The Evolution of Cells potx

CELLS
THE BUILDING
BLOCKS OF LIFE
The Evolution
of Cells
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Cells: The Building Blocks of Life
Cell Structure, Processes, and Reproduction
Cells and Human Health
The Evolution of Cells
How Scientists Research Cells
Plant Cells
Stem Cell Research and Other Cell-Related Controversies
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TERRY L. SMITH
CELLS
THE BUILDING
BLOCKS OF LIFE
The Evolution
of Cells
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THE EVOLUTION OF CELLS
Copyright © 2012 by Infobase Learning

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Library of Congress Cataloging-in-Publication Data
Smith, Terry L. (Terry Lane), 1944-
e evolution of cells / by Terry L. Smith.
p. cm. — (Cells, the building blocks of life)
Includes bibliographical references and index.
ISBN 978-1-61753-006-7 (hardcover)
ISBN 978-1-4381-3906-7 (e-book)
1. Life—Origin. 2. Cells—Evolution. I. Title.
QH325.S59 2011
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Date printed: November 2011
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ContentsContents
• • •• • •

1 The Beginnings of Life 7
2 The Chemistry of Life 19
3 Prokaryotes: the Simplest Cells 30
4 Eukaryotes: the Cells of Complex Life 40
5 Cells in Action 51
6 Genetics and Cell Evolution 66
7 Plant Cells and Evolution 78
8 The Diversity of Complex Cells 89
9 Cells: Key to the Future 99
Glossary 109
Bibliography 113
Further Resources 115
Picture Credits 116
Index 117
About the Author 122
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7
The BeginningsThe Beginnings
of Lifeof Life
 ere is something about being human that instills in us a sense of won-
der. When we stop to think about it, the very idea of life seems such a mys-
tery. Where did we come from? How did life begin? When we look at the
sky, we wonder about the vastness of the universe and whether other life
may exist there. If we look through a microscope at a drop of pond water,
we are amazed at the variety of tiny creatures we see.
Since cells form the very basis of life, it is only natural that our sense
of wonder extends to the cell. Where did the fi rst cells come from? How is
it possible that cells with the same basic components can form creatures
as simple as bacteria or as complex as a human being? How do brain cells

allow us to think, and how do cells of the hand work together to allow us
to play the piano? Video animations of cell interiors let us see the amaz-
ing molecular “machines” that move around inside every cell as they copy
genetic molecules, shuttle nutrients, relay messages, and build or repair
membranes.
What follows is an exploration of these subjects, beginning with the
fi rst cell that started the cascade of events leading to life on Earth as we
know it.  e exact nature of the very fi rst life on our planet and how it
came into being may be forever unknown. Yet research into the nature
of life can help us understand what is in the realm of possibility. Scien-
tists have vastly increased our knowledge about the connectedness of
all life forms through their study of contemporary organisms and their
genetic structures. Investigations into the inner workings of cells now
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8 THE EVOLUTION OF CELLS
hold great promise in the fi elds of evolutionary biology, medicine, and
bioengineering.
THE NATURE OF LIFE AND ITS ORIGINS
Defining Life
 e existence of life might seem to be an obvious fact, but coming up with
a defi nition of life is not so simple. Even biologists have diffi culty agreeing
on a description that can include all possible life forms. How can microbi-
ologists know if they have found life on Mars if they can’t fi rst agree what
defi nes life? Furthermore, how is it possible to look for the fi rst signs of
life on Earth without this defi nition? A chemist may see life as a self-sus-
taining chemical system that can evolve, while a physicist may see life as
an ordered system, in contrast to nature’s tendency toward disorder. Ger-
ald Joyce, a prominent researcher in the origin-of-life fi eld, was quoted in
Smithsonian magazine as joking that life could be defi ned as “that which
is squishy.” Some scientists have even concluded that it is not possible to

