Life Science Flashcards
(260 cards)
1
Q
Human Genome Project (HGP)
A
A worldwide project, completed
in early 2000s, to determine the precise arrangement
of nucleotides in human DNA .
2
Q
fishes
A
Traditionally, a class of vertebrates that
breathe with gills rather than lungs, live in water, and
generally lay eggs, although some bear their young
alive.
Some biologists consider the fishes a “superclass,”
and divide them into three classes: bony fishes,
such as sunfish and cod; fishes with a skeleton formed
of cartilage rather than bone, such as sharks; and
fishes that lack jaws, such as lampreys.
✥ Fishes are cold-blooded animals.
3
Q
xenotransplantation (zen-uh-trans-plan-tay-shun)
A
The transplantation of animal tissue or organs from
one species to another.
✥ With the shortage of human organs available for
transplant, some work has been done to use pig and
nonhuman primate tissues and organs instead. Some
critics charge that this could lead to new, dangerous
forms of disease if a pathogen that now only affects
animals becomes communicable among humans.
4
Q
proteomics (pro-tee-ohm-iks)
A
A branch of genitics that studies the full set of protiens encoded by a genome.
Because
proteins are the product of information coded
for in DNA, proteomics is closely allied to the study of
the genome.
5
Q
gene mapping
A
The process of determining where
genes are located on individual chromosomes.
6
Q
phylum (feye-luhm) plur. phyla
A
One of the major divisions of the kingdoms of living things; the secondlargest standard unit of biological classification.
The
arthropods, chordates, and mollusks are phyla.
Phyla in the plant kingdom are frequently called divisions.
7
Q
physiology
A
The study of the function of living
things, including processes such as nutrition, movement,
and reproduction.
8
Q
arthropods (ahr-thruh-podz)
A
A phylum, or major division of the animal kingdom. Its ab animal with no internal spine, a body made of joined segments, and a shell. Examples re
insects, spiders, centipedes, and crustaceans. There
are more species of arthropods than of any other animal
phylum.
9
Q
sugars
A
Carbohydrates that can supply energy to
living things.
Common table sugar is sucrose. Some
other sugars are fructose, which is found in fruits; lactose,
which is found in milk; and glucose, which is
the most common sugar in the bodies of animals and
plants.
10
Q
aerobic (air-oh-bik)
A
In biology, a descriptive term
for organisms that require the presence of oxygen to
live. (Compare anaerobic.)
✥ Aerobic exercise, such as running, swimming, and
doing calisthenics for an extended time, is designed to
improve the body’s use of oxygen.
11
Q
homologous chromosomes (huh-mol-uh-guhs)
A
A pair of matching chromosomes in an organism, with
one being inherited from each parent.
12
Q
fauna (faw-nuh)
A
Animals, especially the animals of
a particular place and time.
13
Q
pollen
A
The male sex cells in plants.
In flowering
plants, pollen is produced in thin filaments in the flower
called stamens. (See fertilization and pollination.)
✥ When pollen is carried into the air by the wind, it
frequently causes allergic reactions (see allergy) in
humans.
14
Q
Egg
A
A female gamete.
15
Q
recombinant DNA technology (ree-kom-buh-nuhnt)
A
Techniques, usually associated with genetic engineering,
in which strands of DNA from different
sources are spliced together to form DNA for a new life
form. Gene splicing is another name for this process.
16
Q
reproductive cloning
A
The cloning of organisms
with the goal of planting the blastula produced by
the technique into the uterus of an adult female and
thus creating a new organism.
17
Q
second messenger
A
A molecule that relays messages
in a cell from a receptor on a cell membrane to
the final destination where an action within the cell is
to take place.
18
Q
family
A
In biology, the classification lower than an
order and higher than a genus.
Lions, tigers, cheetahs,
and house cats belong to the same biological family.
Human beings belong to the biological family of
hominids.
19
Q
ecological niche
A
The place or function of a given
organism within its ecosystem.
✥ Different organisms may compete for the same
niche. For example, in a forest there may be a niche for
an organism that can fly and eat nectar from blossoms.
This niche may be filled by some sort of bird, or an insect,
or even a mammal such as a bat.
20
Q
photosynthesis
A
Use by green plants of the energy
in sunlight to carry out chemical reactions, such as
the conversion of carbon dioxide into oxygen. Photosynthesis
also produces the sugars that feed the plant.
✥ Green plants depend on chlorophyll to carry
out photosynthesis.
21
Q
mutagen (myooh-tuh-juhn, myooh-tuh-jen)
A
Something that causes mutations in living things. Mutagens
include chemicals, such as drugs or toxins, and
radiation.
22
Q
Rachel Carson
A
An American author and scientist of the twentieth century who was fervently devoted to defending the natural world against pollution. Her best-known books are Silent Spring, concerning the overuse of pesticides and weed killers, and The Sea
Around Us.
✥ She is considered to be the founder of the modern
environmental movement.
23
Q
hominids (hom-uh-nidz)
A
The biological family that includes our species, Homo sapiens.
This family has also included Neanderthals and other forerunners of today’s humans, such as Australopithecus,
Homo erectus, and Homo habilis. Today’s human beings
are the only surviving hominids.
24
Q
glucose (glooh-kohs)
A
The most common form of sugar, found extensively in the bodies of living things; a molecule composed of carbon, oxygen, and hydrogen.
✥ Glucose is involved in the production of energy
in both plants and animals.
25
**nucleic acids** (nooh-klee-ik)
Organic molecules found in the nuclei of cells. DNA and RNA, the bestknown nucleic acids, govern heredity and the chemical processes in the cell.
26
**organic compounds**
The compounds containing
carbon that are typically found in living systems.
✥ Generally, anything made from living systems,
such as cloth, fuels, or wood, is said to be organic. Organic
foods are grown with no fertilizer except the
organic compounds found naturally in plants and animals.
27
**ribosome** (reye-buh-sohm)
A small, ball-like structure
in the cell, made of proteins and RNA molecules,
that serves as a platform on which the cell’s proteins
are made.
28
**stamen** (stay-muhn)
The organ of a flower on
which the pollen grows.
29
**metamorphosis** (met-uh-mawr-fuh-sis)
A change in an animal as it grows, particularly a radical change, such as the transformation of a caterpillar into a butterfly.
30
**Watson and Crick**
The two twentieth-century biologists
(James D. Watson of the United States and
Francis H. C. Crick of England) who discovered the
double helix of DNA.
31
**sexual reproduction**
The production of a new living
thing by two parent organisms, with each parent
contributing half the material in the DNA of the
offspring. The young, genetically different from either
parent, can rapidly adapt to their environment by
means of natual selection.
32
**stem cell**
A cell from which a variety of other cells
can develop through the process of cellular differentiation.
Stem cells can produce only a certain
group of cells (as with skin stem cells) or any cell in the
body (as with embryonic stem cells).
✥ A major controversy involves the question of
whether nonembryonic stem cells should be used for
medical purposes.
33
**chloroplast**
A chlorophyll-containing organelle found in algal and green plant cells.
34
**therapeutic cloning**
A type of cloning with the goal
of harvesting embryonic stem cells from the resulting
blastula to grow tissues and other biological
products with therapeutic value.
35
**vertebrates** (vur-tuh-bruhts, vur-tuh-brayts)
Animals that have a spinal cord enclosed in a backbone.
✥ The five traditional classes of vertebrates are amphibians,
birds, fishes, mammals, and reptiles.
✥ Human beings are vertebrates.
36
**gene therapy**
A promising technology that involves
replacing a defective gene in the body with a healthy
one. This can be done by removing cells from the
body, using genetic engineering techniques to
change defective sequences in the DNA, and then reinserting
the cells.
This technique has been carried out
successfully, for example, on bone marrow cells, in
which defective cells were successfully replaced with
healthy, genetically engineered cells. Scientists hope to find an agent, such as a therapeutic virus, that will
be able to correct defective DNA in situ.
