Environmental Encyclopedia

Подождите немного. Документ загружается.

Environmental Encyclopedia 3

Garbage

Mohandas Karamchand Gandhi. (AP/Wide

World Photos. Reproduced by permission.)

his work seems increasingly relevant to a world with limited

resources and a rapidly growing population.

[Usha Vedagiri and Douglas Smith]

ESOURCES

OOKS

Chadha, Yogesh. Gandhi: A Life. Wiley, 1998.

Fischer, L. The Life of Mahatma Gandhi. New York: Harper, 1950.

Mehta, V. Mahatma Gandhi and His Apostles. New York: Viking, 1977.

Garbage

In 1999, the United States generated 230 million tons of

municipal solid waste

, compared with 195 million tons

in 1990, according to

Environmental Protection Agency

(EPA) estimates. On average, each person generated 4.6 lb

(2.1 kg) of such waste per day in 1999, and the EPA expects

that amount to continue increase. That waste includes cans,

bottles, newspapers, paper and plastic packages, uneaten

food, broken furniture and appliances, old tires, lawn clip-

617

pings, and other refuse. This waste can be placed in landfills,

incinerated, recycled, or in some cases composted.

Landfilling—waste disposed of on land in a series of

layers that are compacted and covered, usually with soil—

is the main method of

waste management

in this country,

accounting for about 57%of the waste. But old landfills

are being closed and new ones are hard to site because of

community opposition. Landfills once were open dumps,

causing unsanitary conditions,

methane

explosions, and re-

leases of hazardous

chemicals

into

groundwater

and air.

Old dumps make up 22% of the sites on the Superfund

National Priorities List

. Today, landfills must have liners,

gas collection systems, and other controls mandated under

Subtitle D of the

Resource Conservation and Recovery

Act

(RCRA).

Incineration

has been popular among

solid waste

managers because it helps to destroy bacteria and toxic chem-

icals and to reduce the volume of waste. But public opposi-

tion, based on fears that toxic metals and other chemical

emissions will be released from incinerators, has made the

siting of new facilities extremely difficult. In the past, garbage

burning was done in open fields, in dumps, or in backyard

drums, but the

Clean Air Act

(1970) banned open burning,

leading to new types of incinerators, most of which are

designed to generate energy.

Recycling

, which consists of collecting materials from

waste streams, preparing them for market, and using those

materials to manufacture new products, is catching national

attention as a desirable waste management method. As of

1999, all states and the District of Columbia had some

type of statewide recycling law aimed at promoting greater

recycling of glass, paper, metals,

plastics

, and other materi-

als. Used oil, household batteries, and lead-acid automotive

batteries are recyclable waste items of particular concern

because of their toxic constituents.

Composting

is a waste management approach that

relies on heat and microorganisms—mostly bacteria and

fungi—to decompose yard wastes and food scraps, turning

them into a nutrient-rich mix called

humus

or compost.

This mix can be used as

fertilizer

. However, as with landfills

and incinerators, composting facilities have been difficult to

site because of community opposition, in part because of the

disagreeable smell generated by some composting practices.

Recently, waste managers have shown interest in

source reduction, reducing either the amount of garbage

generated in the first place or the toxic ingredients of garbage.

Reusable blankets instead of throw-away cardboard packag-

ing for protecting furniture is one example of source reduc-

tion. Businesses are regarded as a prime target for source

reduction, such as implementing double-sided photocopying

Environmental Encyclopedia 3

Garbage Project

to save paper, because the approach offers potentially large

cost savings to companies.

[David Clarke]

ESOURCES

OOKS

Blumberg, L., and R. Gottlieb. War on Waste: Can America Win Its Battle

With Garbage? Washington, DC: Island Press, 1989.

Underwood, J., A. Hershkowitz, and M. de Kadt. Garbage—Practices, Prob-

lems, Remedies. New York: INFORM, 1988.

U.S. Office of Technology Assessment. Facing America’s Trash: What Next

For Municipal Solid Waste? Washington, DC: U.S. Government Printing

Office, 1989.

Garbage Project

The

Garbage

Project was founded in 1973, shortly after

the first

Earth Day

, by William Rathje, professor of anthro-

pology, and fellow archaeologists at the University of Ari-

zona. The objective was to apply the techniques and tools

of their science to the study of modern civilization by analyz-

ing its garbage.

Using sample analysis and assessing biodegradation,

they also hoped to increase their understanding of resource

depletion and environmental and landfill-related problems.

Because it requires sunlight, moisture, and oxygen, as well

as organic material and bacteria, little biodegradation actually

takes place in landfills, resulting in perfectly preserved heads

of lettuce, 40-year-old hot dogs, and completely legible 50-

year-old newspapers.

