Science Brain and Behavior contiuned 9

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WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 87
Eventually, one sprout reaches the intended
target, and this sprout becomes the new axon; all
other sprouts retract. The Schwann cells envelop
the new axon, forming new myelin and restoring
normal function, as shown in Figure 3-10. In
the PNS, then, Schwann cells serve as signposts
to guide axons to their appropriate end points.
Axons can get lost, however, as sometimes happens
after surgeons reattach a severed limb. If
axons destined to innervate one finger end up
innervating another finger instead, the wrong
finger will move when a message is sent along
that neuron.
Unfortunately, glial cells do not provide
much help in allowing neurons in the central
nervous system to regrow, and they may actually
inhibit regrowth. When the CNS is damaged,
as happens, for example, when the spinal
cord is cut, function does not return, even
though the distance that damaged fibers must bridge is short. That recovery should
take place in the peripheral nervous system but not in the central nervous system is
both a puzzle and a challenge in attempts to help people with brain and spinal-cord
injury.
The absence of recovery after spinal-cord injury is especially frustrating, because
the spinal cord contains many axon pathways, just like those found in the PNS. Researchers
investigating how to encourage the regrowth of CNS neurons have focused
on the spinal cord. They have placed tubes across an injured area, trying to get axons
to regrow through the tubes. They have also inserted immature glial cells into injured
areas to facilitate axon regrowth, and they have used chemicals to stimulate the regrowth
of axons. Some success has been obtained with each of these techniques, but
none is as yet sufficiently advanced to treat people with spinal-cord injuries.
INTERNAL STRUCTURE OF A CELL
What is it about the structure of neurons that gives them their remarkable ability to receive,
process, store, and send a seemingly limitless amount of information? To answer
this question, we must look inside a neuron to see what its components are and understand
what these components do. The internal features of a neuron can be colored
with stains and examined under a light microscope that produces an image by reflecting
light waves through the tissue. If the neurons are very small, they can be viewed
In Review .
The two classes of nervous system cells are neurons and glia. The three types of neurons
are sensory neurons, interneurons, and motor neurons. They are the information-conducting
units of the nervous system and either excite or inhibit one another through their
connecting synapses. The five types of glial cells are ependymal cells, astrocytes, microglia,
oligodendroglia, and Schwann cells. Their function is to nourish, insulate, support,
and repair neurons.
(A) When a peripheral axon is
cut, the axon dies.
Cell body
Axon
Axon
sprouts
Cut Schwann cell
Dividing Schwann cells
(B)
Schwann cells first shrink and
then divide, forming glial cells
along the axon's former path.
(C)
The neuron sends out axon
sprouts, one of which finds the
Schwann-cell path and
becomes a new axon.
(D)
Schwann cells envelop the new
axon, forming new myelin.
Degenerating
axon
Schwann cells form myelin
Axon
Myelin
Figure 3-10
Neuron Repair Schwann cells aid the
regrowth of axons in the peripheral
nervous system.
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with an electron microscope, in which electrons take the place of the photons of the
light microscope. Just as we can take apart a robot to see how its pieces work, we can
view the parts of a cell and take apart a cell to understand how its pieces function.
Because a cell is so small, it is sometimes hard to
imagine that it, too, has components.Yet packed inside are
hundreds of interrelated parts that do the cell’s work. This
feature is as true of neurons as it is of any other cell type.
A primary function of a cell is to act as a miniature “factory”
of work centers that manufacture and transport the
proteins that are the cell’s products.
To a large extent, the characteristics of cells are determined
by their proteins. Time and again, when we ask how
a cell performs a certain function, the answer lies in the
structure of a certain protein. Each cell can manufacture
hundreds to thousands of proteins, which variously take
part in building the cell and in communicating with other
cells; when their structures contain errors, they are implicated
in many kinds of brain disease. In the following sections,
we explain how the different parts of a cell contribute
to protein manufacture, describe what a protein is, and detail
some major functions of proteins.
Reviewing some basic chemistry is useful for understanding
this story. The smallest unit of a protein, or any
other chemical substance, is the molecule. Molecules, and
the even smaller atoms that make them up, are the basic
units of a cell factory’s inputs and outputs. Our journey
into the interior of a cell therefore begins with a look at
these basic components. If you already understand the
structure of water and you know what a salt is and what
ions are, this section will serve as a brief review.