37
**DNA polymerase** (pol-uh-muh-rays)
An enzyme that assembles new DNA by copying an existing
strand.
38
**deciduous trees and shrubs** (di-sij-ooh-uhs)
Trees and shrubs that, unlike evergreens, lose their leaves and become dormant during the winter.
39
**horticulture** (hawr-tuh-kul-chuhr)
The science of cultivating garden plants.
40
**transgenic species**
An organism that has had part
of another species’ genome transferred into its own
through the techniques of genetic engineering.
41
**genomics** (juh-noh-miks)
The field of science that
studies the entire DNA sequence of an organism’s genome.
The goal is to find all the genes within each
genome and to use that information to develop improved
medicines as well as answer scientific questions.
42
**synapse** (sin-aps, si-naps)
A gap between two
nerve cells.
Nerve signals are sent across the gap by
neurotransmitters.
43
**invertebrates** (in-vur-tuh-bruhts, in-vur-tuh-brayts)
Animals without backbones.
44
**genetic drift**
A term that describes the random fluctuations
in a gene pool over time. In large populations,
the effects of genetic drift are negligible.
45
**phloem** (floh-em)
The system of vessels in a plant
that carries food from the leaves to the rest of the plant.
46
**cytoskeleton**
The inner structural elements, or backbone, of a cell. It consists of microtubules and various filaments that spread out through the cytoplasm, providing both structural support and a means of transport within the cell.
47
**active site**
The part of an enzyme or antibody
where a chemical reaction occurs.
48
**adaptation**
The changes made by living systems in
response to their environment.
\*Heavy fur, for example,
is one adaptation to a cold climate.
49
**sex-linked trait**
A trait associated with a gene that
is carried only by the male or female parent.
✥ In humans, the gene for colorblindness is carried
by the X-chromosome.
50
**proteins** (proh-teenz, proh-tee-inz)
Complex organic molecules made up of amino acids. Proteins
are basic components of all living cells and are therefore
among the principal substances that make up the
body.
In addition to being necessary for the growth
and repair of the body’s tissues, proteins provide energy
and act as enzymes that control chemical reactions
in the cell.
✥ Foods that contain a high percentage of protein
include meat, fish, poultry, milk products, beans, and
nuts.
51
**microorganisms**
Organisms so small that they can
be seen only through a microscope.
52
**DNA repair**
The way in which a cell corrects potentially
damaging or mutagenic errors in its DNA.
(See mutagen.) DNA bases may be directly replaced by
enzymes, or part of a strand may be replaced by enzymes
using its opposite, paired strand as a template.
53
**genome** (jee-nohm)
The sum of all information contained in the DNA for any living thing. The sequence of all the nucleotides in all the chromosomes of an organism.
54
**natural selection**
A process fundamental to evolution
as described by Charles Darwin. By natural selection,
any characteristic of an individual that allows
it to survive to produce more offspring will eventually
appear in every individual of the species, simply because
those members will have more offspring.
✥ The expression survival of the fittest was used to
describe this process in the nineteenth century but is
not favored by modern scientists.
55
**bacilli** (buh-sil-eye) sing.
They are One of three
forms of bacteria, usually rod-shaped.
56
**eugenics** (yooh-jen-iks)
The idea that one can improve
the human race by careful selection of those who
mate and produce offspring.
✥ Eugenics was a popular theory in the early twentieth
century but is no longer taken seriously, primarily
because of the horrors of the eugenic efforts of the
Nazi regime in Germany.
57
**B-Cell**
One of two main types of immune system lymphocytes (compare T-cell). B-cells originate and develop in the bone marrow and circulate throughout the blood and lymph fluids, recognizing foreign bacteria, viruses, and toxins and binding to them to facilitate their disposal by other cells.
58
**zoology** (zoh-ol-uh-jee)
The scientific study and
classification of animals.
59
**Charles Darwin**
A British naturalist of the nineteenth
century. He and others developed the theory
of evolution. This theory forms the basis for the
modern life sciences. Darwin’s most famous books are
The Origin of Species and The Descent of Man.
✥ Darwin’s ideas were later misrepresented by some
social theorists, who developed the notion of Social Darwinism to justify practices such as child labor in
nineteenth-century England.
60
**carbon cycle**
In ecology, the movement of atoms of carbon through the biosphere. Molecules of carbon dioxide are taken in by plants, to be incorporated into their tissues, which may then be eaten by and incorporated into animals. Animals return the
carbon to the air in the form of carbon dioxide, and
the cycle starts again. (See photosynthesis and respiration.)
61
**population genetics**
The study of the genetic composition
of populations in order to understand the
evolutionary forces that select for a particular gene.
62
**embryology** (em-bree-ol-uh-jee)
The study of the embryo; a major field of research in modern biology
63
**Cell**
The basic unit of all living things except viruses. In advanced organisms, cells consist of a nucleus (which contains genetic material), cytoplasm, and organelles, all of which are surrounded by a cell membrane.
✥ Groups of cells with similar structure and function
form tissues.
64
**symbiosis** (sim-bee-oh-sis, sim-beye-oh-sis)
The
process by which two organisms live together, usually
to their mutual benefit. An example of a symbiotic pair
are cows and the bacteria that live in their digestive
tracts, enabling them to digest cellulose in grass.
65
**action potential**
The rapid change in electric potential
that parts of a nerve cell undergo when a nerve impulse
is generated.
\*Unlike ordinary electric current,
which consists of the flow of electrons, the action
potential involves the movement of sodium and potassium
ions across the cell membrane.
66
**birds**
A class of vertebrates distinguished by their feathers and their two legs and two wings. Birds are warm-blooded animals, and their young hatch from eggs.
✥ Some scientists argue that modern birds are descended
from the dinosaurs.
67
**anabolism**
The chemical reactions that synthesize
molecules in metabolism. (Compare catabolism.)
✥ Athletes often attempt to improve their performance
by speeding up this molecule-building process
through the use of drugs called anabolic steroids, despite
the potential health risks involved. This use is
outlawed in many athletic competitions, such as the
Olympic Games.
68
**fungi** (fun-jeye, fung-geye) sing. fungus
Plantlike organisms lacking chlorophyll, such as mushrooms,
molds, yeasts, and mildews.
Modern biologists tend to
place fungi in their own kingdom, not in the plant
kingdom, because they get their nutrients from other
living things (or from the remains of living things that
have died) rather than from photosynthesis.
69
**nitrogen fixing**
The conversion of atmospheric nitrogen
(which plants cannot absorb) into forms of
nitrogen that plants can absorb.
Bacteria in the topsoil
carry out the conversion.
70
**carbohydrates**
Substances composed of long chains of oxygen, hydrogen, and carbon molecules. Sugar, starch, and cellulose are all carbohydrates. In the human body, carbohydrates play a major role in respiration; in plants, they are important in photosynthesis.
✥ Carbohydrates in food provide energy for the
body and, if present in excess, are stored as fat.
71
**blastula** (blas-chuh-luh)
The stage of an embryo that consists of just over a hundred cells — a stage reached about one week after fertilization. At this stage the cells are just at the very beginning of cellular
differentiation and are said to be totipotent (See totipotency).
✥ This is the stage of development where embryonic
stems cells can be harvested for medical research.
72
**developmental biology**
The study of the processes by which an organism develops from a zygote to its full structure. This field includes the study of cellular differentiation as well as body structure development.
73
**metabolism** (muh-tab-uh-liz-uhm)
The total of the chemical reactions that maintain the life of a living thing.
✥ In humans, metabolism is related to the intake
and use of food; persons with a high metabolism can
eat more without gaining weight.
74
**genetics**
The study of heredity, or how the characteristics
of living things are transmitted from one generation
to the next.