In Rubbish: The Archaeology of Garbage, published in

1992, Rathje and Atlantic Monthly managing editor Cullen

Murphy discuss some of the data gleaned from the project.

For example, the accumulation of refuse has raised the City

of New York 6–30 ft (1.8–9 m) since its founding. Also,

the largest proportion—40%—of landfilled garbage is paper,

followed by the leftovers from building construction and

demolition. In fact, newspapers alone make up about 13%

of the total volume of trash.

Just as interesting as what they found was what they

did not find. Contrary to much of public opinion, fast-food

packaging made up only one-third of 1% of the total volume

of trash landfilled between 1980 and 1989, while expanded

polystyrene

foam accounted for no more than 1%. Even

disposable diapers

averaged out at only 1% by weight of

the total

solid waste

contents (1.4% by volume). Of all

the garbage examined,

plastics

constituted from 20–24%.

Surveys of several national landfills revealed that organic

materials made up 40–52% of the total volume of waste.

The Garbage Project also debunked the idea that the

United States is running out of space for landfills. While it

618

is true that many landfills have been shut down, it is also

true that many of those were quite small to begin with

and that they now pose fewer environmental hazards. It is

estimated that one

landfill

120 ft (35.4 m) deep and measur-

ing 44 mi

(71 km

) would adequately handle the needs of

the entire nation for the next 100 years (assuming current

levels of waste production).

In “A Perverse Law of Garbage,” Rathje extrapolated

from “Parkinson’s Law” to define his Parkinson’s Law of

Garbage: “Garbage expands so as to fill the receptacles avail-

able for its containment.” As evidence he cites a Garbage

Project study of the recent mechanization of garbage pick-

up in some larger cities and the ensuing effects. As users

were provided with increasingly larger receptacles (in order

to accommodate the mechanized trucks), they continued to

fill them up. Rathje attributes this to the newfound conve-

nience of disposing of that which previously had been con-

signed to the basement or secondhand store, and concludes

that the move to automation may be counterproductive to

any attempt to reduce garbage and increase

recycling

[Ellen Link]

ESOURCES

OOKS

Rathje, W. L., and C. Murphy. Rubbish!: The Archaeology of Garbage. New

York: Harper Collins, 1992.

ERIODICALS

Lilienfeld, R. M. “Six Enviro-Myths: Recycling is the Key.” New York

Times (January 21, 1995): 23(L).

Rathje, W. L. “A Perverse Law of Garbage.” Garbage 4, no. 6 (December-

January 1993): 22.

Garbology

The study of

garbage

, through either archaeological exca-

vation of landfills or analysis of fresh garbage, to determine

what the composition of

municipal solid waste

says about

the society that generated it. The term is associated with

the

Garbage Project

of the University of Arizona (Tucson),

co-directed by William Rathje and Wilson Hughes, which

began studying trash in 1973 and excavating landfills in

1987. They found that little degradation occurs in landfills;

New York newspapers from 1949 and an ear of corn dis-

carded in 1971 were found intact. Unexpectedly, the project

also found that

plastics

make up less than one percent of

the total volume of landfills.

Gardens

see

Botanical garden; Organic gardening

and farming

Environmental Encyclopedia 3

Gasoline

Gasohol

Gasohol is a term used for the mixture of 10% ethyl alcohol

(also called

ethanol

or grain alcohol) with

gasoline

. Etha-

nol raises the

octane rating

of lead-free

automobile

fuel

and significantly decreases the

carbon monoxide

released

from tailpipes. It has also been promoted as a means of

reducing corn surpluses. By 2001, 2.2 billion gal (8.3 billion

l) were being produced a year and this number is expected

to rise to 4.6 billion gal (17.4 billion l). However, ethanol

also raises the vapor pressure of gasoline, and it has been

reported to increase the release of “evaporative” volatile

hy-

drocarbons

from the fuel system and oxides of

nitrogen

from the exhaust. These substances are components of urban

smog

, and thus the role of ethanol in reducing

pollution

is controversial.

Gasoline

Crude oil in its natural state has very few practical uses.

However, when it is separated into its component parts by

the process of fractionation, or refining, those parts have an

almost unlimited number of applications.

In the first 60 years after the process of

petroleum

refining was invented, the most important fraction produced

was kerosene, widely used as a home heating product. The

petroleum fraction slightly lighter than kerosene — gaso-

line — was regarded as a waste product and discarded. Not

until the 1920s, when the

automobile

became popular in

the United States, did manufacturers find any significant

use for gasoline. From then on, however, the importance of

gasoline has increased with automobile use.

