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Science Brain and Behavior contiuned 10

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convert one language into another, in contrast
with transcription, in which the language remains
the same.) Transfer RNA (tRNA) assists
in translation. Proteins are just long chains of
amino acids, folded up to form specific shapes.
The flow of information contained in the
genetic code is conceptually quite simple: a DNA
strand is transcribed into an mRNA strand, and
the mRNA strand is translated by ribosomes into
a molecular chain of amino acids. As shown in
Figure 3-18, each group of three consecutive nucleotide
bases along an mRNA molecule encodes
one particular amino acid. These sequences of
three bases are called codons. For example, the base sequence uracil, guanine, guanine
(UGG) encodes the amino acid tryptophan (Trp), whereas the base sequence uracil,
uracil,uracil (UUU) encodes the amino acid phenylalanine (Phe).Codons also direct the
placement of particular amino acids into a polypeptide chain.
Humans require 20 different amino acids for the synthesis of proteins. All 20 are
structurally similar, as illustrated in Figure 3-19A. Each consists of a central carbon
atom (C) bound to a hydrogen atom (H), an amino group (NH3!), a carboxyl group
(COO2"), and a side chain (represented by the letter R). The side chain, which varies
in chemical composition from one amino acid to another, helps to give different protein
molecules their distinctive biochemical properties.
Amino acids are linked together by a special bond called a peptide bond (Figure
3-19B). A series of amino acids is called a polypeptide chain (meaning “many peptides”).
Just as a remarkable number of words can be made from the 26 letters of the
English alphabet, a remarkable number of peptide chains can be made from the 20 different
amino acids. These amino acids can form 400 (20 # 20) different dipeptides
(two-peptide combinations), 8000 (20 # 20 # 20) different tripeptides (three-peptide
combinations), and almost countless polypeptides.
A polypeptide chain and a protein are related, but they are not the same. The relation
is analogous to that between a ribbon and a bow of a particular size and shape
that can be made from the ribbon. Long polypeptide chains have a strong tendency to
twist into a helix (a spiral) or to form pleated sheets, which, in turn, have a strong
tendency to fold together to form more-complex shapes as shown in Figure 3-20. A
folded-up polypeptide chain constitutes a protein. In addition, two or more polypeptide
chains may combine to form a single protein.Many proteins are globular (round)
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 95
DNA
Polypeptide
chain
Amino acids
Codon
C C C C G G
G G G G C
G
C C
TRANSCRIPTION
TRANSLATION
A
T
A
T
A
T
A
T
T
A
Trp Phe Gly Ser
mRNA C G G U U U G G C U C A
Figure 3-18
Transcription and Translation In
protein synthesis (see Figure 3-17), a
strand of DNA is transcribed into mRNA.
Each sequence of three bases in the
mRNA strand (a codon) encodes one
amino acid. Directed by the codons,
the amino acids link together to form
a polypeptide chain. The amino acids
illustrated are tryptophan (Trp),
phenylalanine (Phe), glycine (Gly),
and serine (Ser).
Figure 3-19
Properties of Amino Acids (A) Each
amino acid consists of a central carbon
atom (C) attached to an amine group
(NH3!), a carboxyl group (COO"), and a
distinguishing side chain (R). (B) The
amino acids are linked by peptide bonds
to form a polypeptide chain.
Pleated sheet
Helix
(A) Primary structure
(B) Secondary structures
(C) Tertiary structure (D) Quaternary structure
Amino acid
chains…
…form pleated
sheets or helices.
Sheets and helices…
…fold to form a
protein. A number of
proteins combine…
to form a more
complex protein.
Figure 3-20
Four Levels of Protein Structure
Whether a polypeptide chain forms a
pleated sheet or a helix and its ultimate
three-dimensional shape are determined
by the sequence of amino acids in the
primary structure.
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in shape and others are fibrous, but, within these broad categories, countless variations
are possible. A protein’s shape and ability to change shape and to combine with other
proteins are central to the protein’s function.
GOLGI BODIES AND MICROTUBULES: PROTEIN PACKAGING
AND SHIPMENT
Any one neuron may use as many as 10,000 protein molecules. Some proteins are destined
to be incorporated into the structure of the cell. They become part of the cell
membrane, the nucleus, the ER, and so forth. Other proteins remain in the intracellular
fluid, where they act as enzymes—protein catalysts that facilitate the cell’s chemical
reactions. Still other proteins are excreted by the cell as “messenger molecules” and so
allow the cell to communicate with other cells. Getting all these different proteins to
the right destinations is the task of the cell components that package, label, and ship
them. These components operate much like a postal service.
To reach their appropriate destinations, the protein molecules that have been synthesized
in the cell must first be wrapped in membranes and given labels that indicate
where they are to go. This wrapping and labeling takes place in the organelles called
Golgi bodies. The packaged proteins are then loaded onto motor molecules that “walk”
along the many microtubules radiating through the cell, thus carrying the protein to
its destination. The work of exporting proteins is illustrated in Figure 3-21.
