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Summary molecular cell biology lodish 6th edition

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Molecular cell biology lodish 6th edition

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University Of Rhode Island

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Molecular Cell Biology

L ike ourselves, the individual cells that form our bodies can grow, reproduce, process information, respond to stimuli, and carry out an amazing array of chemical reactions. These abilities define life. We and other multicellular organisms contain billions or trillions of cells organized into ...

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1
LIFE BEGIN S
WITH CELLS

A single ~200 micrometer (␮m) cell, the huma
sperm, w hich are also single cells. From the u
and sperm w ill arise the 10 trillion cells of a h
[Photo Researchers, Inc.]

ike ourselves, the individual cells that form our bodies

L can grow, reproduce, process information, respond to
stimuli, and carry out an amazing array of chemical re-
actions. These abilities define life. We and other multicellular
1.1 The Diversity and Comm
of Cells
Cells come in an amazing variety of sizes an
organisms contain billions or trillions of cells organized into 1-1). Some move rapidly and have fast-chang
complex structures, but many organisms consist of a single we can see in movies of amoebae and rot
cell. Even simple unicellular organisms exhibit all the hall- largely stationary and structurally stable. O
mark properties of life, indicating that the cell is the funda- cells but is an absolute requirement for othe
mental unit of life. As the twenty-first century opens, we face multicellular organisms are intimately invo
an explosion of new data about the components of cells, cells. Although some unicellular organisms
what structures they contain, how they touch and influence others form colonies or live in close associ
each other. Still, an immense amount remains to be learned, types of organisms, such as the bacteria that
particularly about how information flows through cells and tract nitrogen from the air or the bacteria th
how they decide on the most appropriate ways to respond. testines and help us digest food. Despite the
M olecular cell biology is a rich, integrative science that
brings together biochemistry, biophysics, molecular biology,

,2 CH APTER 1 • Life Begins with Cells

(a) (b) (c) (d)

(e) (f) (g) (h)

▲ FIGU RE 1-1 Cells come in an astounding assortment of Purkinje neuron of the cerebellum, w hich can form m
shapes and sizes. Some of the morphological variety of cells is hundred thousand connections w ith other cells throu
illustrated in these photographs. In addition to morphology, cells branched netw ork of dendrites. The cell w as made v
differ in their ability to move, internal organization (prokaryotic introduction of a fluorescent protein; the cell body is
versus eukaryotic cells), and metabolic activities. (a) Eubacteria; the bottom. (g) Cells can form an epithelial sheet, as
note dividing cells. These are Lactococcus lactis, w hich are used through intestine show n here. Each finger-like tow er
to produce cheese such as Roquefort, Brie, and Camembert. villus, contains many cells in a continuous sheet. Nu
(b) A mass of archaebacteria (M ethanosarcina) that produce their transferred from digested food through the epithelia
energy by converting carbon dioxide and hydrogen gas to blood for transport to other parts of the body. New c
methane. Some species that live in the rumen of cattle give rise continuously near the bases of the villi, and old cells
to >150 liters of methane gas/day. (c) Blood cells, show n in false from the top. (h) Plant cells are fixed firmly in place
color. The red blood cells are oxygen-bearing erythrocytes, the plants, supported by a rigid cellulose skeleton. Space
w hite blood cells (leukocytes) are part of the immune system the cells are joined into tubes for transport of w ater
and fight infection, and the green cells are platelets that provide [Part (a) Gary Gaugler/ Photo Researchers, Inc. Part (b) Ralp
substances to make blood clot at a w ound. (d) Large single cells: Visuals Inlimited, Inc. Part (c) NIH/Photo Researchers, Inc. P
fossilized dinosaur eggs. (e) A colonial single-celled green alga, John D. Cunningham/Visuals Unlimited, Inc. Part (e) Carolin
Volvox aureus. The large spheres are made up of many individual Biological/Visuals Unlimited, Inc. Part (f) Helen M . Blau, Sta
cells, visible as blue or green dots. The yellow masses inside are University. Part (g) Jeff Gordon, Washington University Scho
daughter colonies, each made up of many cells. (f) A single M edicine. Part (h) Richard Kessel and C. Shih/Visuals Unlim