Elements and Atoms
Of the earth’s 92 naturally occurring elements, substances
that cannot be broken down into other substances,
the 10 listed in Table 3-2 account for most of a living
cell’s composition. Three elements—oxygen, carbon, and
hydrogen—account for 96 percent of the cell, with the
other 7 elements constituting most of the remaining 4 percent.
Cells also contain many other elements that, although
important, are present in extremely small quantities.
Chemists represent each element with a symbol,
many of which are simply the first one or two letters of
the element’s English name. Examples are the symbols O
for oxygen, C for carbon, and H for hydrogen. Other
symbols, however, come from the element’s Latin name:
K, for instance, is the symbol for potassium, called kalium
in Latin, and Na is the symbol for sodium, in Latin called
natrium.
An atom is the smallest quantity of an element that retains
the properties of that element.An atom has a nucleus
88 ! CHAPTER 3
Chemical Composition of the Brain
Name of Percentage Nucleus and electrons
element Symbol of weight (not to scale)
Hydrogen H 9.5
Carbon C 18.5
Oxygen O 65
Nitrogen N 3.5
Calcium Ca 1.5
Phosphorus P 1.0
Potassium K 0.4
Sulfur S 0.2
Sodium Na 0.2
Chlorine Cl 0.2
– –
–
– –
– –
–
–
–
– –
–
–
–
–
–
–
–
– –
– –
– – – –
–
– –
–
–
–
–
–
–
–
– –
–
–
– –
–
–
– –
– –
–
– –
–
–
– –
– –
– –
–
–
–
–
– –
–
–
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–
–
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–
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– –
–
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–
– –
–
–
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–
–
–
–
– –
– –
–
–
Table 3-2
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that contains neutrons and protons (Figure 3-11). The neutrons are neutral in
charge, but the protons carry a positive charge (!).Orbiting particles called electrons,
each of which carries a negative charge ("), surround the nucleus. The
basic structures of a cell’s most common atoms are shown in the right-hand column
of Table 3-2.
Ordinarily, an atom has an equal number of positive and negative charges
and so is electrically neutral. But elements that are chemically reactive can easily
lose or gain one or more electrons. When an atom gives up an electron, it
becomes positively charged; when it takes on an extra electron, it becomes negatively
charged.
In either case, the charged atom is now an ion. An ion formed by losing one
electron is represented by the element’s symbol and a plus sign. For example, the
symbol K! represents a potassium ion, and Na! represents a sodium ion.An ion
formed by losing two electrons is represented by the element’s symbol followed
by two positive charges (Ca2! for a calcium ion). Some ions that are important
for cell function have gained electrons rather than lost them. Such an ion is represented
by the element’s symbol followed by a negative sign (for example, Cl"
representing an ion of chlorine, called a chloride ion). The positive and negative
charges of ions allow them to interact, a property that is central to cell function.
Molecules
When atoms bind together, they form molecules, the smallest units of a substance that
contain all that substance’s properties. For example, a water molecule (H2O) is the
smallest unit of water that still retains the properties of water. Breaking down water
any further would divide it into its two component elements, the gases hydrogen and
oxygen.
Atomic symbols specify a substance’s formula. For example, the formula H2O indicates
that a water molecule is a union of two hydrogen atoms and one oxygen atom.
Similarly, NaCl, the formula for table salt (sodium chloride), shows that this substance
consists of one sodium atom and one chlorine atom, whereas KCl, the formula for
potassium chloride, another kind of salt, says that this substance is composed of one
potassium atom and one chlorine atom.Water, salts, and ions play prominent parts in
the cell’s functions, as you will learn throughout the next few chapters.
Salts break into their constituent ions in water.When salts such as NaCl and KCl
are formed, the sodium or potassium atom gives up an electron to the chlorine atom.
Therefore these salts are composed of negatively and positively charged ions tightly
held together by their electrical attraction.