The term gasoline refers to a complex mixture of liquid

hydrocarbons

that condense in a fractionating tower at

temperatures between 100° and 400°F (40° and 205°C). The

hydrocarbons in this mixture are primarily single- and dou-

ble-bonded compounds containing five to 12

carbon

atoms.

Gasoline that comes directly from a refining tower,

known as naphtha or “straight-run” gasoline, was an ade-

quate fuel for the earliest motor vehicles. But as improve-

ments in internal

combustion

engines were made, problems

began to arise. The most serious problem was “knocking.”

If a fuel burns too rapidly in an internal combustion

engine, it generates a shock wave that makes a “knocking”

or “pinging” sound. The shock wave will, over time, also

cause damage to the engine. The hydrocarbons that make

up straight-run gasolines proved to burn too rapidly for

automotive engines developed after 1920.

Early in the development of automotive fuels, engi-

neers adopted a standard for the amount of knocking caused

by a fuel and, hence, for the fuel’s efficiency. That standard

619

was known as “octane number.” To establish a fuel’s octane

number, it is compared with a very poor fuel (n-heptane),

assigned an octane number of zero, and a very good fuel

(isooctane), assigned an octane number of 100. The octane

number of straight-run gasoline is anywhere from 50 to 70.

As engineers made more improvements in automotive

engines after the 1920s, chemists tried to keep pace by devel-

oping better fuels. One approach they used was to subject

straight-run gasoline (as well as other crude oil fractions) to

various treatments that changed the shape of hydrocarbon

molecules in the gasoline mixture. One such method, called

cracking, involves the heating of straight-run gasoline or

another petroleum fraction to high temperatures. The pro-

cess results in a better fuel from newly-formed hydrocarbon

molecules.

Another method for improving the quality of gasoline

is catalytic reforming. In this case, the cracking reaction takes

place over a catalyst such as

copper

, platinum, rhodium, or

other “noble” metal, or a form of clay known as zeolite.

Again, hydrocarbon molecules formed in the fraction are

better fuels than straight-run gasoline. Gasoline produced

by catalytic cracking or reforming has an octane number of

at least eighty.

A very different approach to improving gasoline qual-

ity is the use of additives,

chemicals

added to gasoline to

improve the fuel’s efficiency. Automotive engineers learned

more than 50 years ago that adding as little as two grams

tetraethyl lead

, the best-known additive, to one gallon

of gasoline raises its octane number by as much as ten points.

Until the 1970s, most gasolines contained tetraethyl

lead

. Then, concerns began to grow about the release of

lead to the

environment

during the combustion of gasoline.

Lead concentrations in urban air had reached a level five to

10 times that of rural air. Residents of countries with few

automobiles, such as Nepal, had only one-fifth the lead in

their bodies as did residents of nations such as the United

States, with many automotive vehicles.

The toxic effects of lead on the human body have been

known for centuries, and risks posed by leaded gasoline

became a major concern. In addition, leaded gasoline became

a problem because it damaged a car’s

catalytic converter

which reduced air pollutants in exhaust.

Finally, in 1973, the

Environmental Protection

Agency

(EPA) acted on the problem and set a time-scale

for the gradual elimination of leaded fuels. According to

this schedule, the amount of lead was to be reduced from

2 to 3 grams per gallon (the 1973 average) to 0.5 g/gal by

1979. Ultimately, the additive was to be totally eliminated

from all gasoline.

The elimination of leaded fuels has been made possible

by the invention of new and safer additives. One of the most

popular is methyl-t-butyl ether (MTBE). By 1988 MTBE

Environmental Encyclopedia 3

Gasoline tax

had become so popular that it was among the 40 most widely

produced chemicals in the United States. In 2001, MTBE

was asked to be phased out of production in California

by 2003.

Yet another approach to improving fuel efficiency is

the mixing of gasoline and ethyl or

methanol

. This product,

known as

gasohol

, has the advantage of high

octane rating

lower cost, and reduced

emission

of pollutants, compared

to normal gasoline. See also Air pollution; Alternative fuels

[David E. Newton]

ESOURCES

OOKS

Joesten, M. D., et al. World of Chemistry. Philadelphia: Saunders, 1991.

Lapedes, D. N., ed. McGraw-Hill Encyclopedia of Energy. New York:

McGraw-Hill, 1976.

ERIODICALS

“MBTE Growth Limited Despite Lead Phasedown in Gasoline.” Chemi-

cal & Engineering News (July 15, 1985): 12.