If a protein is destined to remain within the cell, it is unloaded into the intracellular
fluid. If it is to be incorporated into the cell membrane, it is carried to the
membrane, where it inserts itself. Suppose that a particular protein is to be excreted
at the cell membrane. In this process, called exocytosis, the membrane, or vesicle, in
which the protein is wrapped first fuses with the membrane of the cell. Now the protein
inside the vesicle can be expelled into the extracellular fluid.Many excreted proteins
travel to other cells to induce chemical reactions and so serve as messenger
molecules.
THE CELL MEMBRANE REVISITED: CHANNELS, GATES,
AND PUMPS
Knowing something about the structure of proteins will help you to understand other
ways that substances can travel across what would otherwise be an impermeable cell
membrane. Recall that some of the proteins that cells manufacture are carried to the
cell membrane, where they become embedded. Hydrophobic parts of a protein molecule
affix themselves within the cell membrane while hydrophilic parts of the protein
96 ! CHAPTER 3
Channel. Opening in a protein
embedded in the cell membrane that
allows the passage of ions.
Gate. Protein embedded in a cell
membrane that allows substances to pass
through the membrane on some
occasions but not on others.
Pump. Protein in the cell membrane that
actively transports a substance across the
membrane.
The protein may
be incorporated
into the
membrane,…
Nucleus
Proteins formed in the ER
enter the Golgi bodies,
where they are wrapped
in a membrane and given
a shipping address.
1
3
5
…or be excreted
from the cell
by exocytosis.
4
…remain within
the cell to act as
an enzyme,...
Golgi bodies
Endoplasmic reticulum
Vesicle
Microtubule
Each protein is attached
to a motor molecule and
moves along the
microtubule to its
destination.
2
Figure 3-21
Protein Export Exporting a protein
entails packaging, transport, and its
function at the destination.
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stick out into the intra- and extracellular fluid. In this way, membrane protein molecules
span the cell membrane.
These membrane proteins play a number of important roles, one of which is transporting
substances across the membrane.We will consider how three such membrane
proteins work: channels, gates, and pumps. In each case, notice how the function of the
particular protein is an emergent property of its shape.
Both the shape of a protein and its ability to change shape are emergent properties
of the precise sequence of amino acids that compose the protein molecule. Some proteins
change shape when other chemicals bind to them, others change shape as a function
of temperature, and still others change shape in response to changes in electrical
charge. The ability of a protein molecule to change shape is analogous to a lock in a
door.When a key of the appropriate size and shape is inserted into the lock and turned,
the locking device activates and changes shape, allowing the door to be closed or
opened.
An example of a shape-changing protein is the enzyme hexokinase, illustrated in
Figure 3-22. The surface of this protein molecule has a groove, called a receptor, which
is analogous to a keyhole. When another molecule—in this case, glucose—enters the
receptor area, it induces a slight change in the shape of the protein, causing the hexokinase
to embrace the glucose. Either small molecules or other proteins can bind to the
receptors of proteins and cause them to change shape. Changes in shape then allow the
proteins to serve some new function.
A cell-membrane protein’s shape or its ability to change shape enable substances
to cross the cell membrane. Some membrane proteins create channels through which
substances can pass. Different-sized channels in different proteins allow the passage of
different substances. Figure 3-23A illustrates a protein with a particular shape forming
a small channel in the cell membrane that is large enough for potassium (K!) ions, but
not other ions, to pass through. Other protein channels allow sodium ions or chloride
ions to pass into or out of the cell.
Figure 3-23B shows a protein molecule that acts as a gate to regulate the passage
of substances across the cell membrane by changing its shape in response to some trigger,
as the protein hexokinase does in Figure 3-22. The protein allows the passage of
substances when its shape forms a channel and prevents the passage of substances
when its shape leaves the channel closed. Thus a part of this protein acts as a gate.
Changes in the shape of a protein can also allow it to act as a pump. Figure 3-23C
shows a protein that, when Na! and K! ions bind to it, changes its shape to carry
(“pump”) the substances across the membrane, exchanging the Na! on one side of the
membrane for the K! on the other side of the membrane.
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 97
Figure 3-23
Transmembrane Proteins Channels,
gates, and pumps are embedded in the
cell membrane.
Figure 3-22
Receptor Binding When substances
bind to a protein’s receptors, the protein
changes shape, which may change its
function.
Glucose
Glucose bound
to hexokinase
receptor site
Hexokinase
Receptor
site
Protein has a receptor
site for glucose.
Protein changes shape
when glucose docks
with the receptor.
(A) Channel
Gates open Gate closed
K+ K+
K+
(B) Gated channel
Na+
(C) Pump
Na+
Na+
Ions can cross a
cell membrane
through the
appropriately
shaped channel.