Every living thing contains the genetic
material that makes up DNA molecules. This
material is passed on when organisms reproduce. The
basic unit of heredity is the gene.
75
**kingdom**
In biology, the largest of the divisions of
living things. The best-known kingdoms are those of
the plants and animals.
Modern biologists recognize
three additional kingdoms:
Monera (or Prokaryotae)(for example, bacteria and blue-green algae), Protoctista
(for example, red algae, slime molds, and
amoebas and other protozoa), and fungi.
76
**dioxin** (deye-ok-sin)
A group of pollutants created
as by-products in many industrial processes.
Dioxins accumulate in human tissue and affect human metabolism.
They are carcinogens. Eliminating dioxins is
an important goal of environmental policy.
77
**biosphere** (beye-uh-sfeer)
The thin outer shell of the Earth and the inner layers of its atmosphere; the place where all living systems are found.
78
**mollusks** (mol-uhsks)
A phylum of invertebrates
with soft bodies and muscular feet. Some mollusks
also have hard shells.
\*Oysters, clams, snails, slugs,
octopuses, and squid are mollusks.
79
**gonads** (goh-nadz)
The organs in animals that produce
sex cells:
ovaries in the female,
testes in the male.
80
**central dogma of molecular biology**
Describes a key assumption of molecular biology, namely, that
each gene in the DNA molecule carries the information needed to construct one protein, which, acting as
an enzyme, controls one chemical reaction in the cell.
81
**hybridization**
Producing offspring from parents of
different stock.
✥ Hybridization is used extensively in agriculture,
where new forms of hardy and disease-resistant plants
are produced commercially.
82
**genus** (jee-nuhs)
In biology, the classification lower
than a family and higher than a species.
Wolves belong to the same genus as dogs. Foxes belong to a
different genus from that of dogs and wolves, but to the same family.
83
**coevolution**
The process that occurs when two species
influence each other during evolution. For example,
an insect may evolve specialized parts that allow
it to feed on a specific flower, whereas the flower
evolves to facilitate pollination by that particular insect.
84
**morphology** (mawr-fol-uh-jee)
The study of the structure of living things.
85
**P53 gene**
A gene that is thought to play a role in
regulating cell death or apoptosis, in suppressing tumors,
in regulating the cell cycle, and in stopping the
cell from dividing when the DNA is damaged.
86
**gel electrophoresis** (i-lek-toh-fuh-ree-sis)
A technique used in DNA fingerprinting and other processes
in which large molecules are to be identified.
Fragments of DNA are placed in a semiporous gel, and
an electrical field is turned on. The fragments
move in response to the field, with smaller fragments
generally moving faster. After a time, the fragments
have separated enough to form a series of separated
lines like a bar code that characterizes the DNA.
87
**genetic engineering**
The manipulation of DNA to
produce new types of organisms, usually by inserting
or deleting genes.
✥ Genetic engineering has been developed commercially,
with uses such as producing human insulin or
bacteria that will keep plants from freezing in a mild
frost. It is also used to produce genetically modified
organisms.
✥ U.S. courts have ruled that the products
of genetic engineering can be patented.
✥ There is often controversy about the risk involved in releasing genetically engineered organisms into the environment.
88
**evolution**
A theory first proposed in the nineteenth
century by Charles Darwin, according to which
the Earth’s species have changed and diversified
through time under the influence of natural selection.
Life on Earth is thought to have evolved in three
stages. First came chemical evolution, in which organic
molecules were formed. This was followed by
the development of single cells capable of reproducing
themselves. This stage led to the development of
complex organisms capable of sexual reproduction.
Evolution is generally accepted as fact by scientists
today, although debates continue over the precise
mechanisms involved in the process.
✥ The first cell is thought to have been formed when
the Earth was less than a billion years old.
89
**meristem** (mer-i-stem)
The region on a plant
where division of cells (and hence growth) occurs.
\*Usually, meristems are found in the shoots and root
tips, and places where branches meet the stem. In trees,
growth occurs in the cambium — the layer just beneath
the bark.
90
**Dolly**
The first mammal successfully cloned —
Dolly, a sheep — was born in 1996 in Scotland as the
result of work by biologist Ian Wilmut .
The procedure that produced Dolly involved removing
the nucleus from an egg cell and placing the
nucleus of an adult sheep’s mammary cell into it. Further
manipulations caused the egg to “turn on” all
genes and develop like a normal zygote.
91
**anatomy**
The structure of an animal or plant; also,
the study of this structure through techniques such as
microscopic observation and dissection. (Compare
morphology and physiology.)
92
**therapeutic virus**
A virus created in the laboratory
whose function is to transfer DNA into the cells of an
organism in the process of gene therapy.
93
**cambium** (kam-bee-uhm)
The layer of a tree where growth occurs, just under the bark.
94
**organic molecules**
The smallest units of organic
compounds. Important examples of these molecules
are based on chains of carbon atoms and come in
four major categories: proteins, carbohydrates,
lipids, and nucleic acids (DNA and RNA).
95
**warm-blooded animals**
Animals, such as mammals and birds, that maintain a constant body temperature
regardless of the temperature of the surroundings.
96
**“junk” DNA**
Segments of DNA along a chromosome
that are not genes, do not code for anything that
we know of, and whose purpose we do not understand.
Approximately ninety-five percent of the human genome
falls into this category. The term junk may be
misleading, however, as this DNA may have other
functions, such as regulating genes during development.
✥ Some scientists speculate that junk DNA may be
archaic material left over from an earlier stage of evolutionary development.
97
**exon**
Stretches of DNA in genes that code for proteins.
In eukaryotes, exons in a given gene are generally
separated from each other by stretches of DNA
that do not contain instructions for constructing proteins.
98
**carnivore** (kahr-nuh-vawr)
A living thing that eats meat.
\*Among mammals, there is an order of carnivores,
including primarily meat-eating animals such as
tigers and dogs. Some plants, such as the Venus’sflytrap,
are carnivores.
99
**reptiles**
```
A class of scaly vertebrates that usually
reproduce by laying eggs.
```
\*Lizards, snakes, turtles, and
alligators are reptiles. Reptiles are cold-blooded animals.
✥ The dinosaurs were reptiles.
100
**cold-blooded animals**
Animals, such as reptiles, fishes, and amphibians, that cannot control their body temperature and therefore become sluggish in cold weather. (Compare warm-blooded animals.)
✥ Cold-blooded animals are often seen sunning
themselves to warm up.
101
**biophysics**
The study of living things using the techniques
of physics.
102
**pheromones** (fer-uh-mohns)
Small molecules that,
when released by one organism, act as chemical signals
to induce a certain behavior in another organism.
Scents that attract animals to each other in a mating
process are an example of pheromones.
103
**lipids** (lip-idz, leye-pidz)
A group of organic molecules
that includes fats, oils, and waxes.
Lipids do not dissolve in water. In animals, including humans, lipids store energy and form parts of cell structures, such
as cell membranes.
104
**cell membrane**
The structure separating an animal cell from its environment or a plant cell from its cell wall.
\*The cell membrane is a complex system that allows nutrients to enter the cell and waste products to leave, usually through osmosis
105
**primates** (preye-mayts)
The order of mammals
that includes monkeys, apes, and human beings.
Primates
are distinguished from other animals in that
they generally possess limbs capable of performing a
variety of functions, hands and feet adapted for grasping
(including opposable thumbs), flattened snouts,
and other anatomical features.
106
**protozoa** (proh-tuh-zoh-uh)
Single-celled animals,
such as amoebas, that are the most primitive
form of animal life.
In modern biology, they are classified
in the kingdom of Protoctista rather than in
the animal kingdom.
✥ Some protozoa are parasites and may be pathogenic,
causing diseases such as malaria and dysentery.
107
**homeotic genes**
Genes that control the development of an animal’s body plan.
108
**carrying capacity**
In ecology, the number of living things that can exist for long periods in a given area without damaging the environment.