agree on a defi nition that includes all possible life forms! Allowing for the
possibility that entire new life forms may exist elsewhere in the universe,
life as we know it on Earth shares certain features in common: the need
to take in energy; production of waste that must be eliminated; growth;
replication into similar life forms; and response to the environment, both
as individual organisms and through evolution across generations.
Early Beliefs
 ere is evidence that even very ancient people were concerned about the
nature of their existence and origins of life. Paintings of animal images
from 10,000 to 30,000 years ago have been found in caves in Altimira,
Spain, and the Vézère Valley of France.  ey suggest that the humans who
painted them were even then grappling with the nature of existence and
their place in nature. Stories about the beginning of life developed in most
cultures, as ancient people struggled to understand their origins. Many of
these beliefs about life’s origin remain alive today in the poetry and reli-
gion of various cultures around the world.
Recorded history of the Dark Ages in Europe (from about 500 to
1100 ..) tells us that mankind also came to depend on their obser-
vations to form their belief systems about the origin of life. If a piece
of meat was le to rot, maggots soon appeared; to a person unfamiliar
with science, it was easy to conclude that the maggots had spontaneously
appeared in the rotten fl esh.  is gave rise to the commonly held belief
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The Beginnings of Life 9
in “spontaneous generation” as a source of many life forms. In fact, there
was even a seventeenth-century recipe for the creation of mice: store
dirty underwear with grains of wheat in an open jar and a er 21 days
mice will spontaneously appear. Although the real source of the mice
seems obvious to us today, this belief was consistent with the knowledge
of that era.

Era of Science
 e idea of spontaneous generation of life held sway for several centu-
ries. Beginning in the sixteenth century, the principles of modern science
were also gradually coming into existence. All over Europe, educated men
began to systematically investigate their observations of the natural world.
Tools such as the microscope were invented, and for the fi rst time, it was
possible to view a hidden world unavailable to the naked eye. Even if they
did not understand what they were observing, these early scientists real-
ized that plants were made of tiny partitions they called cells, for their
resemblance to the tiny residential cells of monks.  ey also observed
organisms they called animalcules, which they believed arose spontane-
ously in water a er it sat for a few days in the laboratory.
Figure 1.1 Cave paintings found in the Vézère Valley in southwestern
France feature images of animals. Discovered in 1940, they are believed to be
about 17,000 years old.
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10 THE EVOLUTION OF CELLS
 en, in 1859, the French Academy of Science sponsored a competi-
tion for the best experiment to prove or disprove the idea of spontane-
ous generation. Louis Pasteur, a prominent scientist of the time, set out
to disprove the idea. His experiment has become a classic example of
what we know as the scientifi c method, which forms the basis of all
modern science.
Figure 1.2 Louis Pasteur used uniquely designed swan-necked fl asks to
dispel the theory of spontaneous generation.
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The Beginnings of Life 11
Pasteur started with two glass fl asks into each of which he poured a
meat broth. One fl ask had a straight neck; Pasteur bent the neck of the
second fl ask into a curved shape.  e contents of both fl asks were then

heated to a high temperature to kill any living matter in the broth.  e
fl asks were le at room temperature and exposed to the air for some
time. Microorganisms present in the air could fall into the straight-
neck fl ask but not into the fl ask with the curved neck. At the end of the
experiment, the broth in the straight-neck fl ask was dark and cloudy,
and microorganisms could be observed in the broth. No evidence of
organisms was found in the curve-necked fl ask, thereby demonstrating
that organisms were not spontaneously generated in the broth but had
fallen in from the air.
To convince the French Academy of the truth of his discovery, Pas-
teur fi rst formed a hypothesis based on his previous observations of the
growth of microorganisms. It stated that microorganisms would not grow
in the sterilized broth if they could not fall into the fl ask from the air.
His next step was to conduct an experiment consisting of a control case
(straight-neck fl ask) and a test case (curved-neck fl ask). A er observing
the results of the experiment, Pasteur concluded, “Never will the doctrine
of spontaneous generation recover from the mortal blow of this simple
experiment.” Today’s scientists who seek evidence of the fi rst life forms on
Earth—whether analyzing fossils or conducting experiments in chemistry
or genetics—continue to adhere to these 150-year-old steps of the scien-
tifi c method.
EARLY EARTH
No one will ever know for certain exactly how the fi rst life began on
Earth. Yet, to think about how life could have begun, how that very fi rst
cell developed, it is essential to understand the physical conditions that
existed on early Earth.
Our solar system is thought to have formed some 4.6 billion years ago
from a giant rotating cloud of gas and dust. As much of the rotating mate-
rial collapsed toward the center, a new star, our planet’s Sun, was formed.
Other large chunks of material collided and eventually attracted addi-