Williams, R. “On the Octane Trail.” Technology Illustrated (May 1983):

52–53.

Gasoline tax

Gasoline

taxes include federal, state, county, and municipal

taxes imposed on gasoline motor vehicle fuel. In the United

States, most of the federal tax is used to fund maintenance

and improvements in such

transportation

infrastructures

as interstate highways. As of mid-2002, the federal excise

tax for gasoline stood at 18.4 cents per gallon, and state

excise taxes ranged from 7.5 cents in Georgia to 29 cents

in Rhode Island (making the weighted national average state

tax 19.97 cents per gallon). In total, the U.S. national average

gasoline tax (combining federal and state) was 38 cents per

gallon.

Oregon became the first state to institute a tax on

gasoline in 1919. By the time the federal government estab-

lished its own 1 cent gas tax in 1932, every state had a gas

tax. After several small increases in the 1930s and 1940s,

the gas tax was raised to 3 cents to finance the Highway

Trust Fund in 1956. The Trust Fund was earmarked to pay

for federal interstate construction and other road work. In

1982, the federal gasoline tax was increased to 9 cents to

fund road maintenance and

mass transit

. The tax was hiked

again in 1990 to 14.1 cents, and to 18.4 cents in 1993—

where it remained as of July 2002.

Over this time, gasoline prices increased from about

20 cents per gallon in 1938 to a U.S. national average of

139.2 cents in May of 2002. The average national gasoline

tax (both federal and a weighted average of state taxes)

accounts for 30.2% of the retail price of a gallon of gas.

620

In some countries, diesel fuels are taxed and priced

less than gasoline. Commercial vehicles are major consumers

of diesel, and lower taxes avoided undue impacts on trucking

and commerce. In the United States, diesel is taxed at a

higher rate than gasoline—an average of 44 cents per gallon

(including 24.3 cents federal tax and the weight average of

state taxes). In contrast,

gasohol

, an alternative fuel con-

sisting of 90% gasoline and 10%

ethanol

, has a lower rate

of taxation (with a 13.1 cent federal excise tax).

Although federal gasoline taxes are a manufacturer’s

excise tax, meaning that the government collects the tax

directly from the manufacturer, rate hikes are often passed

on to consumers at the pump. In this light, gasoline taxes

have been criticized as regressive and thus inequitable—i.e.,

lower income individuals pay a greater share of their income

as tax than higher income individuals. Also, the tax as a

share of the pump price has been increasing. However, it

should be noted that the general trend of the real price of

gasoline (adjusted for inflation and including taxes) has been

declining for many decades. In addition,

automobile

effi-

ciency (mileage) has been improving and thus the fuel re-

quirements and cost-per-mile-traveled have declined.

These factors can be viewed as largely or completely

offsetting the impact of gasoline taxes. For example, Con-

gress attempted in May 1996 to rollback the 4.3 cents tax

increase of 1993. The impact of this repeal for a family of

four who drive 12,000 miles a year at 20 miles per gallon is

a savings of $26, which in the House debates was compared

to the cost of a family dinner at McDonald’s. On the other

hand, a rollback could have bigger consequences for the

future upkeep of the country’s highways and interstates; a

2000 Congressional report estimated that a repeal of the

federal gasoline tax would mean a $5.2 billion annual loss

in revenues for the Highway Trust Fund.

Outside of the United States, both gasoline prices and

gas tax rates are typically far higher (e.g., gasoline taxes were

307 cents per gallon in the United Kingdom as of March

2002). In addition to funding governments, high gasoline

taxes form part of a strategy to encourage the use of public

transportation, reduce

pollution

, conserve energy, and im-

prove national security (since most gasoline is imported).

[Stuart Batterman and Paula Anne Ford-Martin]

ESOURCES

ERIODICALS

Talley, Louis Allen. Congressional Research Service. “The Federal Excise

Tax on Gasoline and the Highway Trust Fund: A Short History” CRS

Report for Congress. Washington, DC: Congressional Research Service,

March 2000.

Environmental Encyclopedia 3

Gene bank

THER

American Petroleum Institute, Policy Analysis and Statistics Department.

How Much We Pay for Gasoline: April 2001 Review. Washington, DC:

API, 2001.

U.S. Department of Energy, Energy Information Administration. Gasoline

and Diesel Fuel Update. [cited July 9, 2002]. <http://tonto.eia.doe.gov/oog/

info/gdu/gasdiesel.asp>.

U.S. Department of Transportation, Federal Highway Administration. Our

Nation’s Highways: Selected Facts and Figures 2000. Publication No. FHWA-

PL-01-1012. [cited July 6, 2002]. <http://www.fhwa.dot.gov/ohim/onh00/

onh.htm>.

