CHAPTER ONE
Cells: The Fundamental
Units of Life
What does it mean to be living? Petunias, people, and pond scum are all UNITY AND DIVERSITY OF CELLS
alive; stones, sand, and summer breezes are not. But what are the fun-
damental properties that characterize living things and distinguish them
from nonliving matter? CELLS UNDER THE MICROSCOPE
The answer hinges on a basic fact that is taken for granted now but
marked a revolution in thinking when first established more than 175 THE PROKARYOTIC CELL
years ago. All living things (or organisms) are built from cells: small,
membrane-enclosed units filled with a concentrated aqueous solution of
chemicals and endowed with the extraordinary ability to create copies of THE EUKARYOTIC CELL
themselves by growing and then dividing in two. The simplest forms of
life are solitary cells. Higher organisms, including ourselves, are commu- MODEL ORGANISMS
nities of cells derived by growth and division from a single founder cell.
Every animal or plant is a vast colony of individual cells, each of which
performs a specialized function that is integrated by intricate systems of
cell-to-cell communication.
Cells, therefore, are the fundamental units of life. Thus it is to cell biol-
ogy-the study of cells and their structure, function , and behavior-that
we look for an answer to the question of what life is and how it works.
With a deeper understanding of cells, we can begin to tackle the grand
historical problems of life on Earth: its mysterious origins, its stunning
diversity produced by billions of years of evolution, and its invasion of
every conceivable habitat on the planet. At the same time, cell biology
can provide us with answers to the questions we have about ourselves:
Where did we come from? How do we develop from a single fertilized egg
cell? How is each of us similar to-yet different from-everyone else on
Earth? Why do we get sick, grow old, and die?
,4 CHAPTER 1 Cells: The Fundamental Units of Life
DNA synthesis orgnnlsrn reproduces. Thus, in _every cell, long polymer chains of DN
( REP LI CATION ) c1rc made from the same set of four monomers, called nuc!eolides t A
c • f , s rung
together In different sequences hke the letters o an alphabet. The .1 f
. d n or
DNA rnatlon encoded in these DNA molecu Ies Is rea out, or transcribed . -
,,
..
rj
""""\ '""
~~
RNA synthesis
TRANSCRIPTION RNA
a related set or polynucleotides called RNA . Although some of these' ~~to
molecules have their own regulatory, structural, or chemical activir A
most arc trans/alee/ into a different type of polymer called a protein. ;~:·
now of information-from DNA to RNA to protein- is so fundam ental is
life that it is referred to as the central dogma (Figure 1- 2). to
.. " Jd l ll bi d db bll oa -· · The appearance and behavior of a cell are dictated largely by its pro.
tein molecules, which serve as structural_ supports,_chemical catalysts,
protein synthesis
j TRANSLATION
PROTEIN
molecular motors, and much more. Protems are built from amino acid-
and all organisms use the same set of 20 amino acids to make their pr; ·
teins. But the amino acids are linked in different sequences, giving eacr
, . •• - :t:'f-e- ... type of protein molecule a different three-dimensional shape, or conjor
amino acids mation, just as different sequences of letters spell different words . In this
Figure 1-2 In all living cells, genetic
way, the same basic biochemical machinery has served to generate the
information flows from DNA to RNA whole gamut of life on Earth (Figure 1-3 ).
(transcription) and from RNA to protein
(translation)-an arrangement known Living Cells Are Self-Replicating Collections of Catalysts
as the central dogma. The sequence of
nucleotides in a particular segment of One of the most commonly cited properties of living things is their abil -
DNA (a gene) is transcribed into an RNA ity to reproduce . For cells, the process involves duplicating their genetic
molecule, which can then be translated into
material and other components and then dividing in two-producing
the linear sequence of amino acids of a
protein . Only a small part of the gene, RNA, a pair of daughter cells that are themselves capable of undergoing tr::
and protein is shown . same cycle of replication.