tional matter until rotating planets formed. Shortly a er Earth’s forma-
tion, in an event known as the Giant Impact, Earth collided with another
large object, resulting in the expulsion of a chunk of vaporized rock that
gave rise to our moon. Earth would have been a violent place over the next
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12 THE EVOLUTION OF CELLS
billion or so years as it underwent frequent collisions with asteroids and
comets, periods of extreme heat, and volcanic eruptions.
Gradually, the Earth cooled, and the oceans, a surface crust, and sur-
rounding atmosphere formed, although the timing of these events remains
controversial.  e chemical content of the oceans, rocks, and atmosphere
would have been critical for the fi rst formation of life, but these also
remain uncertain. It is likely the oceans were highly acidic, with elements
such as iron and calcium dissolved in them.  e atmosphere would have
contained carbon dioxide and hydrogen-containing compounds such as
methane and ammonia, but little oxygen.
SEARCHING FOR THE FIRST CELLS
However or whenever the fi rst life may have originated, many scientists
agree that life existed on Earth by about 3.8 billion years ago, relatively
early in the violent life of a new planet. Several scientifi c approaches are
IS THERE LIFE OUT THERE?
The presence of life somewhere else in the universe is not just the
stuff of science fi ction. Astrobiology is the branch of science that stud-
ies the possibility of life elsewhere in the universe. Astrobiologists also
explore the chemistry of interstellar space, the habitability of planets,
and evolutionary biology of Earth’s organisms.
After much searching through astronomical data, astronomers in
2010 found what may be a “Goldilocks” planet, so named for its pres-
ence of conditions suitable for the existence of life. Until this planet’s
discovery, only Earth was regarded as having ideal conditions—“not too

hot, not too cold, but juuust right,” as the famous fairy tale goes. How-
ever, if the existence of this planet, known as Gliese 581g, is confi rmed,
it may have the ideal temperature that would allow for the presence
of water in liquid form. In addition, its size suggests it could have the
proper gravitational forces and atmosphere to sustain life. Although the
planet is relatively close to Earth, given the vastness of the universe, no
one will be visiting to check for signs of life anytime soon. Its distance
of 120 trillion miles (193 trillion kilometers) away would require several
generations of humans for a spaceship to reach there.
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The Beginnings of Life 13
used to search for evidence of how this fi rst life may have come into being:
fossil analysis, radioactive dating of rocks, evolutionary relationships
among organisms, chemistry experiments, study of present-day organ-
isms living in extreme conditions, and analysis of genomes. Scientists
involved in this search come from the fi elds of biology, physics, chemistry,
geology, paleontology, and astrobiology.
Much of the research into the origins of life involves learning more
about organisms that exist on today’s Earth. In their search for where
the fi rst life may have originated, scientists have focused attention on
extremophiles.  ese are organisms that survive in extreme condi-
tions, such as the high pressure that occurs on the ocean fl oor, or the
high temperatures of hot springs, such as those in Yellowstone National
Park. Much evidence now suggests that cells may have fi rst formed near
Figure 1.3 This view of a geothermal pool in Yellowstone National Park
in Wyoming shows the colors caused by deposited minerals and colonies
of heat-loving bacteria and algae called extremophiles. The pool is heated
because of magma (molten rock) at the bottom.
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14 THE EVOLUTION OF CELLS