, 1.1 • The Diversity and Commo

(a) Prokaryotic cell (b) Eukaryotic cell

Periplasm ic space Nucleus
and cell w all
Golgi v

M ito
Outer m em brane Inner (plasm a) Nucleoid
m em brane 0.5 ␮m

Endoplasm ic reticulum 1

Nucleoid
Nuclear m em bran
Plas

Nucleus

Inner (plasm a) m em brane

Cell w all

Periplasm ic space

Outer m em brane
Rough endoplasm ic
reticulum

▲ FIGU RE 1-2 Prokaryotic cells have a simpler internal characteristic of eukaryotic cells is segregation o
organization than eukaryotic cells. (a) Electron micrograph of a w ithin a defined nucleus, w hich is bounded by a
thin section of Escherichia coli, a common intestinal bacterium. membrane. The outer nuclear membrane is cont
The nucleoid, consisting of the bacterial DNA, is not enclosed rough endoplasmic reticulum, a factory for asse
w ithin a membrane. E. coli and some other bacteria are Golgi vesicles process and modify proteins, mito
surrounded by two membranes separated by the periplasmic energy, lysosomes digest cell materials to recyc
space. The thin cell wall is adjacent to the inner membrane. peroxisomes process molecules using oxygen, a
(b) Electron micrograph of a plasma cell, a type of w hite blood vesicles carry cell materials to the surface to rel
cell that secretes antibodies. Only a single membrane (the plasma [Part (a) courtesy of I. D. J. Burdett and R. G. E. M urra
membrane) surrounds the cell, but the interior contains many P. C. Cross and K. L. M ercer, 1993, Cell and Tissue Ult
membrane-limited compartments, or organelles. The defining A Functional Perspective, W. H. Freeman and Compan

,4 CH APTER 1 • Life Begins with Cells

Anim als Plants so many similarities. In recent years, detailed an
Fungi DN A sequences from a variety of prokaryotic or
Ciliates revealed two distinct types: the so-called “true” ba
Euglena
M icrosporidia EUKARYOTA bacteria, and archaea (also called archaebacteria o
Slim e m olds Working on the assumption that organisms with
Diplom onads
(Giardia lam blia)
genes evolved from a common progenitor more r
those with more dissimilar genes, researchers hav
EUBACTERIA
the evolutionary lineage tree shown in Figure 1-3.
E. coli Sulfolobus this tree, the archaea and the eukaryotes diverged f
ARCHAEA bacteria before they diverged from each other.
B. subtilus Therm ococcus M any archaeans grow in unusual, often ex
Therm otoga
ronments that may resemble ancient condition
M ethanobacterium
first appeared on earth. For instance, halophile
Halococcus ing” ) require high concentrations of salt to s
Flavobacteria thermoacidophiles (“heat and acid loving”) grow i
Green sulfur Halobacterium sulfur springs, where a pH of less than 2 is co
bacteria other archaeans live in oxygen-free milieus a
M ethanococcus methane (CH 4 ) by combining water with carbon
Borrelia
jannaschii
burgdorferi

Unicellular Organisms H elp and H urt U
Presum ed com m on progenitor
Bacteria and archaebacteria, the most abundant
of all extant organism s organisms, are commonly 1–2 ␮m in size. Despit
size and simple architecture, they are remarkab
Presum ed com m on progenitor
of archaebacteria and eukaryotes
cal factories, converting simple chemicals into c
logical molecules. Bacteria are critical to the ear
▲ FIGU RE 1-3 All organisms from simple bacteria to but some cause major diseases: bubonic plague (B
complex mammals probably evolved from a common, single- from Yersinia pestis, strep throat from Streptom
celled progenitor. This family tree depicts the evolutionary culosis from M ycobacterium tuberculosis, an
relations among the three major lineages of organisms. The Bacillus anthracis, cholera from Vibrio cholera
structure of the tree w as initially ascertained from morphological soning from certain types of E. coli and Salm on
criteria: Creatures that look alike w ere put close together. M ore H umans are walking repositories of bacter
recently the sequences of DNA and proteins have been plants and animals. We provide food and shelte
examined as a more information-rich criterion for assigning gering number of “ bugs,” with the greatest conc
relationships. The greater the similarities in these macromolecular
our intestines. Bacteria help us digest our food
sequences, the more closely related organisms are thought to
are able to reproduce. A common gut bacteriu
be. The trees based on morphological comparisons and the fossil
also a favorite experimental organism. In respon
record generally agree w ell w ith those those based on molecular
data. Although all organisms in the eubacterial and archaean
from bacteria such as E. coli, the intestinal cells
lineages are prokaryotes, archaea are more similar to eukaryotes priate shapes to provide a niche where bacteria c
than to eubacteria (“ true” bacteria) in some respects. For facilitating proper digestion by the combined e
instance, archaean and eukaryotic genomes encode homologous bacterial and the intestinal cells. Conversely, exp
histone proteins, w hich associate w ith DNA; in contrast, bacteria testinal cells changes the properties of the bac
lack histones. Likew ise, the RNA and protein components of they participate more effectively in digestion. Su
archaean ribosomes are more like those in eukaryotes than nication and response is a common feature of ce
those in bacteria. The normal, peaceful mutualism of humans
is sometimes violated by one or both parties. W
begin to grow where they are dangerous to us (e.g.,