109
**Australopithecus** (aw-stray-loh-pith-i-kuhs, awstray-
loh-pi-thee-kuhs)
An extinct genus of the hominid family that lived in Africa from about three
to one million years ago. The name means “southern ape.”
✥ Members of this genus were the ancestors of modern
humans. One of the best-known fossils, Lucy, was
a member of this genus.
110
**mutations**
Changes in chromosomes or genes
that cause offspring to have characteristics different
from those of their parents.
\*Mutations can be caused
by the effects of chemicals, radiation, or even ordinary
heat on DNA. Mutations produce some of the
differences between memb
111
**apoptosis** (ap-uhp-toh-sis)
The programmed death of a cell. Scientists believe that this process is governed by chemical signals a given cell receives from its neighbors.
\
✥ It is thought some forms of cancer may result
when this process of cell death is somehow interrupted,
allowing cells to grow unchecked, with the result
being a cancerous tumor.
112
**respiration**
The conversion of oxygen by living
things into the energy by which they continue life.
Respiration is part of metabolism.
✥ Carbon dioxide is a waste product of respiration.
113
**nitrogenous wastes** (neye-troj-uh-nuhs)
```
Animal wastes (particularly urine) that contain materials high
in nitrogen content.
```
✥ Nitrogenous waste can be valuable as fertilizer
114
**order**
```
In biology, the classification lower than a
class and higher than a family.
```
Dogs and cats belong
to the order of carnivores; human beings, monkeys,
and apes belong to the order of primates. Flies and
mosquitoes belong to the same order; so do birch trees
and oak trees.
115
**biology**
The study of life and living systems.
116
**asexual reproduction** (ay-sek-shooh-uhl)
The kind of reproduction in which it is not necessary to have
two parents to produce offspring. The reproduction of
single-celled organisms through fission, and the
production of spores in some plants and plantlike organisms,
are examples of this reproduction.
117
**bacteria** sing. bacterium
Microorganisms made up of a single cell that has no distinct nucleus. They reproduce by fission or by forming spores.
✥ Some are beneficial to humans (for example,
those that live in the stomach and aid digestion),
and some are harmful (for example, those that
cause disease).
118
**embryo** (em-bree-oh)
A developing plant or animal.
A plant embryo is an undeveloped plant inside a seed.
An animal embryo is the animal as it develops from the
single cell of the zygote until birth. Among humans
and most other mammals, the embryo is carried in the
mother’s womb.
✥ The term is occasionally used to denote a new or
developing idea or project: “The idea for the complete
theory was already present in his work, in embryo
form, in 1950.”
119
**hibernation**
Passing the winter in a sleeping or inactive
condition.
\*Bears, ground squirrels, woodchucks,
and several other kinds of animals hibernate.
120
**balance of nature**
A concept in ecology that describes natural systems as being in a state of equilibrium, in which disturbing one element disturbs the entire system. The inference is usually drawn that the natural state of any system is the preferred state and
that it is best to leave it undisturbed. Modern ecologists
no longer believe that a balance of nature exists.
121
**gene splicing**
A term used to refer to the process by
which the DNA of an organism is cut and a gene, perhaps
from another organism, is inserted.
Gene splicing
is often used in industry to allow single-celled organisms
to produce useful products, such as human
insulin. It is also used in the production of genetically
modified organisms
122
**dominant trait**
In genetics, a trait that will appear
in the offspring if one of the parents contributes it.
✥ In humans, dark hair is a dominant trait; if one
parent contributes a gene for dark hair and the other
contributes a gene for light hair, the child will have
dark hair
123
**fertilization**
The joining of sex cells to form a new
living thing.
In humans, a male sperm joins a female
ovum, or egg; the resulting zygote divides into a multicelled
structure that implants in the womb and grows
into an embryo.
In plants, pollen grains, containing
the male sex cells, enter the female sex cells in the pistil;
from this union, fruit eventually grows.
When
fertilization occurs within a single flower, we call it
self-fertilization.
124
**anaerobic** (an-uh-roh-bik, an-air-oh-bik)
A descriptive term for a process, such as fermentation, that can
proceed only in the absence of oxygen, or a living
thing that can survive only in the absence of oxygen.
(Compare aerobic.)
125
**prokaryotes** (proh-kar-ee-oht)
Organisms whose cells do not have a nucleus in which DNA is housed and which lack many of the organelles found in more
advanced cells.
The kingdom of Monera or Prokaryotae
is composed of single-celled prokaryotes.
✥ It is thought that prokaryotes were the first cells to
appear on Earth.
126
**repetitive DNA**
Stretches of DNA that repeat themselves
throughout a genome, either in tandem or interspersed
along the genome.
These stretches can comprise
up to fifty percent or more of an organism’s
DNA. It can code for an end product, perhaps a certain
enzyme that is needed in large numbers; it can have a
structural function (such as a telomere); or it can
comprise sequences with no known function.
127
**transposon** (trans-poh-zon)
Segments of DNA that
shift from one area of a genome to another.
Previously
called jumping genes.
128
**double helix (hee-liks)**
The shape taken by the DNA
molecule. A helix is a three-dimensional spiral, like
the shape of a spring or the railing on a spiral staircase.
A DNA molecule consists of two helixes intertwined.
129
**missing link**
A supposed animal midway in evolution
between apes and humans.
The term is based on
a misunderstanding about the theory of evolution,
which does not state that humans are descended from
apes, but rather maintains that both humans and apes
descended from a common ancestor. Modern evolutionary
scientists do not search for a “missing link.”
130
**botany**
The scientific study and categorization of
plants. (See fruit, photosynthesis, and plant kingdom.)
131
**pollination**
The carrying of pollen grains (the
male sex cells in plants) to the female sex cells for
fertilization.
Pollination can occur between plants
when pollen is carried by the wind or by insects such as
the honeybee (see cross-fertilization), or within the
same plant, in which case it is called self-fertilization.
132
**cellular respiration**
The chemical process that generates most of the energy in the cell, supplying molecules needed to make the metabolic reactions of an organism run.
✥ The main carrier of energy in metabolism is the
molecule ATP.
133
**biochemical pathways**
In biology, the long chains of chemical reactions that take place in the normal operation of living systems.
134
**axon**
The part of a nerve cell or neuron that
transfers a nerve impulse from the nerve cell body to a
synapse with another cell.
Depending on the location of the cell, the length of an
axon can vary widely. In some cases they may be several feet
long.
135
**regenerative medicine**
A term applied to new medical
advances in which an understanding of the human
genome allows us to use the body’s own mechanisms
to heal it.
Expected advances include a host of new
pharmaceuticals and, eventually, the ability to create
new tissues for transplant. (See embryonic stem
cell.)
136
**gamete**
A reproductive cell having a single set of
chromosomes, especially a mature sperm or egg.
137
**embryonic stem cell**
Cells obtained from an embryo
in the blastula phase, when they are still only a
few days old.
Because they have only begun to differentiate,
these cells have the capability of developing into
any cell in the human body, a fact which makes them
potentially important in medicine.
138
**recessive trait**
In genetics, a trait that must be contributed
by both parents in order to appear in the
offspring.
Recessive traits can be carried in a person’s
genes without appearing in that person.
\*For example, a dark-haired person may have one gene for dark hair,
which is a dominant trait, and one gene for light
hair, which is recessive. It is thus possible for two darkhaired
parents to have a light-haired child, provided
each parent contributes a gene for light hair.
139
**protoplasm** (proh-tuh-plaz-uhm)
The jellylike material
in a cell, both inside and outside the nucleus,
where the chemical reactions that support life take
place.
140
**mitosis** (meye-toh-sis)
Division of a single cell into two identical “daughter” cells. Each daughter cell has an identical number of chromosomes as the parent
cell. Mitosis begins when the DNA in the parent
cell replicates itself; it ends with two cells having the
same genes. Most cells in the human
body, and all single-celled organisms, reproduce
through mitosis.