A gated channel
allows the passage
of substances when
gates are open…
…and prevents
the passage when
one or both gates
are closed.
A pump
changes shape…
…to carry
substances
across a cell
membrane.
For more information about how
substances are transported across the
membrane, visit the section on membrane
potential in the neural communication
module on the Foundations CD.
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Channels, gates, and pumps play an important role in allowing substances to enter
and leave a cell. This passage of substances is critical in explaining how neurons send
messages. Chapter 4 explores the topic of neuron communication in detail.
GENES, CELLS, AND BEHAVIOR
Genes are the blueprints for proteins, proteins are essential to the function of cells, and
cells produce behavior. That sequence of connections sounds simple enough. But exactly
how one connection leads to another is one of the big challenges for future research.
If you choose a career in neuroscience research, you will most likely be working
out some aspect of this relation.
Just as the replacement of a malfunctioning part of a robot restores its function,
the identification and replacement of an abnormal gene could provide a cure for the
brain and behavioral abnormalities that it produces. Genetic research, then, promises
a revolutionary effect not only on the study of the brain and behavior but also on the
search for new ways to treat genetic disorders. For these reasons, we will focus on
human genetics in the rest of this chapter.
To review, genes are chromosome segments that encode proteins, and proteins
serve as enzymes,membrane channels, and messenger molecules. This knowledge does
not tell you much about the ultimate structure and function of a cell, because so many
genes and proteins take part. The eventual function of a cell is an emergent property
of all its many constituent parts.
Similarly, knowing that behaviors result from the activity of neurons does not tell
you much about the ultimate form that behaviors will take, because so many neurons
participate in them.Your behavior is an emergent property of the action of all your billions
of neurons. The challenge for future research is to be able to explain how genes,
proteins, cells, and behavior are related.
Understanding the contributions of genes alone is a tremendous challenge. The
field of study directed toward understanding how genes produce proteins is called genomics.
Humans have up to 20,000 genes, about half of which contribute to building
the brain. Although each gene is a code for one protein, the number of proteins that
can be produced is much larger than the number of genes.
The number of proteins produced by the genome is increased in four different ways:
1. Most gene pairs have a number of variants, or alleles, and each allele will produce
a slightly different protein, as explained in the next section. In addition, in an individual
organism, one of the gene variants from one parent may be imprinted so
that it is expressed, whereas the other variant is not.
In Review .
Chemical elements within cells combine to form molecules that in turn organize into the
constituent parts of the cell, including the cell membrane, nucleus, endoplasmic reticulum,
Golgi bodies, tubules, and vesicles. Important products of the cell are proteins, which
serve many functions including acting at the cell membrane as channels, gates, and pumps
to allow substances to cross the membrane. Simply put, the sequence of events in building
a protein is: D NA makes mRNA and mRNA makes protein. When formed, the protein
molecules are wrapped by the Golgi bodies and transported to their designated sites in
the neuron, to its membrane, or for export from the cell by microtubules.
98 ! CHAPTER 3
You can learn more about genetics
by exploring the Chapter 3 Web links on
the Brain and Behavior Web site
(www.worthpublishers.com/kolb).
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WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 99
2. Enzymes in the nucleus can edit the mRNA that carries the message from a gene
to produce a protein, resulting in still more protein variants.
3. Once formed, protein molecules can be cleaved by enzymes, producing two or
more different proteins.
4. As we have described, protein molecules can merge to form still different proteins.
The mergers may include thousands of proteins that form interactions that collaborate
to produce biological functions.
In principle, then, there is no upper limit on the number of proteins that could be
manufactured by a cell, but the number of proteins required for normal cell function
is likely fewer than 100,000. Knowing what functions each of those proteins has would
greatly advance our understanding of how the brain is constructed and produces behavior.
The field of study directed toward understanding what all these proteins do is
called proteomics.
Thus, although the Human Genome Project has cataloged the human genome (all the
genes in our species) and the genomes of many other species also have been described,
identifying the function of every gene and describing the emergent properties of their proteins
and their interactions will take a long time. Interestingly, genome size and chromosome
number seem unrelated to the complexity of the organism (Table 3-3). “Knocking
out Genes”on page 100 describes one technique developed by genomics researchers.
Even though neuroscientists cannot yet explain human behavior in relation to
genes and neurons, we know the severe behavioral consequences of about 2000 genetic
abnormalities that affect the nervous system. For example, an error in a gene could produce
a protein that should be a K! channel but will not allow K! to pass, it may produce
a pump that will not pump, or it may produce a protein that the transportation
system of the cell refuses to transport.
With thousands of different proteins in a cell, a genetic mutation that results in an
abnormality of any one protein could have a beneficial effect, it could have little noticeable
effect, or it could have severe negative consequences. Studying genetic abnormalities
is one source of insight into how genes, neurons, and behaviors are linked.