It is the special relationship between DNA, RNA, and proteins-as
outlined in the central dogma (see Figure 1-2)-that makes this sel f-
replication possible. DNA encodes information that ultimately directs
the assembly of proteins: the sequence of nucleotides in a molecule of
DNA dictates the sequence of amino acids in a protein. Proteins, in turn
catalyze the replication of DNA and the transcription of RNA, and they
participate in the translation of RNA into proteins. This feedback loop
between proteins and polynucleotides underlies the self-reproduci ng
behavior of living things (Figure 1- 4). We discuss this complex inter-
dependence between DNA, RNA, and proteins in detail in Chapters S
through 8.
In addition to their roles in polynucleotide and protein synthesis, protei1:,
also catalyze the many other chemical reactions that keep the self-repl i-
cating system shown in Figure 1-4 running. A living cell can break do\\T
(A) L........J (B) (C) (D)
2µm
Figure 1- 3 All living organisms are constructed from cells. (A) A colony of bacteria, (B) a butterfly, (C) a rose, and (D) a dolphin
are all made of cells that have a fundamentally similar chemistry and operate according to the same basic principles. (A, courtesy
of Janice Carr; D, courtesy of Jonathan Gordon, IFAW.)
, Unity and Diversity of Cells 5
Figure 1- 4 Life is an autocatalytic
ll •<\.:
nucleotides
DNA and RNA
SCtJ UENCE
IN FO HM ATI O N
process. DNA and RNA provide the
sequence information (green arrows) that
is used to produce proteins and to copy
themselves. Proteins, in turn, provide the
catalytic activity (red arrows) needed to
•• synthesize DNA, RNA, and themselves .
• ••
Cl\11\LYT IC
/\CTIVITY Together, these feedback loops create the
proteins self-replicating system that endows living
cells with their ability to reproduce.
nutrients and use the products to both make the building blocks needed
to produce polynucleotides, proteins, and other cell constituents and to
generate the energy needed to power these biosynthetic processes. We
discuss these vital metabolic reactions in detail in Chapters 3 and 13.
Only living cells can perform these astonishing feats of self-replication .
Viruses also contain information in the form of DNA or RNA, but they do
not have the ability to reproduce by their own efforts. Instead, they para-
sitize the reproductive machinery of the cells that they invade to make
copies of themselves. Thus, viruses are not truly considered living. They
are merely chemical zombies: inert and inactive outside their host cells
but able to exert a malign control once they gain entry. We review the !ife
cycle of viruses in Chapter 9.
All Living Cells Have Apparently Evolved from the Same
Ancestral Cell
When a cell replicates its DNA in preparation for cell division, the copy-
ing is not always perfect. On occasion, the instructions are corrupted by
mutations that change the sequence of nucleotides in the DNA. For this
reason, daughter cells are not necessarily exact replicas of their parent.
Mutations can create offspring that are changed for the worse (in that
they are less able to survive and reproduce), changed for the better (in
that they are better able to survive and reproduce), or changed in a neutral
way (in that they are genetically different but equally viable) . The struggle
for survival eliminates the first, favors the second, and tolerates the third .
The genes of the next generation will be the genes of the survivors.
For many organisms, the pattern of heredity may be complicated by sex-
ual reproduction, in which two cells of the same species fuse, pooling
their DNA. The genetic cards are then shuffled, re-dealt, and distributed
in new combinations to the next generation, to be tested again for their
ability to promote survival and reproduction . Mutations are mistakes in the DNA
These simple principles of genetic change and selection, applied repeat- that change the genetic plan from
edly over billions of cell generations, are the basis of evolution-the that of the previous generation.
process by which living species become gradually modified and adapted Imagine a shoe factory. Would you
to their environment in more and more sophisticated ways. Evolution expect mistakes (i.e., unintentional
changes) in copying the shoe
offers a startling but compelling explanation of why present-day cells
design to lead to improvements in
are so similar in their fundamentals : they have all inherited their genetic
the shoes produced? Explain your
instructions from the same common ancestral cell . It is estimated that answer.
this cell existed between 3.5 and 3.8 billion years ago, and we must sup-
pose that it contained a prototype of the universal machinery of all !ife on
Earth today. Through a very long process of mutation and natural selec-
tion, the. descendants of this ancestral cell have gradually diverged to fill
every habitat on Earth with organisms that exploit the potential of the
machinery in a seemingly endless variety of ways.
