land to areas of deeper water during dry spells. On land, it moves using a slithering motion combined with a thrashing of its tail. A stout spine in each pectoral fin digs into the ground to help balance and propel the fish. It is able to breathe air by means of a modified gill that forms an air chamber. This catfish originally occurred in eastern India and Southeast Asia but was discovered near Boca Raton, Florida in 1968, following its importation by tropical-fish dealers. Its maximum length is 56 cm (22 in).Of the numerous catfish species native to North America, the bullhead is commonly fished for eating. Of greatest commercial importance are fish in the Mississippi River valley and the Gulf states, some of which weigh as much as 70 kg (150 lb). The blue catfish, or chucklehead, and the channel catfish, the flesh of which is esteemed as equal to that of black bass, form the major part of the harvest.Many species of catfish have been listed as endangered on the International Union for the Conservation of Nature and Natural Resources (IUCN) Red List of Threatened Species. These include the giant catfish, the cave catfish, and the Andean catfish.

Angler

Angler is one of the common name for any of about 265 related saltwater fishes having appendages resembling fishing rods or lures with which they entice their prey. The common anglerfish is found along the coasts of Europe and North America from the British Isles and Nova Scotia to Barbados. Up to 1.5 m (about 5 ft) long, they live on the ocean floor, creeping along on modified pectoral fins in search of food. With a huge mouth and distensible stomach, an angler can swallow other fish as large as itself. In the United States certain anglers are also known as goosefishes and are often marketed under the name monkfish. Other groups of anglers include batfishes, frogfishes, and sea toads.
One of the most unusual aspects of anglers is their reproductive behavior. In many species of deep-sea anglers, the male is less than one-tenth the size of the female and lacks her characteristic lure. The parasitic male attaches himself to the body of his mate by biting through the skin of the female host. The circulatory systems of the two fish then join, with nutrients from the blood of the female thereafter providing the male angler with his only source of nourishment. Anglers are the only fish that exhibit this type of extreme sexual dimorphism.Anglers make up the order Lophiiformes. Those known as goosefishes make up the family Lophidae in the suborder Lophioidei, including the common anglerfish, or European goosefish, classified as Lophius piscatorius. Sexual dimorphism is characteristic of anglers belonging to the suborder Ceratioidei, often referred to as deep-sea anglers. Other groups of anglers include the batfish family, Ogcocephalidae, the frogfish family, Antennariidae, and the sea toad family, Chaunacidae.