a deep sea hydrothermal vent. Essential chemicals could have been pres-
ent at such a location, and the vent would have provided energy to fuel
chemical reactions. DNA sequencing information indicates that a com-
mon ancestor of various life forms was likely a microorganism living in
extremely high temperatures.
 e oldest fossils that record the fi rst evidence of life were discovered
in western Australia in formations called stromatolites.  ese layered for-
mations were produced by the actions of early cyanobacteria and consist
of calcium carbonate which precipitated over the growing mats of bacte-
rial fi laments. Stromatolites continue to form off the Australian coast, and
the bacteria that form them are being closely studied by scientists of the
Australian Centre for Astrobiology.  e great diversity of bacteria found
Figure 1.4 Stromatolites dot a nature reserve in Shark Bay in western
Australia. This mineralized microbial community formed from cyanobacteria
build up over the last 4,000 years.
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The Beginnings of Life 15
in these present-day stromatolites suggests that the fossil stromatolites
may have been formed by organisms suffi ciently evolved to develop such
diversity. If this is the case, it puts the fi rst appearance of life on Earth
earlier than previously thought.
STANLEY L. MILLER,
A PIONEERING GRADUATE STUDENT
In 1951, Stanley L. Miller, a new graduate student in chemistry at the
University of Chicago, attended a lecture that fi red his imagination. The
lecturer, Professor Harold C. Urey, proposed an outlandish idea that it
might be possible to produce building blocks of organic molecules in
the laboratory by simulating conditions of early Earth. Miller was ready
to take him up on the idea. At fi rst, Professor Urey discouraged him
from undertaking such an experiment, claiming it was much too risky

a project for a graduate student and would delay his academic degree.
Still, Miller persisted, and it was fi nally agreed that he would work on
the project for a year.
In just a short time, Miller succeeded in producing the results that
his professor had proposed. He assembled an apparatus containing a
fl ask of water to simulate the ocean, then heated it to produce water
vapor. A mixture of methane, hydrogen, and ammonia gases, which were
then believed to be the components of early Earth’s atmosphere, was
circulated through the apparatus to mix with the water vapor. Electrical
charges then pelted the gaseous mixture to simulate an energy source in
the form of lightning. Chemical reactions in the mixture soon produced
compounds that colored the waters of the “ocean.” Analysis revealed
that Miller’s experiment had yielded many amino acids, the building
blocks of proteins.
The scientifi c community greeted Miller’s work with surprise and
disbelief. Resistance to publishing the write-up of his experiment was
so strong that it might have gone unpublished had it not been for the
backing of his eminent professor. Soon, however, others were able to
reproduce the now-famous experiment and Miller’s work became widely
accepted. Its radical departure from anything that had gone before
changed the course of scientifi c thinking about the origins of life.
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16 THE EVOLUTION OF CELLS
Fossil hunts are exciting and have provided critical evidence about
the oldest life on Earth. Yet important origin-of-life research also takes
place in the laboratory. Experiments can serve as “proofs of concept” to
form hypotheses about how life may have originated. In other words, if
certain chemical reactions can be observed in the laboratory, it is possible
that similar reactions took place under the conditions present on early
Earth. One of the earliest experiments that searched for how life began

was conducted by Stanley L. Miller and Harold C. Urey, researchers at the
University of Chicago in 1951.  eir experiment, which attempted to cre-
ate amino acids—a basic component of cells—from compounds known
to exist on early Earth, changed the scientifi c approach to the search for
the origin of life. As more has been learned about the environmental con-
ditions on early Earth, scientists have conducted experiments that more
accurately replicate those conditions.
Nobel-laureate Jack Szostak, geneticist and co-director of the Ori-
gins of Life Initiative at Harvard Medical School, heads a team of scien-
tists who seek to understand how life could have arisen spontaneously
from the chemicals and conditions of early Earth. By exploring novel
chemical systems in laboratory conditions, they can learn about possible
pathways that could have led to the formation of a primitive cell. One
by one, Szostak and his fellow scientists have tackled tough questions
about how life may have formed from non-living compounds: Which
came fi rst, proteins to carry out cellular functions or the genetic mate-
rial to produce the proteins? Where did the essential element phospho-
rus come from, since it was thought to be unavailable in a water-soluble
form? What sources of energy could have set off the chemical reactions
required to form complex cellular compounds from simpler molecules?
Which came fi rst, a reproducible genetic system or the membrane to
enclose it? Furthermore, if a membrane existed, how were essential
nutrients able to pass through it?
Using chemical compounds that are thought to have been present on
early Earth, Szostak’s team has assembled what they call a protocell.  ese
protocells have a double-layered spherical membrane made up of fatty
acids, resembling membranes of present-day organisms. Since fatty acids
are hydrophobic (unable to mix with water), the membrane maintains its
identity, separate from the surrounding water-soluble compounds. Small
nucleotide molecules can enter the sphere, where they join with other