, 1.1 • The Diversity and Commo

(a) (b)

Red blood cell
3
Sporulation

M erozoites 4
M erozoites
2
Liver Gam etocytes

Hum an 5

1
M osquito
Sperm 6
Sporozoites Egg

7

8
Zygote
Oocyst

▲ FIGU RE 1-4 Plasmodium organisms, the parasites that event that brings on the fevers and shaking chil
cause malaria, are single-celled protozoans w ith a the w ell-know n symptoms of malaria. Some of
remarkable life cycle. M any Plasmodium species are know n, released merozoites infect additional RBCs, crea
and they can infect a variety of animals, cycling betw een insect cycle of production and infection. Eventually, so
and vertebrate hosts. The four species that cause malaria in merozoites develop into male and female game
humans undergo several dramatic transformations w ithin their 5 , another metamorphosis. These cells, w hich
human and mosquito hosts. (a) Diagram of the life cycle. the usual number of chromosomes, cannot surv
Sporozoites enter a human host w hen an infected Anopheles unless they are transferred in blood to an Anoph
mosquito bites a person 1 . They migrate to the liver w here they mosquito. In the mosquito’s stomach, the game
develop into merozoites, w hich are released into the blood 2 . transformed into sperm or eggs (gametes), yet
M erozoites differ substantially from sporozoites, so this metamorphosis marked by development of long
transformation is a metamorphosis (Greek, “ to transform” or flagella on the sperm 6 . Fusion of sperm and e
“ many shapes” ). Circulating merozoites invade red blood cells generates zygotes 7 , w hich implant into the ce
(RBCs) and reproduce w ithin them 3 . Proteins produced by stomach w all and grow into oocysts, essentially
some Plasmodium species move to the surface of infected for producing sporozoites. Rupture of an oocyst
RBCs, causing the cells to adhere to the w alls of blood vessels. thousands of sporozoites 8 ; these migrate to t
This prevents infected RBCs cells from circulating to the spleen glands, setting the stage for infection of anothe
w here cells of the immune system w ould destroy the RBCs and host. (b) Scanning electron micrograph of matur
the Plasmodium organisms they harbor. After grow ing and and emerging sporozoites. Oocysts abut the ex
reproducing in RBCs for a period of time characteristic of each surface of stomach w all cells and are encased w
Plasmodium species, the merozoites suddenly burst forth in membrane that protects them from the host im
synchrony from large numbers of infected cells 4 . It is this system. [Part (b) courtesy of R. E. Sinden.]

Like bacteria, protozoa are usually beneficial members of tis; and Trypanosom a brucei, sleeping sickne
the food chain. They play key roles in the fertility of soil, con- worst of the protozoa, Plasm odium falcipa