Gastropods

Gastropods are invertebrate animals that make up the largest

class in the phylum Mollusca. Examples of common gastro-

pods include all varieties of snails, abalone, limpets, and land

and sea slugs. There are over 35,000 existing

species

,as

well an additional 15,000 separate fossil species. Gastropods

first appeared in the fossil record during the early Cambrian

period, approximately 550 million years ago.

This diverse group of animals is characterized by a

soft body, made up of three main parts: the head, foot, and

visceral mass. The head contains a mouth and often sensing

tentacles. The lower portion of the body makes up the foot,

which allows slow creeping along rocks and other solid sur-

faces. The visceral mass is the main part of the body, con-

taining most of the internal organs. In addition to these body

parts, gastropods possess a mantle, or fold which secretes a

hard, calcium carbonate shell. The single, asymmetrical shell

of a gastropod is most often spiral shaped, however, it can

be flattened or cone-like. This shell is an important source

of protection. Predators have a difficult time accessing the

soft flesh inside, especially if there are sharp points on the

outside, as there are on the shells of some of the more ornate

gastropods. There are also some gastropods, such as slugs

and sea hares, that do not have shells or have greatly reduced

shells. Some of the shelless types that live in the ocean (i.e.,

nudibranchs or sea slugs) are able to use stinging cells from

prey that they have consumed as a means of protection.

In addition to a spiraling of their shells, the soft bodies

of most gastropods undergo 180 degrees of twisting, or

torsion, during early development, when one side of the

visceral mass grows faster than the other. This characteristic

distinguishes gastropods from other molluscs. Torsion re-

sults in a U-shaped digestive tract, with the anal opening

slightly behind the head. The torsion of the soft body and the

spiraling of the shell are thought to be unrelated evolutionary

events.

Gastropods have evolved to live in a wide variety of

habitats. The great majority are marine, living in the world’s

oceans. Numerous species live in fresh water, while others

live entirely on land. Of those that live in water, most are

621

found on the bottom, attached to rocks or other surfaces.

There are even a few species of gastropods without shells,

including sea butterflies, that are capable of swimming. Liv-

ing in different habitats has resulted in a wide variety of

structural adaptations within the class Gastropoda. For ex-

ample, those gastropods that live in water use gills to obtain

the oxygen necessary for

respiration

, while their terrestrial

relatives have evolved lungs to breathe.

Gastropods are important links in food webs in the

habitats in which they live, employing a wide variety of

feeding strategies. For example, most gastropods move a

rasping row of teeth on a tongue like organ called a radula

back and forth to scrape microscopic algae off rocks or the

surface of plants. Because the teeth on the radula gradually

wear away, new teeth are continuously secreted. Other gas-

tropods have evolved a specialized radula for drilling through

shells of animals to get at their soft flesh. For example, the

oyster drill, a small east coast gastropod, bores a small hole

in the shell of neighboring molluscs such as oysters and

clams so that it can consume the soft flesh. In addition,

some terrestrial gastropods such as snails use their radula to

cut through pieces of leaves for food.

Gastropods are eaten by numerous animals, including

various types of fish, birds, and mammals. They are also

eaten by humans throughout the world. Abalone, muscular

shelled gastropods that cling to rocks, are consumed on the

west coast of the United States and in Asia. Fritters and

chowder are made from the large, snail-like queen conch

on many Caribbean islands. Escargot (snails in a garlicky

butter sauce) are a European delicacy.

[Max Strieb]

ESOURCES

OOKS

Barnes, R. D. Invertebrate Zoology. 5th ed. Philadelphia: Saunders College

Publishing, 1987.

Gene bank

The term gene bank refers to any system by which the

genetic composition of some population is identified and

stored. Many different kinds of gene banks have been estab-

lished for many different purposes. Perhaps the most numer-

ous gene banks are those that consist of plant seeds, known

as germ banks.

The primary purpose for establishing a gene bank is

to preserve examples of threatened or

endangered species

Each year, untold numbers of plant and animal

species

become extinct because of natural processes and more com-

Environmental Encyclopedia 3

Gene pool

monly, as the result of human activities. Once those species

become extinct, their gene pools are lost forever.

Scientists want to retain those gene pools for a number

of reasons. For example, agriculture has been undergoing a

dramatic revolution in many parts of the world over the past

half century. Scientists have been making available to farmers

plants that grow larger, yield more fruit, are more disease-

resistant, and have other desirable characteristics. These

plants have been produced by agricultural research in the

United States and other nations. Such plants are very attrac-

tive to farmers, and they are also important to governments

as a way of meeting the food needs of growing populations,

especially in

Third World

countries.