Such studies may also help us to reduce the negative effects of these abnormalities, perhaps
someday even eliminating them completely.
Chromosomes and Genes
Recall that the nucleus of each human somatic cell contains 23 pairs of chromosomes,
or 46 in all. One member of each pair of chromosomes comes from the mother, and
the other member comes from the father. The chromosome pairs are numbered from
Genome Size and Chromosome Number in Selected Species
Species Genome size (base pairs) Chromosome number
Ameba 670,000,000,000 Several hundred
Lily 90,000,000,000 12
Mouse 3,454,200,000 20
Human 2,850,000,000 23
Carp 1,700,000,000 49
Chicken 1,200,000,000 39
Housefly 900,000,000 6
Tomato 655,000,000 12
Table 3-3
On the Brain and Behavior Web site
(www.worthpublishers.com/kolb)
visit the Chapter 3 Web links for the latest
updates on the Human Genome Project.
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100 ! CHAPTER 3
1 to 23, with chromosome 1 being the largest and chromosome 22 being next to the
smallest (chromosome 21 is the smallest; Figure 3-24).
Chromosome pairs 1 through 22 are called autosomes, and they contain the genes
that contribute to most of our physical appearance and behavioral functions. The 23rd
pair comprises the sex chromosomes, which eventually produce our physical and behavioral
sexual characteristics. There are two types of sex chromosomes, referred to as
X and Y because of their appearance. Female mammals have two X chromosomes,
whereas males have an X and a Y.
Knocking Out Genes
Focus on New Research
A small, common nematode that is found in soil and lives on
bacteria, Caenorhabditis elegans, or C. elegans, was the first
animal genome sequenced. It has 19,757 genes that code for
proteins. To discover what these proteins do, Ravi Kamath
and colleagues (2003) developed a method for selectively
“knocking out” each gene and observing the resulting phenotype
(expressed traits) of the worms thus produced.
Their method capitalizes on the way in which cells have
evolved to protect themselves from invasion by the doublestranded
RNA (dsRNA) of viruses. If dsRNA is introduced into
the cell, a mechanism called RNA-mediated interference
(RNAi) inactivates the gene that would produce that sequence
of RNA. Thus, a cell so infected will not produce the affected
mRNA sequence, and its resulting protein will not be made.
C. elegans is remarkable in that, if it eats a dsRNA, gene
silencing can be produced by the knockout method. Kamath
and colleagues infected different strains of bacteria with different
RNAi and examined the phenotype of the offspring of
worms that had particular genes inactivated. In all, they were
able to inactivate 86 percent of C. elegans’s genes. What do
the resulting phenotypes of these offspring tell us about the
function of its genes? Kamath and colleagues were able to
classify identified genes into a number of groups, including
the following two:
Ancient genes, those also found in more phylogenetically
primitive animals. Deletion of one of these genes is
lethal, suggesting that they are essential for life in all animals.
Animal genes, those also found in many species including
mice and humans. Deletion of these genes results in abnormalities
in body shape or movement. C. elegans with
these genetic deletions could thus serve as animal models
for human genetic disorders.
One use of knockout methodology identifies genes with
specific interesting properties. Kaveh Ashrafi and colleagues
(2003) described a screen for fat-regulatory genes in C. elegans.
These researchers included a fluorescent dye, in addition
to the dsRNA, in the worm’s diet. This dye allowed fat
droplets in the intestinal cells of living worms to be visualized
by measuring fluorescence intensity with a light meter.
The scientists identified 305 genes that reduce the
amount of body fat on inactivation and 112 genes that increase
it. Many of these genes have been identified in humans
and so might be new targets for understanding human
fat regulation and for identifying new antiobesity drugs.
Caenorhabditis elegans is a small roundworm about 1 millimeter
long that lives in the soil. It was the first species to have all of its
neurons, synapses, and its genome described.
© Carolina Biological/visuals Unlimited
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WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 101
Because your chromosomes are “matched” pairs, a cell contains two copies of
every gene, one inherited from your mother, the other from your father. These two
matching copies of a gene are called alleles. The term “matched” here does not necessarily
mean identical. The nucleotide sequences in a pair of alleles may be either identical
or different. If they are identical, the two alleles are homozygous (homo means
“the same”). If they are different, the two alleles are heterozygous (hetero means
“different”).
The nucleotide sequence that is most common in a population is called the
wild-type allele, whereas a less frequently occurring sequence is called a mutation.
Mutant genes often determine genetic disorders.
Genotype and Phenotype
The actions of genes give rise to what we call physical or behavioral traits, but these actions
are not always straightforward. A gene may be “imprinted” by one parent so that
it is not expressed, even though present. The actions of a protein manufactured by one
gene may be suppressed or modified by other genes. Developmental age or experiential
factors also may influence gene expression. For these reasons, as well as others,
some genes are not expressed as traits or may be expressed only incompletely.
The proteins and genes that contribute to human skin color provide a good example.