,6 CH APTER 1 • Life Begins with Cells

occur during the Plasm odium life cycle are governed by in- make numerous antibiotics and are used in the m
structions encoded in the genetic material of this parasite and of bread, beer, wine, and cheese. N ot so pleasan
triggered by environmental inputs. diseases, which range from relatively innocuou
The other group of single-celled eukaryotes, the yeasts, tions, such as jock itch and athlete’s foot, to life
also have their good and bad points, as do their multicellular Pneum ocystis carinii pneumonia, a common ca
cousins, the molds. Yeasts and molds, which collectively con- among AIDS patients.
stitute the fungi, have an important ecological role in break-
ing down plant and animal remains for reuse. They also
Even Single Cells Can H ave Sex
The common yeast used to make bread and b
(a)
rom yces cerevisiae, appears fairly frequently in t
cause it has proven to be a great experimental org
M ating betw een haploid many other unicellular organisms, yeasts have
1 cells of opposite m ating
type Vegetative grow th types that are conceptually like the male and fem
a α 2
of diploid cells (eggs and sperm) of higher organisms. Two yeas
Diploid cells (a/α) posite mating type can fuse, or mate, to produc
type containing the genetic material from each
Bud 1-5). Such sexual life cycles allow more rapid ch
netic inheritance than would be possible withou
ing in valuable adaptations while quickly
detrimental mutations. That, and not just H o
5
probably why sex is so ubiquitous.
Vegetative
grow th
of haploid
Viruses Are the Ultimate Parasites
cells Four haploid Virus-caused diseases are numerous and all t
ascospores chicken pox, influenza, some types of pneum
w ithin ascus
measles, rabies, hepatitis, the common cold, an
Starvation causes ers. Smallpox, once a worldwide scourge, was e
Ascus ruptures,
4 3 ascus form ation,
spores germ inate a decade-long global immunization effort begi
m eiosis
mid-1960s. Viral infections in plants (e.g., dw
virus in corn) have a major economic impact o
(b) duction. Planting of virus-resistant varieties, d
traditional breeding methods and more recentl
engineering techniques, can reduce crop losses s
M ost viruses have a rather limited host range, i
tain bacteria, plants, or animals (Figure 1-6).
Because viruses cannot grow or reproduce o
they are not considered to be alive. To survive,
Budding (S. cerevisiae) infect a host cell and take over its internal mach
thesize viral proteins and in some cases to replic
▲ FIGU RE 1-5 The yeast Saccharomyces cerevisiae genetic material. When newly made viruses are
reproduces sexually and asexually. (a) Tw o cells that differ in cycle starts anew. Viruses are much smaller than
mating type, called a and ␣, can mate to form an a/␣ cell 1 . order of 100 nanometer (nm) in diameter; in c
The a and ␣ cells are haploid, meaning they contain a single copy bacterial cells are usually ⬎1000 nm (1 nm⫽10 ⫺
of each yeast chromosome, half the usual number. M ating yields virus is typically composed of a protein coat tha
a diploid a/␣ cell containing tw o copies of each chromosome. core containing the genetic material, which carr
During vegetative grow th, diploid cells multiply by mitotic

, 1.1 • The Diversity and Commo

(a) T4 bacteriophage (b) Tobacco m osaic virus

(c) Adenovirus

100 nm

▲ FIGU RE 1-6 Viruses must infect a host cell to grow and infected tobacco plants and stunts their grow th
reproduce. These electron micrographs illustrate some of the causes eye and respiratory tract infections in hu
structural variety exhibited by viruses. (a) T4 bacteriophage has an outer membranous envelope from w hich
(bracket) attaches to a bacterial cell via a tail structure. Viruses glycoprotein spikes protrude. [Part (a) from A. Levi
that infect bacteria are called bacteriophages, or simply phages. Scientific American Library, p. 20. Part (b) courtesy of
(b) Tobacco mosaic virus causes a mottling of the leaves of Part (c) courtesy of Robley C. Williams, University of C

viruses to convey genetic material into cells. To do this, the sues, organs, and appendages. O ur two han
portion of the viral genetic material that is potentially harm- kinds of cells, yet their different arrangeme
ful is replaced with other genetic material, including human image—are critical for function. In addition
genes. The altered viruses, or vectors, still can enter cells tot- hibit distinct functional and/or structural
ing the introduced genes with them (Chapter 9). O ne day, dis- property often called polarity. From such po
eases caused by defective genes may be treated by using viral
vectors to introduce a normal copy of a defective gene into
䉳 FIGU RE 1-7
patients. Current research is dedicated to overcoming the con- few cell division
siderable obstacles to this approach, such as getting the in- fertilized egg set
troduced genes to work at the right places and times. stage for all sub
development. A
mouse embryo is
We Develop from a Single Cell (a) the tw o-cell, (b
(a)
In 1827, German physician Karl von Baer discovered that and (c) eight-cell s
mammals grow from eggs that come from the mother’s The embryo is su
ovary. Fertilization of an egg by a sperm cell yields a zygote, by supporting me
a visually unimpressive cell 200 ␮m in diameter. Every The correspondin
human being begins as a zygote, which houses all the neces- in human develop
sary instructions for building the human body containing occur during the f
days after fertiliza
about 100 trillion (10 14 ) cells, an amazing feat. Development
[Claude Edelmann/P
begins with the fertilized egg cell dividing into two, four, then
Researchers, Inc.]
eight cells, forming the very early embryo (Figure 1-7). Con- (b)

,8 CH APTER 1 • Life Begins with Cells

asymmetric, polarized tissues such as the lining of the intes-
tines and structures like hands and hearts. The features that
make some cells polarized, and how they arise, also are cov-
ered in later chapters.