molecules to form something resembling a strand of RNA. At the Scripps
Research Institute, molecular biologist Gerald F. Joyce and colleagues
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The Beginnings of Life 17
have produced forms of RNA molecules that can promote each other’s
synthesis, as they attempt to recreate a key element of life: genetic material
that can reproduce itself.
THE PARADOX OF THE FIRST CELL
Cells of present-day complex organisms rely on three essential com-
pounds: DNA for replication and storage of genetic information, RNA for
various functions, and proteins that serve as the workhorses of the cell.
Certain proteins, called enzymes, have the important function of serving
as catalysts for essential chemical reactions. Since RNA molecules have
much simpler chemical forms than DNA, scientists assume that the fi rst
cells depended on RNA for preservation of genetic material. Still, how
could cells produce new RNA molecules, an essential function for living
things, without proteins to catalyze the chemical reactions?  is question
gave rise to one of the great paradoxes of origin-of-life studies—which
came fi rst, RNA or protein? Progress in the fi eld was slow until a key dis-
covery was made in the 1980s.  omas Cech of the University of Colorado
discovered an RNA molecule within a protozoa that could chemically
manipulate itself without the assistance of a protein. In other words, the
RNA molecules were playing the chemical role of an enzyme. Sidney Alt-
man of Yale University independently discovered a similar RNA molecule,
and the two researchers shared the 1989 Nobel Prize in Chemistry for
their discoveries.  ese RNA molecules were given the name ribozymes
for their ability to fold themselves into biologically active molecules that
were able to play the role of an enzyme in chemical reactions.  us, the
fi rst cell would have depended on a simple RNA molecule, or ribozyme,
that could both transmit genetic material and act in place of proteins to

carry out essential cell functions.  is discovery of the primary role of
RNA led scientists to refer to the world in which life fi rst developed as the
RNA world.
LIFE IN AN RNA WORLD
Scientists have found another reason to think that life evolved in an RNA
world through their study of the structure of modern cells. Proteins are
assembled within our cells by large molecular complexes called ribo-
somes.  ese complexes contain both RNA and protein components.
Recent biochemical analysis indicates that the mechanism for protein
assembly is catalyzed by the RNA portion of the complex, not protein.
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18 THE EVOLUTION OF CELLS
 us, ribosomes within each of our cells carry this “fossil” evidence that
life developed in an RNA world.
 e other essential role of RNA in primitive cells was to replicate itself
in order to produce new cells. Without this ability of RNA to replicate
itself, life could not exist. As more and more cells were produced through
this replication process, some variation would have existed in how well
the molecules could copy themselves.  is variation opened the door for
the process of natural selection—better replicators would produce more
off spring cells. Over time, RNA molecules with superior copying ability
would have dominated, leading to a population with stable and effi cient
replicating capability.
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19
What was it about the conditions on early Earth that made life possible?
To be sure, life might have developed under other conditions, and con-
sisted of diff erent chemical elements. Yet the forms of life that did develop
on Earth are dependent on the element carbon, which happens to have
chemical properties that are ideal for the requirements of cellular func-