The color expressed depends on the precise complement of a number of different
genes. And environmental factors such as exposure to sunlight may modify gene
expression. Genes and expressed traits can thus be very different, and so scientists distinguish
between them:
Genotype refers to the full set of all the genes that an organism possesses.
Phenotype refers to the appearance of an organism that results from the interaction of
genes with one another and with the environment (the prefix pheno comes from the
Greek word meaning “show”).
The extent of phenotypic variation, given the same genotype, can be dramatic. For
example, in strains of genetically identical mice, some develop a brain with no corpus
callosum, the large band of fibers that connects the two hemispheres (Figure 3-25).
This abnormality is similar to a disorder in humans. The absence of a corpus callosum
has a genetic cause, but something happens in the development of the brain that determines
whether the trait is expressed in a particular mouse. Although the precise
causal factors are not known, they affect the embryo at about the time at which the corpus
callosum should form.
Allele. Alternate form of a gene; a gene
pair contains two alleles.
Homozygous. Having two identical
alleles for a trait.
Heterozygous. Having two different
alleles for the same trait.
Wild type. Refers to a normal (most
common in a population) phenotype or
genotype.
Mutation. Alteration of an allele that
yields a different version of that allele.
Figure 3-24
Human Chromosomes The nucleus of
a human cell contains 23 chromosomes
derived from the father and 23 from the
mother. Sexual characteristics are
determined by the 23rd pair, the sex
chromosomes.
Nucleus
x y
Figure 3-25
Genetic Expression Identical sections
through the brains of genetically
identical mice reveal distinctly different
phenotypes. The mouse in part A has a
corpus callosum, whereas the mouse in
part B does not. Adapted from “Defects
of the Fetal Forebrain in Acallosal Mice,” by
D. Wahlsten and H. W. Ozaki, in Callosal Agenesis
(p. 126), edited by M. Lassonde and M. A. Jeeves,
1994, New York: Plenum Press.
Corpus callosum
Anterior commisure
(A)
(B)
CNRI/Science Photo Library/
Photo Researchers
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This example illustrates the importance of distinguishing between genotype and
phenotype. Having identical genes does not mean that those genes will be identically
expressed. By the same token, even if we knew everything about the structure and function
of our own genes, it would be impossible to predict how much of our behavior is
due to our genotype, because so much of our behavior is phenotypical.
Dominant and Recessive Alleles
If both alleles in a pair of genes are homozygous, the two encode the same protein, but,
if the two alleles in a pair are heterozygous, they encode two different proteins. Three
possible outcomes attend the heterozygous condition when these proteins express a
physical or behavioral trait: (1) only the allele from the mother may be expressed;
(2) only the allele from the father may be expressed; or (3) both alleles may be expressed
simultaneously.
A member of a gene pair that is routinely expressed as a trait is called a dominant
allele; a routinely unexpressed allele is recessive. Alleles can vary considerably in their
dominance. In complete dominance, only the allele’s own trait is expressed in the phenotype.
In incomplete dominance, the expression of the allele’s own trait is only partial.
In codominance, both the the allele’s own trait and that of the other allele in the
gene pair are expressed completely.
The concept of dominant and recessive alleles was first introduced by Gregor
Mendel, a nineteenth-century monk who studied pea plants in his monastery garden
(see Chapter 1). Mendel showed that organisms possess discrete units of heredity,
which we now call genes. Each gene makes an independent contribution to the offspring’s
inheritance, even though that contribution may not always be visible in the offspring’s
phenotype.When paired with a dominant allele, a recessive allele is often not
expressed. Still, it can be passed on to future generations and influence their phenotypes
when not masked by the influence of some dominant trait.
Genetic Mutations
As you know, the mechanism for reproducing genes and passing them on to offspring
is fallible. Errors can arise in the nucleotide sequence when reproductive cells make
gene copies. The new versions of the genes are mutations. The number of potential genetic
mutations is enormous.
A mutation may be as small as a change in a single nucleotide base. Because the
average gene has more than 1200 nucleotide bases, an enormous number of mutations
can potentially occur on a single gene. For example, the BRCA1 gene, found on chromosome
17, predisposes women to breast cancer, and more than 100 different mutations
have already been found on this gene. Thus, in principle, there are more than 100
different ways in which to inherit a predisposition to breast cancer.
A change in a nucleotide or the addition of a nucleotide in a gene sequence can be
either beneficial or disruptive. An example of a mutation that is both causes sickle-cell
anemia, a condition in which blood cells have an abnormal sickle shape that offers
some protection against malaria, but they also have poor oxygen-carrying capacity,
thus weakening the person who possesses them.
Other genetic mutations are more purely beneficial in their results, and still others
are seemingly neutral to the functioning of the organism that carries them. Most
mutations, however, have a negative effect. If not lethal, they produce in their carriers
debilitating physical and behavioral abnormalities.