141
**pistil** (pis-tuhl)
The female part of a plant.
In flowering
plants, it is at the center of the flower. When
fertilized with pollen, the pistil develops into fruit.
142
**gene**
A portion of a DNA molecule that serves as
the basic unit of heredity. Genes control the characteristics
that an offspring will have by transmitting information in the sequence of nucleotides on short
sections of DNA.
143
**organ**
Part of a living thing, distinct from the other
parts, that is adapted for a specific function.
Organs are
made up of tissues and are grouped into systems, such
as the digestive system.
✥ The brain, liver, and skin are organs.
144
**fermentation**
A chemical reaction in which sugars
are broken down into smaller molecules that can
be used in living systems.
Alcoholic beverages, such as
beer, wine, and whiskey, are made from the controlled
use of fermentation. Fermentation is an anaerobic
process.
145
**cytoplasm** (seye-tuh-plaz-uhm)
The material within a biological cell that is not contained in the nucleus or other organelles.
146
**memory cell**
A cell in the immune system that,
when exposed to an invading pathogen, replicates itself
and remains in the lymph nodes searching for the
same antigen, resulting in a more efficient and rapid
response to any subsequent attack.
✥ The creation of memory cells is one of the main
goals of vaccination.
147
**PCR** (polymerase chain reaction)
A laboratory technique that can amplify the amount of DNA from a tiny sample to a large amount within just a few hours.
Theoretically, PCR can take one molecule and produce
measurable amounts of identical DNA in a short
period of time. It is used in DNA fingerprinting and DNA sequencing.
✥ An enzyme used in this process was originally
found in bacteria in hot springs.
148
**clone**
A living system that is genetically identical to
its ancestor (that is, it has exactly the same DNA molecules).
Because each cell contains the DNA molecules
that characterize an individual, it is, in principle,
possible to replicate, or reproduce, complex living systems
in the laboratory.
✥ The first cloned mammal, a sheep named Dolly,
was born in Scotland in 1996. DNA from an adult donor
was placed into an egg, which was then implanted
in the uterus of another sheep. Since that time, mice,
cows, and pigs have been cloned.
✥ There is a major
debate on the ethical aspects (see bioethics) of cloning,
especially as applied to human beings. Therapeutic
cloning involves the placing of adult DNA in an
egg for the express purpose of creating stem cells for
medical purposes. Reproductive cloning involves
the placement of adult DNA into an egg and the implantation
of the egg into a uterus for the purpose of
creating a viable fetus.
✥ Clone is often used informally
to indicate a close copy or resemblance: “This
new computer is a clone of the IBM model.”
149
**root**
In biology, the part of a plant that grows
downward and holds the plant in place, absorbs water
and minerals from the soil, and often stores food.
The main root of a plant is called the primary root;
others are called secondary roots. The hard tip is called
the root cap, which protects the growing cells behind it.
Root hairs increase the root’s absorbing surface.
150
**intron**
A stretch of DNA in a gene that does not
code for proteins.
In eukaryotes, introns in a given
gene separate stretches of DNA that contain instructions
for constructing proteins.
151
**organelles** (awr-guh-nelz)
Parts of a cell that store
food, discharge waste, produce energy, or perform
other functions analogous to what organs do in large
living things.
152
**cladistics** (kluh-dis-tiks)
A method of taxonomic classification that groups organisms according to their lines of evolutionary descent. All descendants of a given organism are called a clade.
153
**blood-brain barrier**
The separation of the brain, which is bathed in a clear cerebrospinal fluid, from the bloodstream. The cells near the capillary beds external to the brain selectively filter the molecules that are allowed to enter the brain, creating a more stable, nearly pathogen-free environment.
✥ Oxygen, glucose, and white blood cells are
molecules that are able to pass through this barrier.
Red blood cells cannot.
154
**biochemistry**
The study of the structure and interactions
of the complex organic molecules found in
living systems.
155
**nucleotides** (nooh-klee-uh-teyedz)
The molecules that form the basic modular structure of the double helix of the DNA molecule. A nucleotide consists of
three molecules — a sugar, a phosphate group, and a
molecule called a base.
If the double helix is a twisted
ladder, the sugar and phosphates form the sides of the
ladder and pairs of bases form the rungs. There are
four different bases, usually abbreviated A, C, G, and T
for adenine, cytosine, guanine, and thymine). The order
of bases in DNA determines the genetic code.
156
**The Origin of Species** (1859)
A book by Charles Darwin explaining his theory of evolution.
It provoked great controversy; by casting doubt on the historical accuracy of the biblical accounts of Creation,
it caused many believers to question their faith
in Christianity.
157
**cellulose** (sel-yuh-lohs)
A stringy, fibrous substance that forms the main material in the cell walls of plants. Cellulose is an organic molecule, composed of carbon, hydrogen, and oxygen.
158
**cellular differentiation**
The process by which a cell becomes specialized in order to perform a specific function, as in the case of a liver cell, a blood cell, or a neuron.
There are more than 250 general
types of cells in the human body. Differentiation is the
process that takes place inside an embryo that deterlmines which genes are expressed and hence what type
of cell will result.
✥ The ability of embryonic stem cells to undergo
differentiation into any cell in the body is what makes
them a focus of modern research.
159
**virus** (veye-ruhs) plur. viruses
Microorganisms consisting of DNA and RNA molecules wrapped in a protective coating of proteins.
Viruses are the most primitive form of life. They depend on other living
cells for their reproduction and growth.
✥ Viruses cause many diseases.
160
**crustacean** (kru-stay-shuhn)
A class of arthropods with shells.
✥ Crabs, lobsters, shrimp, and crayfish are crustaceans.
161
**enzyme** (en-zeyem)
A protein molecule that helps
other organic molecules enter into chemical reactions
with one another but is itself unaffected by these
reactions.
\*In other words, enzymes act as catalysts for
organic biochemical reactions.
162
**DNA** (deoxyribonucleic acid)
The molecule that carries genetic information in all living systems . The DNA molecule is formed in the
shape of a double helix from a great number of
smaller molecules (see nucleotides). The workings of
the DNA molecule provide the most fundamental explanation of the laws of genetics.
DNA acts in three important way. First, when a cell
divides, the DNA uncoils, and each strand creates a
new partner from the surrounding material — a process
called replication. The two cells that result from the
cell division have the same DNA as the original ().
Second, in sexual reproduction, each parent
contributes one of the two strands in the DNA of the offspring. Third, inside the cell, the DNA governs the
production of proteins and other molecules essential
to cell function.
163
**genotype**
A combination of alleles situated on
corresponding chromosomes that determines a specific
trait.
164
**basal metabolism** (bay-suhl, bay-zuhl muh-tab-uhliz-
uhm)
The rate at which an inactive, resting organism
expends energy.
165
**Gregor Mendel** (men-dl)
An Austrian biologist and
monk of the nineteenth century. Mendel discovered
the basic laws of genetics by doing experiments with
pea plants.
166
**bioethics**
The application of ethics to the science and practice of biology, especially as modern science is applied to human life and reproduction.
✥ With the advent of cloning and research on embryonic
stem cells, bioethics has become an important
branch of scientific inquiry.
167
**artificial selection**
The breeding of plants and animals
to produce desirable traits. Organisms with the
desired traits, such as size or taste, are artificially mated
or cross-pollinated with organisms with similar desired
traits.
168
**Carolus Linnaeus** (li-nee-uhs, li-nay-uhs)
A Swedish biologist of the eighteenth century. Linnaeus
originated our present scheme of classification of living
things.
Linnaeus started the standard scientific
practice of referring to animals and plants by genus
and species whereby, for example, people are Homo
sapiens and sugar maple trees are Acer saccharum.