In contrast, the atoms that constitute a water molecule are held together by shared
electrons.As you can see in Figure 3-12A, the electrons provided by the H atoms spend
some of their time orbiting the O atom. In this particular case, the electron sharing is
not equal. The shared electrons spend more time orbiting O than they do H, which
gives the oxygen region of the molecule a slight negative charge and leaves the hydrogen
regions with a slight positive charge.Water, therefore, is a polar molecule, meaning
that it has opposite charges on opposite ends (just as the earth does at the North and
South Poles).
Because water molecules are polar, they are electrically attracted to ions and to one
another.A slightly positively charged hydrogen ion of one water molecule is attracted to
the slightly negatively charged oxygen ion of a nearby molecule. This attracting force is
called a hydrogen bond (Figure 3-12B). Each water molecule can form hydrogen bonds
with a maximum of four neighbors. The attraction of water molecules for one another
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 89
Charged chloride ion (Cl–)
The outer orbit gains
an electron.
Charged sodium ion (Na+)
– –
–
– –
–
–
–
–
–
– –
–
–
–
–
–
Chlorine atom (Cl)
Outer orbit contains
7 electrons
The nucleus contains
neutrons and protons. –
– –
–
–
–
– – – –
–
Sodium atom (Na)
Outer orbit contains
1 electron
–
–
–
–
–
–
–
–
–
–
– –
–
– –
–
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–
–
The outer orbit
disappears
because it lost its
only electron.
Figure 3-11
Ion Formation Ions are formed by the
addition or loss of electrons.
Na! sodium ion
K! potassium ion
Ca2! calcium ion
Cl" chlorine ion
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90 ! CHAPTER 3
is also described by the term hydrophilic, or water loving (from the Greek hydro, meaning
“water,” and philic, meaning “love”). Other polar molecules also are hydrophilic—
they, too, are attracted to water molecules.
Hydrogen bonding gives water some interesting properties, such as
high surface tension (small insects can walk across water), strong cohesion
(water droplets cling together), and a high boiling point (the temperature
at which liquid water vaporizes).Water can also break down,
or dissolve, salts.
An example of dissolving is what happens to sodium chloride, or
table salt, in water. As already stated, NaCl is formed by electrical attraction
when sodium atoms give up electrons to chlorine atoms and the
resulting positively and negatively charged ions (Na! and Cl") join together
to form a crystal (Figure 3-13A). Salt cannot retain its crystal
shape in water, however.As shown in Figure 3-13B, the polar water molecules
muscle their way between the Na! and Cl" lattice, surrounding
and separating the ions. The result is salty water. Sodium chloride is one
of the dissolved salts found in the fluid that exists inside and outside
cells.Many other salts, such as KCl (potassium chloride) and CaCl2 (calcium
chloride) are found there as well.
Parts of a Cell
We have compared a cell to a miniature factory, with work centers that
cooperate to make and ship the cell’s products—proteins.We now continue
this analogy as we investigate the internal parts of a cell and how
they function, beginning here with a quick overview of the cell’s internal
structure. Figure 3-14 displays many cellular components.
A factory’s outer wall separates it from the rest of the world and
affords some security. Likewise, a cell’s double-layered outer wall, or cell
H
O
H H
O
H2O
(A) Hydrogen and oxygen share (B)
electrons unequally, which gives
oxygen a negative charge and
hydrogen a positive charge. These
opposite charges on opposite ends
make water a polar molecule.
Hydrogen
bond joins
water
molecules.
O
H
H
+ +
–
–
+ –
+
+
+
+ +
–
–
H
–
– –
–
–
–
–
–
–
– –
–
–
–
–
–
– –
–
Figure 3-12
Chemistry of Water (A) Two hydrogen (H)
atoms share electrons with one oxygen (O) atom.
The resulting water molecule is polar. (B) The
charged regions of a polar water molecule are
attracted to oppositely charged parts of
neighboring water molecules. Each water
molecule can hydrogen bond to a maximum
of four partners.