102 ! CHAPTER 3
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A mutation may have a specific effect on one particular trait or it can have widespread
effects. Most mutant genes responsible for human hereditary disorders cause
multiple symptoms. The abnormal protein produced by the gene takes part in many
different chemical reactions, and so the affected person may have an abnormal appearance
as well as abnormal function.
Mendel’s Principles Apply to Genetic Disorders
Some disorders caused by mutant genes clearly illustrate Mendel’s principles of dominant
and recessive alleles. One is Tay-Sachs disease, caused by a dysfunctional protein
that acts as an enzyme known as HexA (hexosaminidase A), which fails to break down
a class of lipids (fats) in the brain. Symptoms usually appear a few months after birth.
The baby begins to suffer seizures, blindness, and degenerating motor and mental abilities.
Inevitably, the child dies within a few years. The Tay-Sachs mutation appears with
high frequency among certain ethnic groups, including Jews of European origin and
French Canadians.
The dysfunctional Tay-Sachs enzyme is caused by a recessive allele. Distinctive inheritance
patterns result from recessive alleles, because two copies of the allele (one
from the mother and one from the father) are needed for the disorder to develop. A
baby can inherit Tay-Sachs disease only when both parents carry the recessive Tay-Sachs
allele.
Because both parents have survived to adulthood, they must also both possess a
corresponding dominant normal allele for that particular gene pair. The egg and sperm
cells produced by this man and woman will therefore contain a copy of one or the other
of these two alleles.Which allele is passed on is determined completely by chance.
This situation gives rise to three different potential gene combinations in any
child produced by two Tay-Sachs carriers, as diagrammed in Figure 3-26A. The child
may have two normal alleles, in which case he or she will be spared the disorder and
cannot pass on the disease. The child may have one normal and one Tay-Sachs allele,
WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 103
Tay-Sachs disease. Inherited birth
defect caused by the loss of genes that
encode the enzyme necessary for
breaking down certain fatty substances;
appears 4 to 6 months after birth and
results in retardation, physical changes,
and death by about age 5.
" " " "
(A) Recessive gene
Normal
allele
Parents Parents Parents Parents
Offspring Offspring
Normal
carrier
Normal carrier
Normal
Tay-Sachs
allele
Normal
Normal carrier Normal
Normal
carrier
Normal
carrier
Tay-Sachs Normal
Normal carrier Normal carrier
Offspring Offspring
(B) Dominant gene
Carrier
Huntington’s
Normal
Normal Normal
Carrier Carrier
Normal
Huntington’s Huntington’s Huntington’s
Huntington’s
Normal
allele
Huntington
allele
Two copies required
to exhibit trait
Only one copy required
to exhibit trait
Figure 3-26
Inheritance Patterns (A) Recessive
condition: If a parent has one mutant
allele, that parent will not show
symptoms of the disease but will be a
carrier. If both parents carry a mutant
allele, each of their offspring stands a 1
in 4 chance of developing the disease.
(B) Dominant condition: A person with a
single allele will develop the disease. If
this person mates with a normal
partner, offspring have a 50-50 chance
of developing the disease. If both
parents are carriers, both will develop
the disease, and offspring have a
75 percent chance of developing it.
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104 ! CHAPTER 3
in which case he or she, like the parents, will be a carrier of the disorder. Or the child
may have two Tay-Sachs alleles, in which case he or she will develop the disease.
In the recessive condition, the chance of a child of two carriers being normal is 25
percent, the chance of being a carrier is 50 percent, and the chance of having Tay-Sachs
disease is 25 percent. If only one of the parents is a Tay-Sachs carrier and the other is
normal, then any of their children has a 50-50 chance of being either normal or a carrier.
Such a couple has no chance of conceiving a baby with Tay-Sachs disease.
Fortunately, a blood test can detect whether a person carries the recessive Tay-
Sachs allele. This allele operates independently of the dominant allele, just as Mendel
described. As a result, it still produces the defective HexA enzyme, and so the person
who carries it has a higher-than-normal lipid accumulation in the brain.
Because this person also has a normal allele that produces a functional enzyme, the
abnormal lipid accumulation is not enough to cause Tay-Sachs disease. People found
to be carriers can make informed decisions about conceiving children. If they avoid
having children with another Tay-Sachs carrier, none of their children will have the disorder,
although some will probably be carriers.
The one normal allele that a carrier of Tay-Sachs possesses produces enough functional
enzyme to enable the brain to operate in a satisfactory way. It would not be the
case if the normal allele were dominant, however, as happens with the genetic disorder
Huntington’s chorea. In Huntington’s chorea, an abnormal version of a protein known
as huntingtin builds up in nervous system cells. In some way, this protein causes the
death of brain cells, especially cells in the basal ganglia and the cortex, as discussed further
in “Huntington’s Chorea.”