169
**protein folding problem**
One of the main unsolved
problems in molecular biology, the protein folding
problem deals with the question of predicting the
three-dimensional shape of a protein molecule
from the sequence of amino acids that make up the
protein molecules.
The main difficulty arises from the
large number of interactions between different atoms
in a protein molecule, a number that lies outside of the
ability of modern computers to handle. (See also protein
structure and proteomics.)
170
**fluid mosaic model**
A model that describes the
structure of cell membranes.
In this model, a flexible
layer made of lipid molecules is interspersed with
large protein molecules that act as channels through
which other molecules enter and leave the cell.
171
**herbivore** (hur-buh-vawr, ur-buh-vawr)
A living thing that eats only plants.
\*Cattle, sheep, and horses are herbivores.
172
**ecology**
The study of living things, their environment,
and the relation between the two.
173
**nucleus** plur. nuclei
In biology, the central region
of the cell, in which DNA is stored.
The nucleus usually
appears as a dark spot in the interior of the cell.
Primitive cells (such as bacteria and blue-green algae)
have no nuclei.
174
**RNA**
*ribonucleic acid* One of a group of molecules similar in structure
to a single strand of DNA. The function of RNA is
to carry the information from DNA in the cell’s nucleus
into the body of the cell, to use the genetic
code to assemble proteins, and to comprise part of
the ribosomes that serve as the platform on which
protein synthesis takes place.
175
**Homo** (hoh-moh)
The genus to which human beings belong.
The genus Homo includes Neanderthals and other hominids closely related to today’s humans, such as Homo erectus.
176
**telomere** (tel-uh-meer)
The long end sequences of
a DNA strand occuring at the tip of the chromosomes;
a type of repetitive DNA that usually consists
of one hundred to fifteen hundred copies of a single
DNA sequence.
✥ During DNA replication, small parts of the
telomere are lost with each cycle. Scientists think that
this loss may be related to the aging process.
177
**deforestation** (dee-fawr-uh-stay-shuhn)
The process of destroying a forest and replacing it with something else.
The term is used today to refer to the destruction of forests by human beings and their
replacement by agricultural systems.
✥ Deforestation is considered to be a main contributor
to the greenhouse effect.
178
**taxonomy** (tak-son-uh-mee)
The classification of
living things.
179
**cell wall**
The rigid outer covering of a typical plant cell, composed mainly of cellulose and lying outside the cell membrane. Animal cells do not have cell walls.
✥ It is the cell walls that give plant stems and wood
their stiffness.
180
**chlorophyll** (klawr-uh-fil)
The complex chemical that gives a plant its green color and plays an important role in the conversion of sunlight into energy for the plant.
181
**in vitro** (in vee-troh)
In the laboratory; literally, “in
glass” (laboratory experiments are often carried out in
glass containers). In vitro conditions are distinguished
from conditions that actually apply in nature.
✥ In vitro appears in the expression in vitro fertilization,
a way of producing human embryos in a laboratory.
182
**heredity**
The passing of characteristics from parents
to children.
183
**habitat**
The area or type of environment in which a
particular kind of animal or plant usually lives.
184
**mitochondrion** (meye-tuh-kon-dree-uhn)
The cell organelle where much of cellular respiration
takes place; the “power plant” of the cell.
✥ Mitochondria probably entered eukaryotes by
an act of endosymbiosis, in which one simple cell was
absorbed by another.
✥ Mitochondria contain their
own DNA. It is by tracing the mitochondrial DNA,
which individuals inherit only from their mothers, that
genetic linkeages are often traced.
185
**cleavage**
The process by which an animal cell divides into two daughter cells after mitosis. In an embryo, this process is repeated many times and leads to the formation of the blastula.
186
**karyotype** (kar-ee-uh-teyep)
The complete set of chromosomes that constitutes the entire genome of a species.
The human karyotype contains forty-six
chromosomes, twenty-three from each parent. This set is contained in the nucleus of almost every cell in
the body
187
**sex chromosomes**
The two chromosomes in each
body cell of a living thing that determine what sex
it is.
✥ As with other pairs of chromosomes, one of the
sex chromosomes is contributed by each parent; they
are of two types, X and Y. The mother supplies only an
X-chromosome, but the father can transmit both the
X- and the Y-chromosome. At fertilization, if the father’s
sperm is also carrying an X-chromosome, the
child will be female. If the father’s sperm is carrying a
Y-chromosome, the child will be male.
188
**plasmid**
A circular bacterial DNA, sometimes used
as a vector for gene insertion or genetic engineering.
Plasmids are often the site of genes that code for
resistance to antibiotics.
189
**T-cell**
Key to the immune system, these cells originate
in the bone marrow but mature in the thymus
(the t stands for thymus).
T-cells attack other body
cells that are infected by some bacteria, a virus, or
another pathogen. (Compare B-cell.)
✥ The HIV/AIDS virus destroys a type of T-cell,
leading to the syndrome characterized by a defective
immune system.
✥ T-cell counts are used as a diagnostic
test to indicate the strength of the immune system
in AIDS patients.
190
191
**molecular biology** (muh-lek-yuh-luhr)
The branch of science devoted to studies of the structure, function, and reactions of DNA, RNA, proteins, and other
molecules involved in the life processes.
192
**taproot**
The single deep root of many deciduous
trees that forms the basis for their root systems.
✥ Figuratively, a “taproot” is the source of an idea or
work: “His childhood in Wales is the taproot of his poetry.”
193
**class**
In biology, the classification beneath a phylum and above an order.
✥ Mammals, reptiles, and insects are classes.
194
**instinct**
Behavior that is not learned but passed between
generations by heredity.
195
**single-celled**
Made up of only one cell.
Bacteria
are single-celled organisms.
196
**plant kingdom**
One of the five kingdoms of living
things. Most plants derive energy from photosynthesis.
197
**omnivore** (om-nuh-vawr)
An animal whose normal
diet includes both plants and animals.
\*Human beings
and bears, for instance, are omnivores.
198
**amino acids** (uh-mee-noh)
Basic organic molecules
that combine to form proteins. Amino acids
are made up of hydrogen, carbon, oxygen, and nitrogen.
\*Some examples of amino acids are lysine,
phenylalanine, and tryptophan.
✥ Amino acids are the basic molecular building
blocks of proteins.
199
**cross-fertilization**
The fertilization of the ovum of one plant by the sperm of another plant.
200
**gene patent**
The controversial legal practice of patenting
a newly discovered gene.
It allows unique segments
of DNA, which perhaps code for a certain
disease or a certain protein, to be owned by an individual
or corporation.
201
**sexual selection**
In evolution, the selection of a
mate based on secondary sex characteristics.
Sexual
selection is thought to lead distinct differences in
the appearance of the two sexes within a species
For example, the tail of the male peacock may be the result
of sexual selection.
202
**fruit**
In botany, the part of a seed-bearing plant that
contains the fertilized seeds capable of generating a
new plant (see fertilization).
Fruit develops from the
female part of the plant.
\*Apples, peaches, tomatoes,
and many other familiar foods are fruits.
203
**catabolism**
Biochemical reactions that break down molecules in metabolism. Molecules may be broken down to gain their energy or to prepare them for disposal from the body. (Compare anabolism.)
204
**chemical evolution**
The formation of complex organic molecules from simpler inorganic molecules through chemical reactions in the oceans during the early history of the Earth; the first step in
the development of life on this planet. The period of
chemical evolution lasted less than a billion years.
✥ Many of the steps in chemical evolution can now
be reproduced in the laboratory.
✥ Some scientists believe
that all or most of the Earth’s original organic
molecules were created in space and were brought to
the Earth’s oceans by meteorites.
205
**Brontosaurus** (bron-tuh-sawr-uhs)
A large herbivorous (see herbivore) dinosaur, perhaps the most familiar of the dinosaurs. The scientific name has recently been changed to Apatosaurus, but Brontosaurus
is still used popularly. The word is from the Greek,
meaning “thunder lizard.”