– –
–
– –
–
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(A)
(B)
– –
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+
+
+
+
+ + +
+
+
+ +
+
+
+ +
+
+ +
+
+
+
+
+ + +
+
Chloride ion (Cl–)
NaCl
Sodium loses (sodium chloride)
one electron to
become a cation
Chlorine gains
one electron to
become a anion
Salt (NaCl)
Water (H2O)
Sodium ion (Na+)
–
Electrical
attraction
–
–
+
– –
–
– –
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–
Sodium atom (11 e–) Chlorine atom (17 e–)
– –
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– – –
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– –
– –
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–
Figure 3-13
Salts Dissolve (A) Crystals of table salt are formed by the electrical
attraction of sodium and chlorine. The positively charged sodium ion (Na!) is
short one electron, whereas the chlorine gains an electron and so has a
negative charge (Cl"). (B) Weakly bound, polar water molecules surround
Na! ions and Cl" ions, dissolving the salt. The positive part of the water
molecule is attracted to the negative Cl" ion, and the negative part of the
water molecule is attracted to the positive Na! ion.
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membrane separates it from its surroundings and allows it to regulate what enters and
leaves its domain. The cell membrane surrounds the neuron’s cell body, dendrites and
their spines, and the axon and its terminals and so forms a boundary around a continuous
intracellular compartment.
Very few substances can enter or leave a cell, because the cell membrane is almost
impenetrable. Proteins made by the cell are embedded in the cell membrane to facilitate
the transport of substances into and out of the cell. Some proteins thus serve as
the cellular factory’s gates.
Although the neurons and glia of the brain appear to be tightly packed together,
they, like all cells, are separated by extracellular fluid composed mainly of water with
dissolved salts and many other chemicals. A similar fluid is found inside a cell as well.
The important point is that the concentrations of substances inside and outside the cell
are different. The fluid inside a cell is known as the intracellular fluid.
Within the cell are other membranes that surround its interior compartments (see
Figure 13-14), similar to the work areas demarcated by the inner walls of a factory. In
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 91
Tubule: Tiny tube that
transports molecules
and helps give the
cell its shape
Intracellular fluid: Fluid
in which the cell’s
internal structures
are suspended
Mitochondrion: Structure
that gathers, stores, and
releases energy
Endoplasmic reticulum:
Folded layers of
membrane where
proteins are assembled
Nuclear membrane:
Membrane surrounding
the nucleus
Nucleus: Structure
containing the
chromosomes and genes
Dendritic spine: Small
protrusion on dendrites
that increases surface area
Dendrite: Cell extension
that collects information
from other cells
Cell membrane:
Membrane
surrounding
Axon: Extension that the cell
transmits information
from cell body to
other cells
Golgi body:
Membranous structure
that packages protein
molecules for transport
Lysosomes: Sacs containing
enzymes that break down
wastes
Microfilaments: Threadlike
fibers making up much of
the cell’s “skeleton” Figure 3-14
Typical Nerve Cell This view inside a
neuron reveals its organelles and other
internal components.
The section on the cell body in the
neural communication module on the
Foundations CD further details the
internal structure of a neuron.
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each inner compartment, the cell concentrates chemicals that it needs while keeping out
unneeded ones. The prominent nuclear membrane surrounds the cell’s nucleus, where
the genetic blueprints for the cell’s proteins are stored, copied, and sent to the “factory
floor.” The endoplasmic reticulum (ER) is an extension of the nuclear membrane where
the cell’s protein products are assembled in accord with instructions from the nucleus.
When those proteins are assembled, they are packaged and sent throughout the
cell. Parts of the cell called the Golgi bodies provide the packaging rooms where the proteins
are wrapped, addressed, and shipped. Other cell components are called tubules,
of which there are a number of kinds. Some (microfilaments) reinforce the cell’s structure,
others aid in the cell’s movements, and still others (microtubules) form the transportation
network that carries the proteins to their destinations, much as roads allow
a factory’s trucks and forklifts to deliver goods to their destinations.
Two other important parts of the cellular factory shown in Figure 3-14 are the mitochondria,
the cell’s power plants that supply its energy needs, and lysosomes, sacklike
vesicles that transport incoming supplies and move and store wastes. Interestingly,
more lysosomes are found in old cells than in young ones. Cells apparently have trouble
disposing of their garbage, just as we do.
With this brief overview of the cell’s internal structure in mind, you can now examine
its parts in more detail, beginning with the cell membrane.