Cockatiel

Cockatiel is the common name for a small, crested parrot native to Australia that is a popular pet. It is also known as a quarrion, weero, cockatoo parrot, or crested parrot. Cockatiels are gentle and affectionate birds that make good companions, especially when kept as solitary pets. Their soft call consists of a long, rolling kweel-kweel, but they can be taught to whistle basic melodies or speak simple words.Cockatiels in the wild live throughout Australia, but they are found mainly in the interior regions. They prefer open fields with groups of trees or bushes and a nearby water source. Australia bans the commercial export of this bird species. Cockatiels purchased as pets in other countries are the offspring of captive birds.From beak to tip of tail an adult cockatiel is a small bird, ranging in length from 29 to 34 cm (11 to 13 in) and weighing 75 to 125 g (3 to 4 oz). Male cockatiels (cocks) are larger than females (hens). In the wild, cockatiels display mostly slate-gray coloring. Cocks have a bright, lemon-yellow face and throat with prominent orange cheek patches and dark brown to black eyes. The pointed crest of cocks is predominantly yellow, with a grayish-yellow tint at the tip. Their white shoulder and wing patches contrast with a gray body; the beak and feet are also gray. The underside of the tail is black. Hens display similar but more subdued coloring, but the back and rump are a light gray with narrow white stripes. The underside of the tail is also striped with yellow and white markings.
Genetic mutations and selective breeding have produced eight color varieties in captive cockatiels, described as pied, pearl, cinnamon, fallow, silver, lutino, whiteface, and albino. The pied cockatiel may be predominantly white or gray, with an irregular pattern of yellow, white, or gray feathers. The pearl cockatiel is primarily yellow with a checkered or scalloped pattern on the back and wings created by feathers with a yellow or white center and darker edges. Cinnamon and fallow cockatiels are both brown in color, with fallow a lighter brown than cinnamon. The silver cockatiel has metallic gray coloring and red eyes. The lutino cockatiel is mostly white or yellow, with dark red eyes, while the whiteface cockatiel is mostly gray and white with no yellow or orange. Albinos result from the combination of whiteface and lutino coloring. Combinations of these eight color variations are also common.In the wild, cockatiels band together in flocks of 12 to 100, but they may also occasionally travel in pairs. They are nomadic or migratory birds that follow the availability of food and water. Their diet consists of grains, fruits and berries, seedling grasses, and seeds. Cockatiels forage on the ground but are quick to fly up into trees to avoid birds of prey, their most common predators. Because large cockatiel flocks may devour fields of crops, some farmers consider cockatiels as pests.
In northern Australia, the cockatiel mating season occurs from April to June; in southern Australia, cockatiels breed from August to December. During courtship, the cock displays his white shoulder patches to the hen, and he may drum his feet on the ground or a tree branch. If the hen flies away, the cock follows and repeats his display. Nests are built in hollow trees, usually eucalyptus trees, near fresh water. Both cock and hen take turns incubating the four to seven eggs, which hatch in about three weeks. Cockatiels are one of the few species of parrots in which both sexes share brooding responsibilities.Cockatiels do not display adult coloring until after their first molting (seasonal shedding of feathers) at six to nine months. They reach sexual maturity between 6 and 12 months. Cockatiels have an average life span of 12 to 15 years.

Gene Flow and Migration

Another source of genetic change in human populations is gene flow, the exchange of genes between populations. Gene flow occurs directly when individuals from one population mate with members of another population, thereby introducing their genes into the population. Increased gene flow between populations generally makes them more alike than they had been previously. Gene flow also occurs indirectly. For example, if population A interbreeds with population B, and population B interbreeds with population C, some genes from population A will pass to population C. In this way, gene flow occurs across vast geographic regions and connects distant populations. In fact, global gene flow maintains the unity of the human species, ensuring that people from any two populations in the world can successfully mate. If a human population became isolated and no longer shared gene flow with other populations, it might, over hundreds of thousands of years, lose the ability to breed successfully with other human populations. At that point the isolated population would be considered a new species.
In humans, gene flow often occurs as a result of migration. Migrations most frequently occur on a small scale, as when individuals or families move to a neighboring village, town, or city. Small-scale migration usually takes place at short distances and is reciprocal—that is, members of neighboring populations each migrate to the region of the other population. Large-scale or mass migrations occur when a large group of people moves to a new region, often because of the effects of war or natural disaster.
Mass migration and major population resettlements dramatically increase gene flow. For example, Africans who were brought to the United States as slaves, as well as their descendants, intermixed with white populations. Today the gene pool of those who identify themselves as African American is intermediate between that of American whites and African blacks. On average, African Americans in the United States have 30 percent European ancestry. Those African Americans in the northern United States may have up to 50 percent European ancestry while those in the Southern states—where laws and cultural values long prohibited racial mixing—may have as little as 10 percent European ancestry. This difference illustrates the power that psychological and cultural barriers can have in decreasing gene flow. People who feel deeply rooted in a particular racial or ethnic group may have some animosity toward the mating of people with different physical appearances or from different cultural backgrounds. Religious and socioeconomic differences can also act as barriers to gene flow. However, people are highly social by nature. Even with the effects of racism and ethnocentrism (a belief in the superiority of one’s own social or cultural group), people have always intermarried and interbred with members of neighboring groups.
Historically, natural barriers such as large rivers, seas, deserts, and mountain ranges have prevented migration and reduced gene flow between certain regions. Geographic distance also impeded migrations; people preferred to migrate only short distances. Over the course of the past several centuries, technological improvements in transportation have reduced the influence of geography and distance. For instance, people now can travel easily from one side of the world to the other within a day by airplane. In general, however, populations tend to be more similar to their neighbors and more different from populations that live far away.