When farmers switch to these new plants, however,

they often abandon older, more traditional crops that may

then become extinct. Although the traditional plants may

be less productive, they have other desirable characteristics.

They may, for example, be able to survive droughts or other

extreme environmental conditions that new plants cannot.

Placing seeds from traditional plants in a gene bank

allows them to be preserved. At some later time, scientists

may want to study these plants further and perhaps identify

the genes that are responsible for various desirable properties

of the plants. The

U.S. Department of Agriculture

(USDA)

has long maintained a

seed bank

of plants native to the

United States. About 200,000 varieties of seeds are stored at

the USDA’s Station at Fort Collins, Colorado, and another

100,000 varieties are kept at other locations around the

country.

Efforts are now underway to establish gene banks for

animals, too. Such banks consist of small colonies of the

animals themselves. Animal gene banks are desirable as a

way of maintaining species whose natural population is very

low. Sometimes the purpose of the bank is simply to maintain

the species to prevent its becoming extinct. In other cases,

species are being preserved because they were once used as

farm animals although they have since been replaced by

more productive modern hybrid species. The Fayoumi

chicken native to Egypt, for example, has now been aban-

doned by farmers in favor of imported species. The Fayoumi,

without some form of protection, is likely to become extinct.

Nonetheless, it may well have some characteristics (genes)

that are worth preserving.

In recent years, another type of gene bank has become

possible. In this kind of gene bank, the actual base sequence

of important genes in the human body will be determined,

collected, and catalogued. This effort, begun in 1990, is a

part of the Human Genome Project effort to map all human

genes. See also Agricultural revolution; Extinction; Genetic

engineering; Population growth

[David E. Newton]

622

ESOURCES

ERIODICALS

Anderson, C. “Genetic Resources: A Gene Library That Goes ’Moo’.”

Nature 355 (January 30, 1992): 382.

Crawford, M. “USDA Bows to Rifkin Call for Review of Seed Bank.”

Science 230 (December 6, 1985): 1146–1147.

Roberts, L. “DOE to Map Expressed Genes.” Science 250 (November 16,

1990): 913.

Gene pool

The term gene pool refers to the sum total of all the genetic

information stored within any given population. A gene is

a specific portion of a DNA (

deoxyribose nucleic acid

)

molecule, so a gene pool is the sum total of all of the DNA

contained within a population of individuals.

The concept of gene pool is important in ecological

studies because it reveals changes that may or may not be

taking place within a population. In a population living in

an ideal

environment

for its needs, the gene pool is likely

to undergo little or no change. If individuals are able to

obtain all the food, water, energy, and other resources they

need, they experience relatively little stress and there is no

pressure to select one or another characteristic.

Changes do occur in gene frequency because of natural

factors in the environment. For example, natural

radiation

exposure

causes changes in DNA molecules that are re-

vealed as genetic changes. These natural mutations are one

of the factors that make possible continuous changes in the

genetic constitution of a population that, in turn, allows for

evolution

to occur.

Natural populations seldom live in ideal situations,

however, and so they experience various kinds of stress that

lead to changes in the gene pool. A classical example of this

kind of change was reported by J. B. S. Haldane in 1937.

Haldane found that a population of moths gradually became

darker in color over time as the trees on which they lived also

became darker because of

pollution

from factories. Moths in

the population who carried genes for darker color were better

able to survive and reproduce than were their lighter-colored

cousins, so the composition of the gene pool changed to

relieve stress.

Humans have the ability to make conscious changes

in gene pools that no other

species

has. Sometimes we

make those changes in the gene pools of plants or animals

to serve our own needs for food or other resource. Hybridiza-

tion of plants to produce populations that have some desir-

able quality such as

resistance

to disease, shorter growing

season, or better-tasting fruit. The modern science of

ge-

netic engineering

is perhaps the most specific and deliber-

ate way of changing in gene pools today.

Environmental Encyclopedia 3

Genetic engineering

Humans can also change the gene pool of their own

species. For example, individuals with various genetic disor-

ders were at one time doomed to death. Our inability to

treat diabetes, sickle-cell

anemia

, phenylketonuria, and

other hereditary conditions meant that the frequency of the

genes causing those disorders in the human gene pool was

kept under control by natural forces.