206
**sociobiology** (soh-see-oh-beye-ol-uh-jee)
An area of inquiry in biology, still somewhat controversial
but gaining wider acceptance.
Its central tenet is that
many human behavioral and social traits are genetically
inherited and are thus determined by genetic
makeup and not culture.
✥ Sociobiology is on the “nature” side in the naturenurture
controversy.
207
**in vivo** (in vee-voh)
In nature; literally, “in life.” In vivo conditions are distinguished from those that might exist only in a laboratory.
208
**toxic waste**
A general term used to refer to chemical
compounds produced by industry which, if they are
ingested or breathed in by humans, can cause physiological
damage.
The disposal of toxic wastes is a major
environmental problem in the United States.
209
**vivisection** (viv-uh-sek-shuhn, viv-uh-sek-shuhn)
The cutting up or dissection of animals, including
anesthetized live animals, in scientific research.
Vivisection
is also a general term for the use of animals as
subjects in laboratory experiments, especially in the
development of new medical techniques and drugs.
✥ Vivisection, as well as the general use of animals in
medical research, is a target of protest by animal rights
advocates.
210
**Monera** (muh-neer-uh) (or Prokaryotae)
The kingdom
of single-celled organisms without a cell nucleus
(see also prokaryotes). Monera are the most
primitive living things and are thought to have been
the first to evolve.
211
**DNA methylation** (meth-uh-lay-shuhn)
The chemical reactions that place a methyl group (a combination of one carbon atom and three hydrogen atoms)
at a particular spot on DNA during organismal development.
The effect of this process is probably to “turn
off” various genes during the process of cellular
differentiation, causing the cell to develop into a
specific type.
✥ It is thought that during cloning, the methyl
groups are removed from the DNA, turning the genes
back “on” again.
✥ There is evidence that embryonic
stem cells are cells in which this process has not yet
occurred.
212
**flora** (flawr-uh)
Plants, especially the plants of a
particular place and time.
213
**opposable thumb**
A thumb that can be used for grasping.
✥ Opposable thumbs are one of the distinguishing
features of primates.
✥ The opposable thumb of human
beings allows us to use tools.
214
**mammals**
```
A class of vertebrates characterized
by the production of milk by the females and in most
cases, by a hairy body covering.
```
Most mammals give
live birth to their young. Human beings are mammals.
215
**Tyrannosaurus rex** (ti-ran-uh-sawr-uhs reks)
A large, carnivorous dinosaur that
walked on two legs.
Its name is from the Greek words
meaning “tyrant” and “lizard” and the Latin word for
“king.”
216
**closed ecosystem**
An ecosystem in which no materials can leave or enter but through which energy from external sources can flow.
✥ The Earth is a closed ecosystem.
217
**ecosystem** (ee-koh-sis-tuhm, ek-oh-sis-tuhm)
A collection
of living things and the environment in which
they live. For example, a prairie ecosystem includes
coyotes, the rabbits on which they feed, and the grasses
that feed the rabbits.
✥ Chemical substances move through ecosystems
on the Earth in cycles (see carbon cycle).
✥ Thesource of energy for almost every ecosystem on Earthis the sun.
218
**flower**
The part of a plant that produces the seed. It
usually contains petals, a pistil, and pollen-bearing
stamens.
219
**gene pool**
The total number of genes available in a
given species.
✥ Loosely speaking, the gene pool represents the total
breeding stock available to the species.
220
**mitochondrial Eve**
When the original analysis of
the DNA in the mitochondria of modern humans
was carried out, the results suggested that all modern
humans share the DNA of a single individual female
who lived a few hundred thousand years ago. This female
was named Eve in reference to the Creation
story in the Book of Genesis in the Bible.
✥ Further research has largely discredited the notion
of a single Eve, although scientists have found that modern
humans are descended from a very small population,
perhaps as few as five thousand individuals.
221
**biodegradable** (beye-oh-di-gray-duh-buhl)
Material that, left to itself, will be decomposed by natural
processes.
✥ The use of biodegradable packaging is supposed
to reduce the volume of waste in landfills.
222
**eukaryote** (yooh-kar-ee-oht)
An organism whose
cells contain a nucleus. All multicelled organisms
are eukaryotes, as is one superkingdom of singlecelled
organisms. Eukaryotes also have organelles
enclosed by membranes. (Compare prokaryote.)
✥ Eukaryotes evolved in a process in which one early
prokaryote consumed another, forming a more complex
structure.
✥ The word eukaryote comes from the
Greek for “true nucleus.”
223
**amphibians** (am-fib-ee-uhnz)
Vertebrate animals,
such as frogs, that live part of their life cycle in the
water and the other part on land.
✥ The word is also used to describe things such as
vehicles that can operate both on land and in the water.
✥ They were the first land-dwelling animals to
evolve.
224
**neurotransmitter**
Any one of a number of chemicals
that are used to transmit nerve signals across a
synapse.
They are sprayed from the end of the “upstream”
nerve cell and absorbed by receptors in the
“downstream” cell.
✥ Drugs like Prozac and alcohol affect the emission
and reception of neurotransmitters.
225
**cloning vector**
An agent used to transfer bits of DNA into the DNA of a cell in genetic engineering. Common types of vectors include the genetically engineered viruses that infect a cell.
226
**animal kingdom**
The group of living things typically distinguished from members of the plant kingdom by the power of moving from place to place and by a metabolism that does not use photosynthesis.
227
**sex cells**
The sperm and egg of living things.
```
Sex cells have only half the number of chromosomes that
other cells (body cells) have.
```
228
**chordates** (kawr-dayts, kawr-duhts)
Animals that have a central nerve like the human spinal cord.
✥ Chordates make up a phylum in the animal kingdom that includes all the vertebrates, along with some primitive wormlike sea animals. (See Linnean classification.)
229
**protein structure**
The three-dimensional structure
of a protein that is often key to its enzyme function.
The structure of a protein is characterized in four
ways: The primary structure is the order of the different
amino acids in a protein chain, whereas the secondary
structure consists of the geometry of chain segments
in forms such as helices or sheets. The tertiary
structure describes how a protein folds in on itself; the
quaternary structure of a protein describes how different
protein chains hook up with each other.
230
**system**
A group of bodily organs that have similar
structures or work together to perform some function,
such as the digestive system, nervous system, and
respiratory system.
231
**GMO**
The abbreviation for genetically modified
organism. A GMO is an organism whose genome has
been altered by the techniques of genetic engineering
so that its DNA contains one or more genes not
normally found there.
✥ A high percentage of food crops, such as corn and
soybeans, are genetically modified.
232
**biomass**
Material in growing or dead plants.
✥ The term biomass is most often encountered in
discussions of sources of energy, as biomass can be
used to supply energy needs directly (as fuel wood, for
example) or indirectly (by being converted to alcohol;
see gasohol).
233
**phenotype** (fee-nuh-teyep)
The outward appearance
of an organism;
the expression of a genotype in the
form of traits that can be seen and measured, such as
hair or eye color.
234
**genetic code**
The code that translates the sequence
of nucleotides in genes along the DNA strand into
the structure of protein, which, through its action as
an enzyme, governs one chemical reaction in the
cell. A simple mnemonic is “One gene codes for one
protein which runs one reaction.”
✥ All living things share the same genetic code, a fact
that represents strong evidence for evolution. Unraveling
the genetic code was one of the great scientific
achievements of the twentieth century, and it opened
the way to genetic engineering.
235
**meiosis** (meye-oh-sis)
Division of cells in which
four “daughter” cells are produced from one “parent”
cell, each with half the genes of the parent. Meiosis is a
key process in sexual reproduction. In the ovaries
and testes, meiosis produces a great variety of sex
cells (sperm and ova), because the genes of the parent
cell can be split in many different ways. The sex
cells combine in fertilization to produce a new individual
with the full number of genes — half from each
parent. Because the sex cells come in such variety, and
come from two parents, there is an enormous number
of possible forms for the offspring.