Human origin

The differences among modern human populations developed in the evolutionary past. Scientists believe that humans evolved from apelike ancestors beginning about 5 million years ago. The predecessor of modern humans, Homo erectus, lived in Africa and migrated to Asia and Europe 1 million to 2 million years ago. Scientists generally agree that anatomically modern humans, Homo sapiens, evolved within the last 200,000 years. However, anthropologists disagree about how and where modern humans evolved. There are two major hypotheses about how modern humans evolved: the out of Africa hypothesis and the multiregional hypothesis.
According to the out of Africa hypothesis, modern humans originated in Africa in the last 200,000 years and spread from there to the rest of the world, including the Americas and Australia. This migration out of Africa to the rest of the world took place within the last 100,000 years and may have begun as recently as 50,000 to 70,000 years ago. Based on this hypothesis, the differences among modern humans today originated relatively recently—mostly after the great dispersal out of Africa, although some differences may have formed in Africa. According to the competing multiregional hypothesis, modern humans developed in parallel in Africa, Europe, and Asia over 1 million or 2 million years from existing populations of Homo erectus. In this scenario, differences between human populations originated in the distant past. The original support for the multiregional hypothesis derived from fossil evidence that suggested continuity of evolution between archaic humans in Europe, known as Neandertals, and modern Europeans. Certain fossils suggested similar continuity between archaic and modern humans in East Asia. The out of Africa hypothesis was first proposed based on genetic studies of a type of DNA known as mitochondrial DNA, which is inherited through the maternal line. Since then, studies of the Y chromosome, which is inherited through the paternal line, have confirmed the results of mitochondrial DNA studies. These studies show that living African populations have more genetic diversity than any other human groups, and that this diversity has been accumulating for perhaps 100,000 to 200,000 years. This finding implies that all modern humans are descended from a small population of Homo sapiens that lived in Africa 100,000 to 200,000 years ago. Analysis of mitochondrial DNA from a Neandertal fossil found in Germany also suggests that Neandertals did not contribute DNA to modern Europeans. Thus, evidence has been accumulating that modern humans are not descended from Neandertals living outside of Africa. Today, many geneticists and physical anthropologists see the balance of the evidence as strongly favoring the out of Africa hypothesis. For more information on the evolution of modern humans we can also find about Human Evolution in: "Theories of Modern Human Origins and Diversity".
Another important finding is that human genetic variation between groups, however defined, is small compared to that within groups. The data strongly support the idea that all living humans originated recently from a relatively small population—on the order of thousands or tens of thousands of individuals. All people share a strong genetic heritage, and are much more alike.

Embryology

Embryology is the branch of biology dealing with the development of the animal embryo. (For the embryology of plants, see Fertilization; Plant; Seed.) Embryology includes within its province the development of the fertilized egg and embryo and the growth of the fetus.
Until the second half of the 18th century, embryology was a matter of speculation rather than of knowledge. One generally accepted theory was that of preformation: The complete animal with all its organs was believed to exist in the germ in miniature, needing only to unfold like a flower. It followed that each germ must contain within itself the germs of all its future descendants, one within another, as in a nest of boxes. Many naturalists believed the germ to be contained in the ovum, the female germ cell, but after the microscope had revealed spermatozoa, the male germ cells, in 1677, a school of so-called spermists advanced the hypothesis that the germ was contained in the spermatozoon. Their drawings show the spermatozoon encasing a minute human figure, called the homunculus.
Little attention was given to the theory, called the theory of epigenesis, that the English physician and anatomist William Harvey had stated in 1651. This theory, which had been vaguely expressed much earlier by Aristotle, held that the specialized structures of the individual develop step by step from unspecialized antecedents in the egg. Proof of this theory was not forthcoming, however, until 1759 when the German anatomist Kaspar Friedrich Wolff reported on his study of the development of the chick in the egg and showed that the organs arise from undifferentiated material. The basic potential nature and organization of the structures of the organism are determined by the genetic constitution of the fertilized egg (see Heredity). Wolff is called the founder of modern embryology, a title also sometimes given to the Estonian naturalist Karl Ernst von Baer, who in the 19th century described the principal phases in the development of the chick and pioneered in comparative embryology.