THE CELL MEMBRANE: BARRIER AND GATEKEEPER
The cell membrane separates the intracellular from the extracellular fluid and so allows
the cell to function as an independent unit. The structure of the membrane, shown in
Figure 3-15A, also regulates the movement of substances into and out of the cell.One of
these substances is water. If too much water enters a cell, it will burst, and, if too much
water leaves a cell, it will shrivel.The cell membrane helps ensure that neither will happen.
92 ! CHAPTER 3
The hydrophilic
head has
polar regions. The phosphate
groups attract one
another.
Extra cellular fluid
Int rac ellular f lui d
Fatty acid tails
have no binding
sites for water.
(A) Phospholipid bilayer
(B) Representation of a
phospholipid molecule
(C) More detailed model of a
phospholipid molecule
The hydrophobic
tails have no
polar regions.
…from intracellular
fluid (inside the cell).
+
–
+
–
Cell
membrane
A phospholipid bilayer
separates extracellular
fluid (outside the cell)…
Figure 3-15
Structure of the Cell Membrane
(A) The membrane’s bilayer. (B) Detail of a
phospholipid molecule’s polar head and
electrically neutral tails. (C) Space-filling
model shows why the phosphate head’s
polar regions (positive and negative
poles).
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The cell membrane also regulates the differing concentrations of salts and other
chemicals on its inner and outer sides. This regulation is important because, if the concentrations
of chemicals within a cell become unbalanced, the cell will not function
normally.What properties of a cell membrane allow it to regulate water and salt concentrations
within the cell? One is its special molecular construction. These molecules,
called phospholipids, are named for their structure, shown close up in Figure 3-15B.
The phosopholipid molecule’s “head” contains the element phosphorus (P) bound
to some other atoms, and its two “tails” are lipids, or fat molecules. The head region is
polar, with a slight positive charge in one location and a slight negative charge in
another, like water molecules. The tails consist of hydrogen and carbon atoms that
are tightly bound to one another by their shared electrons; hence there are no polar
regions in the fatty tail. Figure 3-15C shows a chemical model of the phosopholipid
molecule.
The polar head and the nonpolar tails are the underlying reasons that a phospholipid
molecule can form membranes. The heads are hydrophilic and so are attracted to
one another and to polar water molecules. The nonpolar tails have no such attraction
for water. They are hydrophobic, or water hating (the suffix phobic comes from the
Greek word phobia, meaning “fear”). Quite literally, then, the head of a phospholipid
loves water and the tails hate it. To avoid water, the tails of phospholipid molecules
point toward each other, and the hydrophilic heads align with one another and point
outward to the intracellular and extracellular fluid. In this way, the cell membrane consists
of a bilayer (two layers) of phospholipid molecules (see Figure 3-15A).
The bilayer cell membrane is flexible while still forming a remarkable barrier to a
wide variety of substances. It is impenetrable to intracellular and extracellular water,
because polar water molecules cannot pass through the hydrophobic tails on the interior
of the membrane. Ions in the extracellular and intracellular fluid also cannot
penetrate this membrane, because they carry charges and thus cannot pass the phospholipid
heads. In fact, only a few small molecules, such as oxygen (O2), can pass
through a phospholipid bilayer.
Recall that the cell-membrane barrier is punctuated with embedded protein
“doors” that receive its supplies, dispose of its wastes, and ship its products. Before we
describe these mechanisms in detail, we consider how proteins are manufactured and
transported within the cell.
THE NUCLEUS: SITE OF GENE
TRANSCRIPTION
In our factory analogy, the nucleus is the cell’s
executive office where the blueprints for making
proteins are stored, copied, and sent to the factory
floor. These blueprints are called genes,
segments of DNA that encode the synthesis of
particular proteins. Genes are contained within
the chromosomes, the double-helix structures
that hold an organism’s entire DNA sequence. (The name chromosome
means “colored body,” referring to the fact that chromosomes can be readily
stained with certain dyes.)
The chromosomes are like a book of blueprints for making a complex
building, whereas a gene is like one page of the book containing the plan for a
door or a corridor between rooms. Each chromosome contains thousands of
genes. The location of the chromosomes in the nucleus of the cell, the appearance
of a chromosome, and the structure of the DNA in a chromosome are
shown in Figure 3-16.
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 93
Each chromosome is
a double-stranded
molecule of DNA.
Adenine (A) binds
with thymine (T).