236
**gene amplification**
A process in a cell by which a
particular gene is replicated so that more copies are
available to produce a protein for the cell’s use.
\*For example, the genes that code for proteins involved in
ribosomes are amplified early in the process of cell development
so that there are sufficient numbers of them
to assemble the cell.
✥ PCR, polymerase chain reaction, can be considered
a type of man-made gene amplification process.
237
**cross-breeding**
hybridization
238
**algae** (al-jee)
Primitive organisms that contain
chlorophyll but do not have structures, such as xylem
and phloem, to transport fluids. Algae sometimes
contain only a single cell, and nowadays they
are not considered members of the plant kingdom.
✥ The most familiar algae are the greenish scum that
collects in still water. ✥ Algae supply a considerable
part of the world’s oxygen.
239
**signal transduction pathway**
A set of chemical
reactions in a cell that occurs when a molecule,
such as a hormone, attaches to a receptor on the cell
membrane.
The pathway is actually a cascade of biochemical
reactions inside the cell that eventually reach
the target molecule or reaction. Thus, the pathway is
a method by which molecules inside the cell can be altered
by molecules on the outside.
240
**biodiversity**
A term that describes the number of different species that live within a particular ecosystem.
✥ The preservation of biodiversity is considered by
environmentalists to be a major goal of environmental
policy.
241
**Linnean classification**
A way of organizing living things.
In biology, plants and animals have traditionally
been classified by the structure of their bodies, in a
descending hierarchy of categories: kingdom, phylum,
class, order, family, genus, and species. For
example, human beings are classified as belonging to
the animal kingdom, the phylum of chordates, the
class of mammals, the order of primates, the genus
Homo, and the species sapiens. The scheme is based
on a system developed by the Swedish scientist Carolus
Linnaeus in the eighteenth century.
There is a debate among scientists about whether
the traditional classification system should be retained.
Some argue that classifying living things by their descent,
with all descendants of a single ancestor being
grouped together, is more in keeping with the ideas of
evolution (see cladistics). Others want to classify
organisms by their genetic makeup, using sequences in
DNA or RNA. Each of these schemes provides a different
way of ordering living things, but there is no
“right” way to accomplish this task.
✥ Plants and animals are usually identified merely
by genus and species; thus, human beings are given the
scientific name Homo sapiens.
242
**species** (spee-sheez, spee-seez)
A group of closely related and interbreeding living things;
the smallest standard unit of biological classification. Species can
be divided into varieties, races, breeds, or subspecies.
Red pines, sugar maples, cats, dogs, chimpanzees, and
people are species; Siamese cats and beagles are varieties,
not species.
✥ The term can be used to refer to any group of related
things: “This species of novel has become quite
popular in recent years.”
243
**restriction enzymes**
Chemicals used in the lab to
cut up DNA at specific sites so that it may be sequenced.
They function in nature as a form of bacterial
self-protection that can cut up foreign DNA. The use
of restriction enzymes is crucial in DNA fingerprinting.
244
**convergent evolution**
The development through evolution of similar features by organisms with distinctly different ancestors.
\*A common example of this is the evolution of wings in insects and birds.
245
**hydroponics** (heye-druh-pon-iks)
Cultivating plants in an artificial environment in which the necessary nutrients are carried to the roots in a liquid mixture.
246
**Dinosaurs**
Reptiles, now extinct, that were the
dominant life form on Earth for many millions of
years. The name dinosaur comes from the Greek words
for “monstrous lizard.” Dinosaurs became extinct suddenly,
about sixty-five million years ago. Scientists now
believe that their extinction was caused by the impact
of a large asteroid on the Earth.
✥ Some dinosaurs were very large and had small
brains — factors that may in part have led to their extinction.
The term is often used to refer to something
or someone that is antiquated and unable to adapt to
change: “The old cavalry generals couldn’t adjust to
the use of tanks — they became dinosaurs.”
✥ Commonly
known dinosaurs include Tyrannosaurus
rex, Brontosaurus, Stegosaurus, and Triceratops.
✥ Some scientists believe that modern birds are the
descendants of dinosaurs.
247
**food chain**
The series of steps by which energy is
obtained, used, and transformed by living things.
For example: sunlight helps grain to grow, the grain feeds
cattle, and humans eat the cattle. (See illustration, next
page.)
✥ Harmful chemicals can become concentrated as
they move up the food chain.
248
**allele** (ul-leel)
The sequence of nucleotides on a
DNA molecule that constitutes the form of a gene at
a specific spot or a chromosome.
\*There can be several
variations of this sequence, and each of these is called
an allele. In the case of the gene for eye color, for example,
one allele codes for blue eyes, whereas the other
may code for brown eyes.
249
**totipotency**
The ability of cells such as an embryonic
stem cell to differentiate into any type of body
cell.
Plant cells are also totipotent, which helps to
explain why a graft of a plant can generate a whole new
individual out of just a small branch cutting.
250
**sperm**
The male sex cell, typically consisting of a
head, midpiece, and tail.
✥ Sperm are much smaller than the ova they fertilize.
251
**spore**
A reproductive cell or group of cells, produced
by some plants, that is capable of developing
into an adult plant without combining with another
reproductive cell.
Plants also produce sperm cells. The
spores of nonflowering plants are analogous to the
seeds of flowering plants.
Fungi and
algae typically reproduce by means of spores that are
carried by the wind or some other agency to a new location
for growth.
252
**fetus**
The embryo of an animal that bears its young
alive (rather than laying eggs).
\*In humans, the embryo
is called a fetus after all major body structures have
formed; this stage is reached about sixty days after fertilization.
253
**Protoctista** (pruh-tohk-tis-ta)
The kingdom of organisms,
mostly single celled, whose cells contain a
nucleus.
The amoeba is a member of this kingdom
254
**codon** (koh-don)
A group of three bases on the DNA molecule. Each codon determines the identity of one amino acid in proteins made by the cell.
255
**xylem** (zeye-luhm)
The system of vessels that transports
water in a plant.
256
**amoeba** (uh-mee-buh)
An animal composed of
only one cell that has no fixed shape. It is the best
known of the single-celled animals, or protozoa.
✥ The term is sometimes used to refer to
something with an indefinite, changeable shape.
257
**green revolution**
The increase in the world production
of cereals such as wheat and rice during the 1960s
and 1970s because of better seed and new agricultural
technology.
✥ The green revolution greatly increased the availability
of food and confounded predictions of worldwide
famine that had been made in the early 1970s.
258
**chromosomes** (kroh-muh-sohmz)
The small bodies in the nucleus of a cell that carry the chemical “instructions” for reproduction of the cell. They consist of strands of DNA wrapped in a double helix
around a core of proteins. Each species of plant or animal has a characteristic number of chromosomes.
For human beings, for example, it is forty-six.
✥ In humans, sex is determined by two chromosomes:
an X-chromosome, which is female, and a Ychromosome,
which is male.
259
**DNA sequencing**
A process by which the sequence
of nucleotides along a strand of DNA is determined.
Originally a difficult process to carry out, DNA sequencing
can now be done routinely by machines. The
completion of the Human Genome Project in 2000
produced the largest DNA sequence ever assembled.
To carry out the sequencing of the human genome,
scientists cut the DNA up into short fragments, sequenced
these fragments simultaneously, and then assembled
the entire genome by using sophisticated
computer techniques to match the fragments to each
other.
✥ Many believe that knowledge of the human
genome will lead to enormous advances in medicine.
(Compare gene mapping and DNA fingerprinting.)
260
**ATP** (adenosine triphosphate)
A key energy-carrying molecule in biological systems. It is produced in the body through the process of cell respiration
and in plants through photosynthesis