Today, many of those same disorders can be treated

by medical or genetic techniques. That results in positive

benefit for the individuals who are cured, but raises questions

about the quality of the human gene pool overall. Instead

of having many of those deleterious genes being lost naturally

by an individual’s death, they are now retained as part of

the gene pool. This fact has at times raised questions about

the best way in which medical science should deal with

genetic disorders. See also Agricultural revolution; Birth de-

fects; Extinction; Gene bank; Population growth

[David E. Newton]

ESOURCES

OOKS

Patt, D. I., and G. R. Patt. An Introduction to Modern Genetics. Reading,

MA: Addison-Wesley, 1976.

Genetic engineering

Genetic engineering is the manipulation of the hereditary

material of organisms at the molecular level. The hereditary

material of most cells is found in the chromosomes, and it

is made of

deoxyribose nucleic acid

(DNA). The total

DNA of an organism is referred to as its genome. In the

1950s, scientists first discovered how the structure of DNA

molecules worked and how they stored and transmitted ge-

netic information.

Genetic engineering relies on recombinant DNA tech-

nology to manipulate genes. Methods are now available for

rapidly sequencing the nucleotides of pieces of DNA, as

well as for identifying particular genes of interest, and for

isolating individual genes from complex genomes. This

allows genetic engineers to alter genetic materials to produce

new substances or create new functions.

The biochemical tools used by genetic engineers or

molecular biologists include a series of enzymes that can

“cut and paste” genes. Enzymes are used to cut a piece of

DNA, insert into it a new piece of DNA from another

organism, and then seal the joint.

One important group of these are restriction enzymes,

of which well over 500 are known. Most restriction enzymes

623

are endonucleases—enzymes that break the double helix of

DNA within the molecule, rather than attacking the ends

of the helix. Every restriction

enzyme

is given a specific

name to identify it uniquely. The first three letters, in italics,

indicate the biological source of the enzyme, the first letter

being the initial of the genus, the second and third letters

being the first two letters of the

species

name. Thus restric-

tion enzymes from

Escherichia coli

are called Eco, those

from Haemophilus influenzae are Hin, from Diplococcus pneu-

moniae comes Dpn, and so on.

The genetic engineer can use a restriction enzyme to

locate and cut almost any sequence of bases. Cuts can be

made anywhere along the DNA, dividing it into many small

fragments or a few longer ones. The results are repeatable:

cuts made by the same enzyme on a given sort of DNA will

always be the same. Some enzymes recognize sequences as

long as six or seven bases; these are used for opening a

circular strand of DNA at just one point. Other enzymes

have a smaller recognition site, three or four bases long;

these produce small fragments that can then be used to

determine the sequence of bases along the DNA.

The cut that each enzyme makes varies from enzyme

to enzyme. Some, like Hin dII, make a clean cut straight

across the double helix, leaving DNA fragments with ends

that are flush. Other enzymes (Eco RI) make a staggered

break, leaving single strands with protruding cohesive ends

("sticky ends") that are complementary in base sequence.

Following breakage, and under the right conditions, the

complementary bases from different sources can be rejoined

to form recombinant DNA.

Another important biochemical tool used by genetic

engineers is DNA polymerase, an enzyme that normally

catalyses the growth of a

nucleic acid

chain. DNA polymer-

ase is used by genetic engineers to seal the gaps between

the two sets of fragments in newly joined chimera molecules

of recombinant DNA. DNA polymerase is also used to

label DNA fragments, for DNA polymerase labels practically

every base, allowing minute quantities of DNA to be studied

in detail. If a piece of

ribonucleic acid

(RNA) of the target

gene is the starting point, then the enzyme reverse tran-

scriptase is used to produce a strand of complementary DNA

(cDNA).

Genetic engineers usually need large numbers of ge-

netically identical copies of the DNA fragment of interest.

One way of doing this is to insert the gene into a suitable gene

carrier, called a

cloning

vector. Common cloning vectors

are bacterial plasmids or viruses such as the bacteriophage

lambda, which are small circles of DNA found in bacterial

cells independently of the main DNA molecule. When the

cloning vectors divide, they replicate both themselves and

the foreign DNA segment linked to it.

Environmental Encyclopedia 3

Genetic engineering

DNA injected into a mouse embryo. (Photograph by Jon Gordon. Phototake. Reproduced by permission.)

In the plasmid insertion method, restriction enzymes

are used to cleave the plasmid double helix so that a stretch

of DNA (previously cleaved with the same enzyme) can be

inserted into the plasmid. As a result, the “sticky ends” of

the plasmid DNA and the foreign DNA are complementary

and base-pair when mixed together. The fragments held

together by base pairing are permanently joined by DNA

ligase. The host bacterium, with its 20- to 30-minute repro-

ductive cycle, is like a manufacturing plant. With repeated

doublings of its offspring on a controlled culture medium,

millions of clones of the purified DNA fragments can be

produced overnight.