Symptoms can begin anytime from infancy to old age, but they most often start in
midlife. These symptoms include abnormal involuntary movements, which is why the
disorder is called a chorea (from the Greek, meaning “dance”). Other symptoms are
memory loss and eventually a complete deterioration of behavior, followed by death.
The abnormal huntingtin allele is dominant to a normal allele, and so only one defective
allele is needed to cause the disorder.
Figure 3-26B illustrates the inheritance patterns associated with a dominant allele
that produces a disorder such as Huntington’s chorea. If one parent carries the defective
allele, offspring have a 50 percent chance of inheriting the disorder. If both parents
have the defective allele, the chance of inheriting it increases to 75 percent. Because the
abnormal huntingtin allele is usually not expressed until midlife, after the people who
possess it have already had children, it can be passed from generation to generation
even though it is lethal.
As with the Tay-Sachs allele, there is now a test for determining if a person possesses
the allele that causes Huntington’s chorea. If a person is found to have the allele,
he or she can elect not to procreate. A decision not to have children in this case will reduce
the incidence of the abnormal huntingtin allele in the human gene pool.
Chromosome Abnormalities
Genetic disorders are not caused only by single defective alleles. Some nervous system
disorders are caused by aberrations in a part of a chromosome or even an entire chromosome.
Changes in the number of chromosomes, even a doubling of chromosomes
is one way in which new species are produced.
In humans, one condition due to a change in chromosome number is Down’s
syndrome, which affects approximately 1 in 700 children. Down’s syndrome is usually
the result of an extra copy of chromosome 21. One parent (usually the mother) passes
on two of these chromosomes to the child, rather than the normal single chromosome.
Huntington’s chorea. Autosomal
genetic disorder that results in motor
and cognitive disturbances; caused by
an increase in the number of CAG
(cytosine-adenine-guanine) repeats on
chromosome 4.
Down’s syndrome. Chromosomal
abnormality resulting in mental
retardation and other abnormalities,
usually caused by an extra chromosome
21.
CH03.qxd 1/28/05 9:54 AM Page 104

WHAT ARE THE UNITS OF NERVOUS SYSTEM FUNCTION? ! 105
Combining these two chromosomes with one from the other parent yields three chromosomes
21, an abnormal number called a trisomy (Figure 3-27).
Although chromosome 21 is the smallest human chromosome, its trisomy severely
alters a person’s phenotype. As illustrated in Figure 3-27 (bottom), people with
Down’s syndrome have characteristic facial features and short stature. They also endure
heart defects, susceptibility to respiratory infections, and mental retardation.
Huntington’s Chorea
Focus on Disorders
Woody Guthrie, whose protest songs made him a spokesman
for farm workers during the Great Depression of the 1930s,
is revered as one of the founders of American folk music. His
best-known song is “This Land Is Your Land.” Bob Dylan,
who gave his first concert wearing Woody Guthrie’s suit, was
instrumental in reviving Woody’s popularity in the 1960s.
Guthrie died in 1967 after struggling with the symptoms
of what was eventually diagnosed as Huntingon’s chorea.
His mother had died of a similar condition, although her illness
was never diagnosed. Two of Guthrie’s five children,
from two marriages, developed the disease, and his second
wife, Marjorie, became active in promoting its study.
Huntington’s chorea is devastating, characterized by
memory impairment, abnormal uncontrollable movements,
and marked changes in personality, eventually leading to virtually
total loss of normal behavioral, emotional, and intellectual
functioning. Fortunately, it is rare, with an incidence
of only 5 to 10 victims in 100,000 people and is most common
in people of European ancestry.
The symptoms of Huntington’s chorea result from the
degeneration of neurons in the basal ganglia and cortex.
Those symptoms can appear at any age but typically start in
midlife. In 1983, the gene, called the huntingtin gene, responsible
for Huntington’s chorea was located on chromosome
4 and, 10 years later, its abnormality was identified as
an expanded region of the huntingtin protein that it encodes,
a region characterized by many repeats of the codon CAG.
(Normally, people have fewer than 30 CAG repeats.)
The CAG nucleotide sequence encodes the amino acid
glutamine. As a result, the huntingtin protein produced by the
defective huntingtin gene contains many repeats of glutamine
in its polypeptide chain. As the number of repeats increases
beyond 30, the onset of the disease is earlier and earlier. Thus
the disease can begin from very early to very late in life, depending
on the number of repeats. Typically, non-Europeans
have fewer CAG repeats than do Europeans, which accounts
for their decreased susceptibility to Huntington’s chorea.
The area of CAG repeats is also prone to expansion in
transmission from the father, when it can double or even triple
in size. In inheritance from the mother, the area of repeats remains
stable. Despite our current insights into its causes,
many unanswered questions about Huntington’s chorea remain.