Stem Cells, Cloning, and Related Techniques
Offer Exciting Possibilities but Raise
Some Concerns
Identical twins occur naturally when the mass of cells com-
posing an early embryo divides into two parts, each of which
develops and grows into an individual animal. Each cell in
an eight-cell-stage mouse embryo has the potential to give
rise to any part of the entire animal. Cells with this capabil-
ity are referred to as em bryonic stem (ES) cells. As we learn
in Chapter 22, ES cells can be grown in the laboratory (cul- ▲ FIGU RE 1-8 Five genetically identical cloned
early sheep embryo w as divided into five groups of
tured) and will develop into various types of differentiated
each w as separately implanted into a surrogate moth
cells under appropriate conditions.
like the natural process of tw inning. At an early stag
The ability to make and manipulate mammalian embryos are able to adjust and form an entire animal; later in
in the laboratory has led to new medical opportunities as the cells become progressively restricted and can no
well as various social and ethical concerns. In vitro fertiliza- so. An alternative w ay to clone animals is to replace
tion, for instance, has allowed many otherwise infertile cou- multiple single-celled embryos w ith donor nuclei from
ples to have children. A new technique involves extraction of adult sheep. Each embryo w ill be genetically identica
nuclei from defective sperm incapable of normally fertiliz- adult from w hich the nucleus w as obtained. Low pe
ing an egg, injection of the nuclei into eggs, and implantation embryos survive these procedures to give healthy an
of the resulting fertilized eggs into the mother. the full impact of the techniques on the animals is no
In recent years, nuclei taken from cells of adult animals [Geoff Tompkinson/Science Photo Library/Photo Researchers
have been used to produce new animals. In this procedure,
the nucleus is removed from a body cell (e.g., skin or blood
cell) of a donor animal and introduced into an unfertilized merous diseases in which particular cell types a
mammalian egg that has been deprived of its own nucleus. or missing, and of repairing wounds more comp
This manipulated egg, which is equivalent to a fertilized egg,
is then implanted into a foster mother. The ability of such a
donor nucleus to direct the development of an entire animal
suggests that all the information required for life is retained 1.2 The M olecules of a Cell
in the nuclei of some adult cells. Since all the cells in an ani- M olecular cell biologists explore how all the
mal produced in this way have the genes of the single origi- properties of the cell arise from underlying mole
nal donor cell, the new animal is a clone of the donor (Figure the assembly of large molecules, binding of larg
1-8). Repeating the process can give rise to many clones. So to each other, catalytic effects that promote part
far, however, the majority of embryos produced by this tech- ical reactions, and the deployment of informatio
nique of nuclear-transfer cloning do not survive due to birth giant molecules. H ere we review the most impor
defects. Even those animals that are born live have shown molecules that form the chemical foundations o
abnormalities, including accelerated aging. The “ rooting” ture and function.
of plants, in contrast, is a type of cloning that is readily ac-
complished by gardeners, farmers, and laboratory technicians.
Small M olecules Carry Energy, Transmit
The technical difficulties and possible hazards of nuclear-

, 1.2 • The M olec

O ne of the best-known small molecules is adenosine polysaccharides. These macromolecules are c
triphosphate (ATP), which stores readily available chemical components of plant cell walls and insect ske
energy in two of its chemical bonds (see Figure 2-24). When polysaccharide is a linear or branched ch
cells split apart these energy-rich bonds in ATP, the released identical sugar units. Such a chain carries
energy can be harnessed to power an energy-requiring number of units. H owever if the units are n
process like muscle contraction or protein biosynthesis. To the order and type of units carry additional
obtain energy for making ATP, cells break down food mole- we see in Chapter 6, some polysaccharides ex
cules. For instance, when sugar is degraded to carbon diox- informational complexity associated with a
ide and water, the energy stored in the original chemical up of different units assembled in a partic
bonds is released and much of it can be “ captured” in ATP property, however, is most typical of the tw
(Chapter 8). Bacterial, plant, and animal cells can all make biological macromolecules—proteins and n
ATP by this process. In addition, plants and a few other or-
ganisms can harvest energy from sunlight to form ATP in
photosynthesis. Proteins Give Cells Structure and Pe
O ther small molecules act as signals both within and be- Cellular Tasks
tween cells; such signals direct numerous cellular activities
(Chapters 13–15). The powerful effect on our bodies of a The varied, intricate structures of proteins
frightening event comes from the instantaneous flooding of carry out numerous functions. Cells string
the body with epinephrine, a small-molecule hormone that ferent amino acids in a linear chain to for
mobilizes the “ fight or flight” response. The movements Figure 2-13). Proteins commonly range in len
needed to fight or flee are triggered by nerve impulses that 1000 amino acids, but some are much sho
flow from the brain to our muscles with the aid of neuro- longer. We obtain amino acids either by sy
transmitters, another type of small-molecule signal that we from other molecules or by breaking down
discuss in Chapter 7. eat. The “ essential” amino acids, from a die
Certain small molecules (monomers) in the cellular soup are the eight that we cannot synthesize and m
can be joined to form polymers through repetition of a single food. Beans and corn together have all eig
type of chemical-linkage reaction (see Figure 2-11). Cells combination particularly nutritious. O nce a
produce three types of large polymers, commonly called acids is formed, it folds into a complex sha
macromolecules: polysaccharides, proteins, and nucleic distinctive three-dimensional structure and
acids. Sugars, for example, are the monomers used to form protein (Figure 1-9).