Similarly, if viruses (bacteriophages) are used as clon-

ing vectors, the gene of interest is inserted into the phage

DNA, and the

virus

is allowed to enter the host bacterial

cell where it multiplies. A single parental lambda phage

particle containing recombinant DNA can multiply to sev-

eral hundred progeny particles inside the bacterial cell (E.

coli) within roughly 20 minutes.

Cosmids are another type of viral cloning vehicle that

attaches foreign DNA to the packaging sites of a virus and

thus introduces the foreign DNA into an infective viral

624

particle. Cosmids allow researchers to insert very long

stretches of DNA into host cells where cell multiplication

amplifies the amount of DNA available. Large artificial

chromosomes of yeast (called megaYACs) are also used as

cloning vehicles, since they can store even larger pieces of

DNA, 35 times more than can be stored conventionally in

bacteria.

The polymerase

chain reaction

(PCR) technique is

an important new development in the field of genetic engi-

neering, since it allows the mass production of short seg-

ments of DNA directly, and offers the advantage of by-

passing the several steps involved in using bacterial and

viruses as cloning vectors.

DNA fragments can be introduced into mammalian

cells, but a different method must be used. Here, genes

packed in solid calcium phosphate are placed next to a cell

membrane that surrounds the fragment and transfers it to

the cytoplasm. The gene is delivered to the nucleus during

mitosis (when the nuclear membrane has disappeared) and

the DNA fragments are incorporated into daughter nuclei,

then into daughter cells. A mouse containing human

cancer

genes (the onchomouse) was patented in 1988.

Environmental Encyclopedia 3

Genetic resistance (or genetic tolerance)

The potential benefits of recombinant DNA research

are enormous. In recent years, scientists have identified the

genetic basis of a number of medical disorders. Genetic

engineering helps scientists replace a particular missing or

defective gene with correct copies of that gene. If that gene

then begins functioning in an individual, a genetic disorder

may be cured. Researchers have already met with great suc-

cess in finding the single genes that are responsible for

common diseases like cystic

fibrosis

and hemochromatosis.

In the early twenty-first century, scientists were applying

knowledge from the human genome project to try to map

multiple genes responsible for diseases like diabetes, hyper-

tension, and schizophrenia.

Genetic engineering has also led to a great deal of

controversy, however. Since scientists can duplicate genes,

they can also duplicate, or “clone” animals and humans. In

December 2001, scientists announced the birth of the first

cloned female cat, although the 1990s saw big headlines for

the first big mammal cloning of sheep. The announcement

was made in April 2002 that the first human baby produced

by a human cloning program would be born in November,

sparking considerable medical and ethical controversy. In

August 2001, President George W. Bush struggled with the

debate over allowing federal funding for embryonic stem cell

research. He allowed funding for only existing lines of cells,

leaving further research in the hands of those who could

seek private funding.

Genetic engineering also presents positive and nega-

tive controversy in its application to agriculture and the

environment

. Recombinant DNA techniques help scien-

tists produce plants that offer medicinal value too. For exam-

ple, calcium-fortified orange juice or vitamin-enriched milk

boost the nutritional value of these foods. However, scientists

can now further genetically modify foods, offering benefits

and risks to the environment. The benefits mean possible

cheaper and easier production of certain medicines, while

the risks include unnatural introduction of plants and animals

produced in a laboratory environment. At the University

of Arizona, scientists have modified tomatoes to produce

vaccines for diarrhea and hepatitis B, and that these vaccines

will likely be much cheaper to produce than current drugs.

Some critics worry that once the new crops move out of the

safety of locked greenhouses and into crop fields, them may

cross-pollinate with conventional tomato crops and contami-

nate them with modified genes.

A poll reported on in 2002 showed that Americans

were fairly evenly split on their feelings about the risks and

benefits of genetically modified foods and

biotechnology

Most like the idea that scientists can create plants that will

help clean up toxic soils, reduce

soil erosion

and reduce

fertilizer

run-off into streams and lakes. They also favor

production of genetically engineered methods to reduce the

625

amount of water used to grow crops and development of

disease-resistant tree varieties to replace those that might be

threatened or endangered. Americans also favor use of ge-

netic engineering to reduce the need to log in native forests

and to reduce the amount of chemical pesticides used by

farmers. On the other hand, Americans express concern over

possible environmental effects of genetically modified plants

or fish contaminating ordinary plants and fish, reducing

genetic diversity, increasing the number of insects that might

become resistant to pesticides, or changing the

ecosystem