Guanine (G) binds
with cytosine (C).
A
T
C
G
Chromosome
DNA
Figure 3-16
A Chromosome The nerve-cell nucleus
contains paired chromosomes of doublestranded
DNA molecules bound together
by a sequence of nucleotide bases.
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This “static” picture of chromosomes does not represent the way that they look in
living cells.Video recordings of the cell nucleus show that chromosomes are constantly
changing shape and moving in relation to one another so as to occupy the best locations
within the nucleus for collecting the building blocks of proteins and making proteins.
By changing shape, chromosomes expose different genes to the surrounding
fluid, thus allowing the processes of protein formation to begin.
A human somatic (body) cell has 23 pairs of chromosomes, or 46 in all (in contrast,
the 23 chromosomes within a reproductive cell are not paired).
Each chromosome is a double-stranded molecule of deoxyribonucleic
acid (DNA), which is capable of replicating and determining the inherited
structure of a cell’s proteins. The two strands of a DNA molecule
coil around each other, as shown in Figure 3-16. Each strand
possesses a variable sequence of four nucleotide bases, the constituent
molecules of the genetic code: adenine (A), thymine (T), guanine (G),
and cytosine (C).
Adenine on one strand always pairs with thymine on the other,
whereas guanine on one strand always pairs with cytosine on the
other. The two strands of the DNA helix are bonded together by the
attraction that these paired bases have for each other. Sequences of
hundreds of nucleotide bases within the chromosomes spell out the
genetic code—for example, ATGCCG, and so forth.
Now you are ready to understand exactly how genes work. Recall
that a gene is a segment of a DNA strand that encodes the synthesis
of a particular protein. The code is contained in the sequence of the
nucleotide bases,much as a sequence of letters spells out a word. The
sequence of bases “spells out” the particular order in which amino
acids, the constituent molecules of proteins, should be assembled to
construct a certain protein.
To initiate the process, the appropriate gene segment of the DNA
strands first unwinds.The exposed sequence of nucleotide bases on one
of the DNA strands then serves as a template to attract free-floating
molecules called nucleotides. The nucleotides thus attached form a complementary
strand of ribonucleic acid (RNA), the single-stranded nucleic acid molecule required
for protein synthesis. This process, called transcription, is shown in steps 1 and 2 of Figure
3-17. (To transcribe means “to copy,” as one would copy a piece of text in a wordprocessing
program.)
The RNA produced through transcription is much like a single strand of DNA except
that the base uracil (U, which also is attracted to adenine) takes the place of
thymine. The transcribed strand of RNA is called messenger RNA (mRNA) because it
carries the genetic code out of the nucleus to the endoplasmic reticulum, where proteins
are manufactured. The sequence for this process is
DNA ! mRNA ! protein
THE ENDOPLASMIC RETICULUM: SITE OF
PROTEIN SYNTHESIS
Steps 3 and 4 in Figure 3-17 show that the ER consists ofmembranous sheets folded to form
numerous channels.A distinguishing feature of the ER is that it may be studded with ribosomes,
protein structures that act as catalysts in the building of proteins.When an mRNA
molecule reaches the ER, it passes through a ribosome,where its genetic code is “read.”
In this process, called translation, a particular sequence of nucleotide bases in the
mRNA is translated into a particular sequence of amino acids. (To translate means to
94 ! CHAPTER 3
Nucleus
Protein
Amino
acid
Ribosome
DNA mRNA
reticulum
Endoplasmic
Gene
mRNA
mRNA
Ribosomes
Nucleus
Endoplasmic
reticulum
DNA uncoils to expose
a gene, a sequence of
nucleotide bases that
codes for a protein.
1
The mRNA leaves
the nucleus and
comes in contact with
ribosomes in the
endoplasmic reticulum.
3
As a ribosome moves
along the mRNA, it
translates the bases
into a specific amino
acid chain, which
forms the protein.
4
The gene serves as a
template for
transcribing a strand
of mRNA.
2
Figure 3-17
Protein Synthesis The flow of
information in a cell is from DNA to
mRNA to protein (peptide chain).
For an animation showing protein
synthesis in action, visit the Chapter 3 Web
links on the Brain and Behavior Web site at
(www.worthpublishers.com/kolb).