Paleoanthropology

The scientific study of human evolution is called paleoanthropology. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it illuminates the origins of the defining traits of the human species, as well as the fundamental connections between humans and other living organisms on Earth. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world came to be. Yet many people have come to reconcile such beliefs with the scientific evidence.
All species of organisms originate through the process of biological evolution. In this process, new species arise from a series of natural changes. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring—that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens.

Hermaphroditism

In some relatively simple animals such as the earthworms and leeches, organs producing sperm and ova occur in the same individual (see Hermaphroditism). Although such animals produce both male and female gametes, the production of sperm and ova usually occurs at different times, so that these animals generally do not fertilize themselves but rather other individuals of the same species. Certain hermaphroditic animals, such as the planarian flatworms, habitually undergo self-fertilization. Among plants, one individual may bear reproductive organs of only one sex, separate reproductive organs of both sexes, or reproductive organs containing both male and female elements (see Flower). Individuals among higher animals bear reproductive organs of only one sex.

Metabolism (chemistry)

Metabolism (chemistry), inclusive term for the chemical reactions by which the cells of an organism transform energy, maintain their identity, and reproduce. All life forms—from single-celled algae to mammals—are dependent on many hundreds of simultaneous and precisely regulated metabolic reactions to support them from conception through growth and maturity to the final stages of death. Each of these reactions is triggered, controlled, and terminated by specific cell enzymes or catalysts, and each reaction is coordinated with the numerous other reactions throughout the organism.

All rights reserved.In keeping with the first two laws of thermodynamics, organisms can neither create nor destroy energy but can only transform it from one form to another. Thus, the chlorophyll of plants, at the foundation of almost all food and energy-transfer webs (see Food Web), captures energy from sunlight and uses it to power the synthesis of living plant cells from inorganic substances such as carbon dioxide, water, and ammonia. This energy, in the form of high-energy products (carbohydrates, fats, and proteins), is then ingested by herbivores and secondarily by carnivores, providing these animals with their only source of energy and cell-building chemicals.

Virtually all living organisms, therefore, ultimately derive their energy from the sun. On reproducing, each species member—whether green plant, herbivore, or carnivore—passes on specific genetic instructions on how to intercept, transform, and finally release energy back into the environment during its life span. Metabolism, from a thermodynamic point of view, embraces the processes by which cells chemically intercept and distribute energy as it continuously passes through the organism.

Bacteriology

Bacteriology, study of bacteria, including their classification and the prevention of diseases that arise from bacterial infection. The subject matter of bacteriology is distributed not only among bacteriologists but also among chemists, biochemists, geneticists, pathologists, immunologists, and public-health physicians. Bacteriology is part of the broader field of microbiology, the study of microorganisms.

Bacteria were first observed by the Dutch naturalist Antoni van Leeuwenhoek with the aid of a simple microscope of his own construction. He reported his discovery to the Royal Society of London in 1683, but the science of bacteriology was not firmly established until the middle of the 19th century. For nearly 200 years it was believed that bacteria are produced by spontaneous generation. The efforts of several generations of chemists and biologists were required to prove that bacteria, like all living organisms, arise only from other similar organisms. This fundamental fact was finally established in 1860 by the French scientist Louis Pasteur, who also discovered that fermentation and many infectious diseases are caused by bacteria. The first systematic classification of bacteria was published in 1872 by the German biologist Ferdinand J. Cohn, who placed them in the plant kingdom. They are now usually included in the kingdom Prokaryote. In 1876 Robert Koch, who had devised the method of inoculating bacteria directly into nutrient media as a means of studying them, found that a bacterium was the cause of the disease anthrax.