tions.  is knowledge involves an understanding of organic chemistry,
the branch of chemistry associated with carbon-containing compounds.
 ese compounds are, for the most part, those associated with living
organisms.
Carbon was a common element in the universe that produced our
solar system. It would have been in plentiful supply as the crust of Earth
was developing billions of years ago. Volcanic eruptions circulated ele-
ments from Earth’s interior to its surface. Gravitational forces pulled in
debris from space, contributing to the variety of elements available for
the fi rst organic chemical reactions. In addition to carbon, other major
components of organic compounds, such as nitrogen, oxygen, and hydro-
gen, all existed on Earth’s surface. Eruptions spewed steam and gases into
the atmosphere, creating shi s in how much heat from the sun reached
Earth’s surface. Water varied in form from atmospheric vapor to liquid
oceans to ice as Earth rotated through cycles of heat and cold. At some
point, the right chemicals got together under just the right conditions for
life to begin.
The Chemistry The Chemistry
of Lifeof Life
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20 THE EVOLUTION OF CELLS
Whenever we picnic under the shade of a tree on a hot summer day,
we sit surrounded by organic chemistry and never give it a thought.  e
wood of the picnic table, the corn chips we eat, the leaves providing our
shade, the ants crawling about, our very selves—they are all containers of
organic chemicals that form the core of our existence. All living things are
composed of cells, and the business of a cell is chemistry. Our human cells
contain the same chemical compounds and undergo similar chemical pro-
cesses as the cells in the corn we eat and the bacteria that live all around us.
 e next time you eat a good meal, consider that your body’s billions of cells

count on the energy your food provides them. What follows is an explora-
tion of some basic principles of organic chemistry and an explanation how
they have contributed to the development of cells of increasing complexity.
CARBON, THE BACKBONE OF LIFE
Cells are composed of a huge variety of complex molecules containing
carbon. Why carbon?  e elemental structure of carbon makes it ideal for
ARSENIC:
COMPATIBLE WITH LIFE?
Six chemical elements provide the basis for the molecules of life: car-
bon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus. Can arsenic be
added to that list? It is common knowledge that arsenic is toxic to life. In
fact, it has been a favorite murder weapon for centuries.
Astrobiologist Felisa Wolfe-Simon, working with the U.S. Geological
Survey, noted the chemical similarity between the elements phosphorus
and arsenic. She wondered if there might not be an organism able to
substitute arsenic for the phosphorus that is contained within DNA of
all known life forms.
She set out to look for such an organism in the arsenic-rich waters
of Mono Lake in California. Among the microorganisms she found there
was one that goes by the unglamorous name of GFAJ-1. The organism
grew in the laboratory, even when the concentration of arsenic was
gradually increased. If Dr. Wolfe-Simon’s evidence that the bacteria are
incorporating arsenic into their DNA is confi rmed by other researchers,
her discovery would represent a milestone in our knowledge about con-
ditions that make life possible.
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The Chemistry of Life 21
forming strong chemical bonds with other carbon atoms and also with
other chemical elements.  e versatility of carbon bonds allows organic
molecules to take on such diverse forms as the long chains that make up

fatty acids, the ring structures that form the basis of sugar compounds,
and even gaseous molecules such as carbon dioxide.  e covalent (mean-
ing that atoms share electrons) chemical bonds that carbon atoms form
with other atoms are relatively strong, making the organic molecules more
stable.
Given the abundance of carbon available on early Earth, and its ideal
chemical properties, it is not surprising that life developed as a carbon-
based system. Another property of carbon molecules that works to the
advantage of essential chemical reactions is their “handedness,” or chiral-
ity. Organic carbon molecules can exist in either a “right-handed” or a
“le -handed” form, which are mirror opposites of each other. Life forms
have developed in a way that incorporates only the right-handed forms of
sugars and the le -handed forms of amino acids. Biochemical reactions
FIGURE 1.7 During a news conference at NASA headquarters in Washington, DC,
in December 2010, NASA astrobiology research fellow Felisa Wolfe-Simon
announces finding a potential new form of life. Wolfe-Simon said that after a two-
year study at Mono Lake in California, she found a bacterium that could eat and
grow on arsenic instead of phosphorus, one of the basic building blocks of life.
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22 THE EVOLUTION OF CELLS
are highly precise in the way that complex molecules must fi t together
and are dependent on the particular right- or le -handed forms of these
molecules being available.
SHAPE MAKES ALL THE DIFFERENCE
Every cellular molecule, no matter how large and complex, is made up of
a series of smaller chemical modules that follow an orderly arrangement
of atoms. A small number of relatively simple chemical building blocks
forms these modules, yet they are capable of arranging themselves in an
almost endless variety of complex three-dimensional forms. Biochemi-
cal reactions take place by means of chemical bonds that form or break