,6 CHAPTER 1 Cells: The Fundamental Units of Life
ti ns for the Form, Function, and
Genes Provide lnstruc O . ms
Behdvlor of Cells and Organ1s .
, en lire sequence of nucleotides in an or
/\ cell's genome -Ll wt Is, lh c . r "•rn lhat instructs a ce ll h gan.
I I . 0 gcncll c prog u ow l
Ism's DN/\- prov c cs . . d anlrnal embryos, the genome di , 0
behove. ror the cel ls ol pl onl u,.n
adult organi sm with hundreds rfec.ts
0 d1f-
I - d I vclop rncnl o an .
the growl 1 an c. e . . . 'ndividual pl ant or animal, these cells can .
th I O
rcrent cell l~pes. ~i ln an discuss in detail in Chapter 20 . Fa t cells, sk·<:
I
extraordlna11ly varied, as we lls see m as dissimilar as any cell s cc n
I bone ce s, . . . ntiated cell types are gen erated durin g embryonicd
eels,
II and nerve ce . iu 1
be . Yet all these dij]er ein le fertilized egg cell, and they co ntain idcnu-
development from a s g • s Their varied characters stem f
cal co ies of the DNA of the specie · . . . . rorn
P . . .d use their genetic mstruct1ons. Different cell··
the way that md1v1 ua 1ce 11 s • )
. . that is they use their genes to produce sorne
express different genes. ' . h · · t I
. d t others depending on t e1r m erna state and
RNAs and proteins an no ' •
ir ancestor cells have received from their sur-
on cues that th ey an d the . .
roundings-mainly signals from other cells m the orgamsm .
The DNA, therefore, is not just a shopping li~t specifying the molecules
and a cell is not Just an assembly of ail the
th at every ce II muSt make , . . . .
·
1tems on th e 11s
· t . Each cell is capable of carrymg
. out
. a vanety
d · of b1ologi-
.
cal tasks, depending on its environment and its _h1st?ry, a~ . 1_t select1ve.iy
uses the information encoded in its DNA to guide its act1v1t1es .. Later in
this book, we will see in detail how DNA defines both the parts list of the
cell and the rules that decide when and where these parts are to be made.
CELLS UNDER THE MICROSCOPE
Today, we have access to many powerful technologies for deciphering
the principles that govern the structure and activity of the cell. But cell
biology started without these modern tools. The earliest cell biologists
began by simply looking at tissues and cells, and later breaking them
open or slicing them up, attempting to view their contents. What they
saw was to them profoundly baffling-a collection of tiny objects whose
relationship to the properties of living matter seemed an impenetrable
mystery. Nevertheless, this type of visual investigation was the fi rst step
toward understanding tissues and cells, and it remains essential today in
the study of cell biology.
Cells were not made visible until the seventeenth century, when the
microscope was invented. For hundreds of years afterward, all tl:Jt
was known about cells was discovered using this instrumen t. L{1:
microscopes use visible light to illuminate specimens, and they allO\\c'J
biologists to see for the first time the intricate structure that underpin:,; -1ll
living things.
Although these instruments now incorporate many sophistiLJ.,'cl
improvements, the properties of light-specifically its wavelength-I··":'.
the fineness of detail these microscopes reveal. Electron micrusL", 'r \'.:-
!nvented in the l 930~, go beyond this limit by using beams of ele~tl.l::::
instead of beams of hght as the source of illumination; because c::lc:dt l ··;
ha~~ a much shorter wav:length, these instruments greatly ~xtc: 11d .::~'.;
ab1hty to see the fine details of cells and even render some ot the l:it_,
molecules visible individually.
In this section, we describe various forms of light and electron n1 i.L1 \1,
sc~py. These vital tools in the modern cell biology laboratory contnH'~
to improve,_revealing new and sometimes surprising details about 110 11
cells are built and how they operate.
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