Since 1880, immunity against bacterial diseases has been systematically studied. In that year, Pasteur discovered by accident that Bacillus anthracis, cultivated at a temperature of 42° to 43° C (108° to 110° F), lost its virulence after a few generations. Later it was found that animals inoculated with these enfeebled bacteria showed resistance to the virulent bacilli. From this beginning date the prevention, modification, and treatment of disease by immunization, one of the most important modern medical advances. See Antitoxin.

Other significant developments in bacteriology were the discoveries of the organisms causing glanders (1862), relapsing fever (1868), typhoid fever (1880), tetanus (1885), tuberculosis (1890), plague (1894), bacillary dysentery (1898), syphilis (1905), and tularemia (1912).

Genetics

Genetics, study of the function and behavior of genes. Genes are bits of biochemical instructions found inside the cells of every organism from bacteria to humans. Offspring receive a mixture of genetic information from both parents. This process contributes to the great variation of traits that we see in nature, such as the color of a flower’s petals, the markings on a butterfly’s wings, or such human behavioral traits as personality or musical talent. Geneticists seek to understand how the information encoded in genes is used and controlled by cells and how it is transmitted from one generation to the next. Geneticists also study how tiny variations in genes can disrupt an organism’s development or cause disease. Increasingly, modern genetics involves genetic engineering, a technique used by scientists to manipulate genes. Genetic engineering has produced many advances in medicine and industry, but the potential for abuse of this technique has also presented society with many ethical and legal controversies.

Genetic information is encoded and transmitted from generation to generation in deoxyribonucleic acid (DNA). DNA is a coiled molecule organized into structures called chromosomes within cells. Segments along the length of a DNA molecule form genes. Genes direct the synthesis of proteins, the molecular laborers that carry out all life-supporting activities in the cell. Although all humans share the same set of genes, individuals can inherit different forms of a given gene, making each person genetically unique.This photomicrograph shows a specialized type of giant chromosomes called polytene chromosomes. Polytene chromosomes occur in many species of two-winged flies. They are formed when the strands of DNA within normal chromosomes undergo numerous rounds of replication without separating from one another.

Since the earliest days of plant and animal domestication, around 10,000 years ago, humans have understood that characteristic traits of parents could be transmitted to their offspring. The first to speculate about how this process worked were Greek scholars around the 4th century BC, who promoted theories based on conjecture or superstition. Some of these theories remained in favor for several centuries. The scientific study of genetics did not begin until the late 19th century. In experiments with garden peas, Austrian monk Gregor Mendel described the patterns of inheritance, observing that traits were inherited as separate units. These units are now known as genes. Mendel’s work formed the foundation for later scientific achievements that heralded the era of modern genetics.

Protista

Protista, group of comparatively simple organisms, called protists, that have characteristics of both plants and animals. Most protists are unicellular (consisting of a single cell) and can only be seen with a microscope, although there are some that are composed of more than one cell. There are a wide variety of protists, and they inhabit many different environments—fresh water, seawater, soils, and the intestinal tracts of animals, where they perform crucial digestive processes. Like plants, many species of protists can make their own food by the process of photosynthesis. Like animals, many protists can move around under their own power. Unlike plants and animals, however, protists do not have cells organized into specialized tissues.The protists include such familiar organisms as seaweeds, amoebas, and slime molds.
The kingdom Protista contains many economically important members, including organisms that cause diseases, such as malaria. Biologists theorize that members of the kingdom Protista gave rise to the kingdoms Plantae, Animalia, and Fungi about 600 million years ago.All protists are eukaryotes. This means that their cells contain a nucleus, a membrane-bounded structure that encloses the cell’s genetic material. (Organisms without nuclei—the bacteria and cyanobacteria, or blue-green algae—are called prokaryotes.) Although most protists have a single nucleus, protists are unique in that some contain multiple nuclei—up to ten thousand in a single cell—and others, such as ciliates, have two different-sized nuclei in a single cell. The deoxyribonucleic acid (DNA) of protists (like that of other eukaryotes) is organized in long molecules called chromosomes within the nucleus. When the cell divides, these chromosomes replicate (duplicate themselves) and then divide in a process called mitosis. Among some protists, the nuclear membrane (the thin layer around the nucleus) stays intact during mitosis, whereas in plants, animals, and fungi the nuclear membrane breaks down.Some protists form structures known as spores that are resistant to chemicals and drying and that disperse in the environment. Spores are often the infectious stage of organisms that cause diseases in humans when ingested.