Insulin DNA

, 10 CH APTER 1 • Life Begins with Cells

Some proteins are similar to one another and therefore lecular weight of 52,700 (g/mol). Assuming t
can be considered members of a protein family. A few hun- typical of eukaryotic proteins, we can calcula
dred such families have been identified. M ost proteins are de- number of protein molecules per liver cell as a
signed to work in particular places within a cell or to be 10 9 from the total protein weight and Avogadr
released into the extracellular (ex tra, “ outside” ) space. Elab- the number of molecules per mole of any che
orate cellular pathways ensure that proteins are transported pound (6.02 ⫻ 10 2 3 ). To carry this calculati
to their proper intracellular (intra, within) locations or se- further, consider that a liver cell contains ab
creted (Chapters 16 and 17). different proteins; thus, a cell contains close
Proteins can serve as structural components of a cell, for molecules of each type of protein on average.
example, by forming an internal skeleton (Chapters 5, 19, and the abundance of different proteins varies wide
20). They can be sensors that change shape as temperature, ion quite rare insulin-binding receptor protein (20
concentrations, or other properties of the cell change. They cules) to the abundant structural protein act
can import and export substances across the plasma mem- molecules).
brane (Chapter 7). They can be enzymes, causing chemical re-
actions to occur much more rapidly than they would without N ucleic Acids Carry Coded Information
the aid of these protein catalysts (Chapter 3). They can bind to for M aking Proteins at the Right Time a
a specific gene, turning it on or off (Chapter 11). They can be
extracellular signals, released from one cell to communicate The information about how, when, and where to p
with other cells, or intracellular signals, carrying information kind of protein is carried in the genetic materia
within the cell (Chapters 13–15). They can be motors that called deoxyribonucleic acid (DN A). The three-
move other molecules around, burning chemical energy (ATP) structure of DNA consists of two long helical stra
to do so (Chapters 19 and 20). coiled around a common axis, forming a double
H ow can 20 amino acids form all the different proteins strands are composed of monomers called nucle
needed to perform these varied tasks? Seems impossible at often are referred to as bases because their struct
first glance. But if a “ typical” protein is about 400 amino cyclic organic bases (Chapter 4).
acids long, there are 20 400 possible different protein se- Four different nucleotides, abbreviated A, T
quences. Even assuming that many of these would be func- are joined end to end in a DN A strand, with th
tionally equivalent, unstable, or otherwise discountable, the projecting out from the helical backbone of the
number of possible proteins is well along toward infinity. DN A double helix has a simple construction: wh
N ext we might ask how many protein molecules a cell is an A in one strand there is a T in the other, a
needs to operate and maintain itself. To estimate this num- matched with a G (Figure 1-10). This complemen
ber, let’s take a typical eukaryotic cell, such as a hepatocyte ing of the two strands is so strong that if com
(liver cell). This cell, roughly a cube 15 ␮m (0.0015 cm) on strands are separated, they will spontaneously
a side, has a volume of 3.4 ⫻ 10 ⫺9 cm 3 (or milliliters). As- gether in the right salt and temperature cond
suming a cell density of 1.03 g/ml, the cell would weigh hybridization is extremely useful for detecting one
3.5 ⫻ 10 ⫺9 g. Since protein accounts for approximately 20 the other. For example, if one strand is purified a
percent of a cell’s weight, the total weight of cellular pro- to a piece of paper, soaking the paper in a solut
tein is 7 ⫻ 10 ⫺10 g. The average yeast protein has a mo- ing the other complementary strand will lead t

Parental Daughter
strands strands

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