between electrons of nearby atoms. For two complex molecules to interact,
their shapes must fi t together precisely in order to bring electrons of the
appropriate atoms close enough to interact.
THE FOUR MOLECULES OF LIFE
Every cell can be thought of as a miniature factory that takes in raw mate-
rials and processes them to manufacture some product needed by the cell
or for the larger organism of which it forms a part.  e “loading dock”
of the factory, or cell membrane, is highly selective, taking in only the
required raw materials.  e “command center,” or cell nucleus, issues
orders regarding what products the cell will manufacture, and various
transport systems convey materials around to the cell’s “assembly lines,”
or organelles.  e products are then sent out through the cell membrane,
along with waste accumulated during processing.  ese diverse cell func-
tions are carried out by four primary types of organic molecules: nucleic
acids, proteins, carbohydrates, and lipids.  ese are o en referred to as
macromolecules because of their large size, and because they are typically
composed of many smaller molecules chemically bonded together.
Nucleic acids
If cellular molecules had an “all-star” list, DNA and RNA would be at the
top.  ese molecules have become so familiar to those who study science
that we rarely take the time to call them by their full chemical names,
deoxyribonucleic acid and ribonucleic acid.  ese highly important
nucleic acids (NAs) direct a cell’s production of proteins and also store
its genetic code. Although the molecules form long, complex chains, their
chemical structures are fairly easy to understand because they consist of
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The Chemistry of Life 23
repeated modules called nucleotides. Each nucleotide molecule is formed
from three building blocks: a sugar unit, a phosphate unit, and a base unit.
In DNA molecules, the sugar unit is named deoxyribose (D), while ribose

Figure 2.2 Note the base pairs of nucleotides that make up the “rungs” of
the ladder in the structure of DNA.
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24 THE EVOLUTION OF CELLS
(R) serves as the sugar unit in RNA.  e base units, all of which con-
tain nitrogen, may take on one of fi ve chemical structures: cytosine (C),
thymine (T), uracil (U), guanine (G), or adenine (A).  ymine bases are
found only in DNA molecules, uracil bases are found only in RNA mol-
ecules, and the others occur in both DNA and RNA. Although these base
molecules are similar in size, they take on quite diff erent chemical shapes,
and their pattern along the length of the NA molecule provides for the
wide diversity of coded information that these molecules contain.
RNA has a much simpler, and chemically less stable, form than DNA.
Early forms of cells likely relied on RNA both for conveying genetic infor-
mation and for cell maintenance. In the modular structure of RNA, the
ribose sugar unit is linked to a phosphate unit. One of the four bases—
either C, U, G, or A—chemically bonds to each sugar-phosphate unit,
forming a nucleotide. Many sugar-phosphate units then bond together to
form the backbone of a nucleic acid, with the base units projecting to the
side. Despite the relatively simple single-chain structure of RNA, these
chains can be quite long and take various looped and twisted forms, allow-
ing them to perform diff erent roles within a cell. Complex cells depend on
three major forms of RNA to transport information and to manufacture
proteins: messenger RNA, transfer RNA, and ribosomal RNA.
DNA molecules can be thought of as two chains of nucleotides that are
lined up to form a ladder.  e sugar-phosphate units of the chains form
the sides of the ladder, while the projecting base pairs link in the middle
to form the rungs. However, this is a very long ladder; DNA molecules
may contain hundreds of thousands of base-pair rungs. What’s more, the
form of the ladder is twisted, forming the well-known double helix struc-

ture of DNA. Chemical bonding limits the combinations of base pairs to
either adenine + thymine or guanine + cytosine (or thymine + adenine
and cytosine + guanine). Despite this limited number of base pair types,
the coded information they can convey is almost unlimited because there
are so many of them and because of the stable helical structure in which
they are bound.
Proteins
Proteins form another of the macromolecule groups that take a variety of
complex forms, but they are made up of simple modular structures that
are bonded tightly together.  ese chemical modules are amino acids.
 e basic structure of an amino acid consists of a carboxyl group of car-
bon, oxygen, and hydrogen that is linked to an amino group consisting of
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