Fungus

Fungus, any member of a diverse group of organisms that—unlike plants and animals—obtain food by absorbing nutrients from an external source. The fossil record suggests that fungi were present 550 million years ago and may have evolved even earlier. Today thousands of different types of fungi grow on and absorb food from substances such as soil, wood, decaying organic matter, or living plants and other organisms. They range from tiny, single-celled organisms invisible to the naked eye to some of the largest living multicellular organisms. In Michigan for example, the underground portion of an individual Armillaria mushroom, a type of fungus, extends more than 12 hectares (30 acres). Other fungi are among the longest-lived organisms on Earth—some lichens, a living partnership of a fungus and an alga, are thought to be more than 4,500 years old.

A large and widely distributed group of organisms, fungi perform activities essential to the functioning of all natural ecosystems. They are among the foremost decomposers of organic matter, breaking down plant and animal remains and wastes into their chemical components. As such, fungi play a critical role in the recycling of minerals and carbon. Fungi’ s value to humankind is inestimable. Certain types of fungi, including several types of mold, have proven extremely valuable in the synthesis of antibiotics and hormones used in medicine and of enzymes used in certain manufacturing processes. Some fungi, such as mushrooms and truffles, are considered tasty delicacies that enhance a wide variety of recipes. Not all fungi are beneficial—some damage agricultural crops, cause disease in animals and humans, and form poisonous toxins in food.Common fungi include mushrooms, puffballs, truffles, yeasts, and most mildews, as well as various plant and animal pathogens (disease agents), such as plant rusts and smuts. Some experts estimate that there are 1.5 million fungus species, of which approximately 100,000 have been identified. The unique characteristics of fungi led scientists to classify these important organisms into a separate kingdom, Kingdom Fungi (also known as Mycetae). Certain fungus-like organisms, such as downy mildews, water molds (also known as oomycetes), and slime molds, once classified as fungi, are now placed in the Kingdom Protista.

Bacteria

Bacteria, one-celled organisms visible only through a microscope. Bacteria live all around us and within us. The air is filled with bacteria, and they have even entered outer space in spacecraft. Bacteria live in the deepest parts of the ocean and deep within Earth. They are in the soil, in our food, and on plants and animals. Even our bodies are home to many different kinds of bacteria. Our lives are closely intertwined with theirs, and the health of our planet depends very much on their activities.Bacterial cells are so small that scientists measure them in units called micrometers (µm). One micrometer equals a millionth of a meter (0.0000001 m or about 0.000039 in), and an average bacterium is about one micrometer long. Hundreds of thousands of bacteria would fit on a rounded dot made by a pencil.

Bacteria lack a true nucleus, a feature that distinguishes them from plant and animal cells. In plants and animals the saclike nucleus carries genetic material in the form of deoxyribonucleic acid (DNA). Bacteria also have DNA but it floats within the cell, usually in a loop or coil. A tough but resilient protective shell surrounds the bacterial cell.Bacteria inhabited Earth long before human beings or other living things appeared. The earliest bacteria that scientists have discovered, in fossil remains in rocks, probably lived about 3.5 billion years ago. These early bacteria inhabited a harsh world: It was extremely hot, with high levels of ultraviolet radiation from the sun and with no oxygen to breathe.Bacteria have remarkable abilities to adapt to extreme environments and thrive in parts of Earth that are inhospitable to other forms of life. Anywhere there is life, it includes bacterial life.