One is why symptoms take so long to develop even with
many CAG repeats. A possible answer is that the extra glutamine
segments in the abnormal huntingtin protein cause it to
fold in abnormal ways, rendering it resistant to removal from
the cell. Another question is why the abnormal huntingtin protein
causes cell death only in certain regions of the brain. As
yet, researchers also know little about how the progress of the
disease might be stopped, but finding ways to remove the abnormal
protein is one possibility.
Woody Guthrie.
Photofest
CH03.qxd 1/28/05 9:54 AM Page 105

106 ! CHAPTER 3
They are prone to developing leukemia and Alzheimer’s disease.Although people with
Down’s syndrome usually have a much shorter-than-normal life span, some live to
middle age or beyond. Improved education for children with Down’s syndrome shows
that they can learn to compensate greatly for the brain changes that cause mental
handicaps.
Genetic Engineering
Despite enormous advances in understanding the structure and function of genes,
there remains a huge gap in understanding how genes produce behavior. Nevertheless,
geneticists have invented a number of methods to influence the traits that genes express.
The most recent of these methods, as well as the most direct avenue for the study
of gene expression, is genetic engineering. In its simplest form, genetic engineering entails
either removing a gene from a genome or adding a gene to it. In so-called transgenic
animals, a gene added to the genome is passed along and expressed in subsequent
generations.
Probably the oldest means of influencing genetic traits is the selective breeding of
animals and plants. Beginning with the domestication of wolves into dogs more than
15,000 years ago,many species of animals have been domesticated by selectively breeding
males and females that display particular traits. For example, the selective breeding
of dogs has produced breeds that can run fast, haul heavy loads, retrieve prey, dig for
burrowing animals, climb rocky cliffs in search of sea birds, herd sheep and cattle, or
sit on an owner’s lap and cuddle.
Selective breeding is an effective way to alter gene expression. As is described by
Heidi Parker and her colleagues (2004) in regard to the dog genome, insights into the
relations between genes and the different behaviors displayed by different dog breeds
are possible.
Maintaining spontaneous mutations is another method of affecting genetic traits.
By using this method, researchers create whole populations of animals possessing some
unusual trait that originally arose as an unexpected mutation in only one or a few individual
animals. In laboratory colonies of mice, for example, large numbers of spontaneous
mutations have been discovered and maintained.
There are strains of mice that have abnormal movements, such as reeling, staggering,
and jumping. Some have diseases of the immune system; others have sensory
deficits and are blind or cannot hear. Many of these genetic abnormalities can also be
found in humans. As a result, the neural and genetic bases of the altered behavior in
the mice can be studied systematically to develop treatments for human disorders.
More direct approaches to manipulating the expression of genetic traits include altering
early embryonic development.One such method is cloning. One form of cloning
can produce an offspring that is nearly genetically identical with another animal.
To clone an animal, scientists begin with a cell nucleus containing DNA, usually
from a living animal, place it into an egg from which the nucleus has been removed,
and, after stimulating the egg to start dividing, implant the new embyro into the uterus
of a female. Because each individual animal that develops from these cells is genetically
identical with the donor of the nucleus, clones can be used to preserve valuable traits,
to study the relative influences of heredity and environment, or to produce new tissue
or organs for transplant to the donor.
Dolly, a female sheep, was the first mammal to be cloned (Figure 3-28). It must be
noted that cloned animals are not always normal. Dolly died at quite a young age for a
sheep and displayed a number of symptoms characteristic of premature aging.
Figure 3-27
Chromosome Aberration (Top)
Down’s syndrome, also known as trisomy
21, is caused by an extra copy of
chromosome 21. (Bottom) Chris Burke,
who lives with Down’s syndrome, played
a leading role on the television series
Life Goes On in the 1990s.
Ron Batzdorf/Everett Collection Dr. Dennis Kunkel/Phototake

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Kesehatan bagun Pagi dan Meditasi

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kesehatan biology antara Meditasi

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sprituality antara biology dan hubungan Meditasi kesadaran
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kesehatan biology dan Meditasi

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Saya seorang Buddhist yang sedang menjalani kehidupan Spritual sesuai dengan ajaran Buddha.akan tetapi saya berusaha dengan tekun untuk manfaat bagi umat Buddha supaya terus melestarikan Buddha,Dharmma,dan Sangha dimana saja,sehingga perbuatan karmma baik dapat berbuah dalam ketenangan dan kebahagiaan diri sendiri dan semua makhluk hidup di dunia ini.Agama Buddha adalah merupakan Ajaran yang mengajarkan kita untuk melaksanakan Danasikha,silasikah,Samadhisikha,dan Pannasikha,Demikianlah suatu hujud prilaku dan moral etika dapat berjalan dengan baik di dalam kehidupan dimana berada untuk hidup tenang dan Bahagia sewaktu hidup sebagai umat manusia dimana berada,jadi jalankan kehidupan ini sebaik-baiknya supaya kehidupan dapat mengikuti aturan-aturan kehidupan yang berkeTuhanan Yang Maha Esa.
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