Cloning

Cloning, process of creating an exact copy of a single gene, cell, or organism. The copies produced through cloning have identical genetic makeup and are known as clones. Many organisms in nature reproduce by cloning. Scientists use cloning techniques in the laboratory to create copies of cells or organisms with valuable traits. Their work aims to find practical applications for cloning that will produce advances in medicine, biological research, and industry. Gene cloning, for example, is often used to study human disease.Cloning techniques can also be applied to animals. Scientists generate genetically modified animals with new traits, such as the ability to resist disease, and they use cloning techniques to reproduce these genetically modified animals. In the near future scientists hope to bolster populations of endangered species by cloning members from existing populations. Someday scientists may even resurrect extinct species by cloning cells from preserved specimens.Perhaps most important from a human perspective, cloning promises great advances in medicine. Scientists have already inserted fragments of DNA containing the human gene for a blood-clotting protein into cells of a sheep. Through cloning techniques, scientists have generated new sheep whose milk contains the protein, which is needed by people with the blood-clotting disorder known as hemophilia.In the near future, researchers hope to use cloning to develop animals with human diseases and use these cloned animals to test the safety and effectiveness of new treatments devised for humans. Biomedical scientists hope to take cells from an ill patient, genetically modify them, and clone the modified cells to grow exactly the cells that the patient needs to regain health. Some scientists even imagine a day when cloning could be part of a process that grows entire organs for transplants.

Cell

Cell (biology), basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life.Cells vary considerably in size. The smallest cell, a type of bacterium known as a mycoplasma, measures 0.0001 mm (0.000004 in) in diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a human hair. Among the largest cells are the nerve cells that run down a giraffe’s neck; these cells can exceed 3 m (9.7 ft) in length. Human cells also display a variety of sizes, from small red blood cells that measure 0.00076 mm (0.00003 in) to liver cells that may be ten times larger. About 10,000 average-sized human cells can fit on the head of a pin.

Biotechnology

Biotechnology, the manipulation of biological organisms to make products that benefit human beings. Biotechnology contributes to such diverse areas as food production, waste disposal, mining, and medicine.Although biotechnology has existed since ancient times, some of its most dramatic advances have come in more recent years. Modern achievements include the transferal of a specific gene from one organism to another (by means of a set of genetic engineering techniques known as transgenics); the maintenance and growth of genetically uniform plant- and animal-cell cultures, called clones; and the fusing of different types of cells to produce beneficial medical products such as monoclonal antibodies, which are designed to attack a specific type of foreign substance.

Biology

Biology, the science of life. The term was introduced in Germany in 1800 and popularized by the French naturalist Jean-Baptiste de Lamarck as a means of encompassing the growing number of disciplines involved with the study of living forms. The limits of the science, however, have always been difficult to determine, and as the scope of biology has shifted over the years, its subject areas have been changed and reorganized. Today biology is subdivided into hierarchies based on the molecule, the cell, the organism, and the population.
Molecular biology, which spans biophysics and biochemistry, has made the most fundamental contributions to modern biology.Cellular biology is closely linked with molecular biology. To understand the functions of the cell—the basic structural unit of living matter—cell biologists study its components on the molecular level.Biology also includes the study of humans at the molecular, cellular, and organismal levels.The boundaries and subdivisions of biology, however, are as fluid today as they have always been, and further shifts may be expected.

Endangered Species

Endangered Species, plant and animal species that are in danger of extinction (dying out). Over 8,300 plant species and 7,200 animal species around the globe are threatened with extinction, and many thousands more become extinct each year before biologists can identify them. The primary causes of species extinction or endangerment are habitat destruction, commercial exploitation (such as plant collecting, hunting, and trade in animal parts), damage caused by nonnative plants and animals introduced into an area, and pollution. Of these causes, direct habitat destruction threatens the greatest number of species. Extinction is a normal process in the course of evolution. Species have slowly evolved and disappeared throughout geologic time as the result of climate changes and the inability to adapt to survive competition and predation. Since the 1600s, however, the rate of extinction has accelerated rapidly because of human population growth and human resource consumption. Today, most of the world’s habitats are changing faster than most species can adapt to such changes through evolution, or natural selection. The current global extinction rate is exponentially greater than the background extinction rate. Many biologists believe that we are in the middle of the greatest mass extinction episode since the disappearance of the dinosaurs 65 million years ago.

Disease

Most viral infections cause no symptoms and do not result in disease. For example, only a small percentage of individuals who become infected with Epstein-Barr virus or western equine encephalomyelitis virus ever develop disease symptoms. In contrast, most people who are infected with measles, rabies, or influenza viruses develop the disease. A wide variety of viral and host factors determine the outcome of virus infections. A small genetic variation can produce a virus with increased capacity to cause disease. Such a virus is said to have increased virulence.
Viruses can enter the body by several routes. Herpes simplex virus and poxviruses enter through the skin by direct contact with virus-containing skin lesions on infected individuals. Ebola, hepatitis B, and HIV can be contracted from infected blood products. Hypodermic needles and animal and insect bites can transmit a variety of viruses through the skin. Viruses that infect through the respiratory tract are usually transmitted by airborne droplets of mucus or saliva from infected individuals who cough or sneeze. Viruses that enter through the respiratory tract include orthomyxovirus (influenza), rhinovirus and adenovirus (common cold), and varicella-zoster virus (chicken pox). Viruses such as rotavirus, coronavirus, poliovirus, hepatitis A, and some adenoviruses enter the host through the gastrointestinal tract. Sexually transmitted viruses, such as herpes simplex, HIV, and human papillomaviruses (HPV), gain entry through the genitourinary route. Other viruses, including some adenoviruses, echoviruses, Coxsackie viruses, and herpesviruses, can infect through the eye.

Virus

Virus (life science), infectious agent found in virtually all life forms, including humans, animals, plants, fungi, and bacteria. Viruses consist of genetic material—either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)—surrounded by a protective coating of protein, called a capsid, with or without an outer lipid envelope. Viruses are between 20 and 100 times smaller than bacteria and hence are too small to be seen by light microscopy. Viruses vary in size from the largest poxviruses of about 450 nanometers (about 0.000014 in) in length to the smallest polioviruses of about 30 nanometers (about 0.000001 in). Viruses are not considered free-living, since they cannot reproduce outside of a living cell; they have evolved to transmit their genetic information from one cell to another for the purpose of replication.Viruses often damage or kill the cells that they infect, causing disease in infected organisms. A few viruses stimulate cells to grow uncontrollably and produce cancers. Although many infectious diseases, such as the common cold, are caused by viruses, there are no cures for these illnesses. The difficulty in developing antiviral therapies stems from the large number of variant viruses that can cause the
same disease, as well as the inability of drugs to disable a virus without disabling healthy cells. However, the development of antiviral agents is a major focus of current research, and the study of viruses has led to many discoveries important to human health.

Human Evolution

Human Evolution, lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people evolved over a period of at least 6 million years.Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the so-called great apes (large apes) of Africa—chimpanzees (including bonobos, or so-called pygmy chimpanzees) and gorillas—share a common ancestor that lived sometime between 8 million and 6 million years ago. The earliest humans evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 million and 2 million years ago come entirely from Africa.Early humans first migrated out of Africa into Asia probably between 2 million and 1.7 million years ago. They entered Europe somewhat later, generally within the past 1 million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years, and to the Americas within the past 35,000 years. The beginnings of agriculture and the rise of the first civilizations occurred within the past 10,000 years.The scientific study of human evolution is called paleoanthropology. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. Scientists have abundant evidence of human evolution from fossils, artifacts, and genetic studies. However, some people find the concept of human evolution troubling because it can seem to conflict with religious and other traditional beliefs about how people, other living things, and the world came to be. Yet many people have come to reconcile such beliefs with the scientific evidence.