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Chapter 11
Gene Expression
and Regulation
Lectures by
Gregory Ahearn
University of North Florida
Copyright © 2009 Pearson Education, Inc..
11.1 How Is The Information In DNA Used
In A Cell?
 Most genes contain information for the
synthesis of a single protein.
• A gene is a stretch of DNA encoding the
instructions for the synthesis of a single
protein.
• Proteins form cellular structures and the
enzymes that catalyze cellular chemical
reactions.
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
 Proteins are synthesized through the
processes of transcription and translation.
• DNA does not directly guide protein synthesis,
but rather, an intermediary—ribonucleic acid
(RNA)—carries information from the nucleus
to the cytoplasm.
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11.1 How Is The Information In DNA Used
In A Cell?
 RNA is different than DNA in three respects:
• RNA is single stranded; DNA is double
stranded.
• RNA has the sugar ribose; DNA has
deoxyribose.
• RNA contains the base uracil; DNA has
thymine.
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11.1 How Is The Information In DNA Used
In A Cell?
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11.1 How Is The Information In DNA Used
In A Cell?
 Protein synthesis
occurs in two steps,
called transcription
and translation.
gene
DNA
(nucleus)
(a) Transcription
messenger RNA
(cytoplasm)
Transcription of the
gene produces an
mRNA with a
nucleotide sequence
complementary to one
of the DNA strands
Translation of the mRNA
produces a protein molecule
with an amino acid sequence
determined by the nucleotide
sequence in the mRNA
(b) Translation
ribosome
protein
Fig. 11-1
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11.1 How Is The Information In DNA Used
In A Cell?
 Transcription: the information contained in
the DNA of a specific gene is copied into
one of three types of RNA
• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Ribosomal RNA (rRNA)
 In eukaryotic cells, transcription occurs in
the nucleus.
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11.1 How Is The Information In DNA Used
In A Cell?
 Translation: ribosomes convert the base
sequence in mRNA to the amino acid
sequence of a protein
• In eukaryotic cells, translation occurs in the
cytoplasm.
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11.2 What Are The Functions Of RNA?
 Messenger RNA carries the code for a
protein from the nucleus to the cytoplasm.
• All RNA is produced by transcription from
DNA, but only mRNA carries the code for
amino acid sequence of a protein.
• mRNA is synthesized in the nucleus and
enters the cytoplasm through nuclear
envelope pores.
• In the cytoplasm, mRNA binds to ribosomes,
which synthesize a protein specified by the
mRNA base sequence; DNA remains in the
nucleus.
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11.2 What Are The Functions Of RNA?
 Messenger RNA (mRNA)
A
U G U
G C
G A G
(a) Messenger RNA (mRNA)
U
U A
The base sequence
of mRNA carries the
information for the
amino acid sequence
of a protein
Fig. 11-2a
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11.2 What Are The Functions Of RNA?
 Ribosomal RNA and proteins form ribosomes.
• Each ribosome consists of two subunits—one small
and one large.
• The small subunit has binding sites for mRNA, a
“start” tRNA, and other proteins that cooperate to
read mRNA to start protein synthesis.
• The large subunit has two binding sites for tRNA
molecules, and one catalytic site where peptide
bonds join amino acids together into a protein.
• During protein synthesis, the two subunits come
together, clasping an mRNA molecule between them.
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11.2 What Are The Functions Of RNA?
 Ribosomal RNA (rRNA)
catalytic site
large
subunit
1
small
subunit
2
tRNA/amino acid
binding sites
rRNA combines with
proteins to form ribosomes;
the small subunit binds
mRNA; the large subunit
binds tRNA and catalyzes
peptide bond formation
between amino acids
during protein synthesis
(b) Ribosome: contains ribosomal RNA (rRNA)
Fig. 11-2b
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11.2 What Are The Functions Of RNA?
 Transfer RNA molecules carry amino acids
to the ribosomes.
• Each cell synthesizes many different kinds of
transfer RNA, one or more for each amino
acid.
• Twenty different kinds of enzymes in the
cytoplasm, one for each amino acid, recognize
the rRNA and attach the correct amino acid.
• These “loaded” tRNA molecules deliver their
amino acids to the ribosome, where they are
incorporated into the growing protein chain.
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11.2 What Are The Functions Of RNA?
 Transfer RNA (tRNA)
tyr
tRNA
attached
amino acid
anticodon
(c) Transfer RNA (tRNA)
Each tRNA carries a specific
amino acid (in this example,
tyrosine [tyr]) to a ribosome
during protein synthesis;
the anticodon of tRNA pairs
with a codon of mRNA,
ensuring that the correct
amino acid is incorporated
into the protein
Fig. 11-2c
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11.3 What Is The Genetic Code?
 The genetic code translates the sequence of
bases in nucleic acids into the sequence of
amino acids in proteins.
• A sequence of three bases codes for an
amino acid; the triplet is called a codon.
• There are 64 possible combinations of
codons, which is more than enough to code
for the 20 amino acids in proteins.
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11.3 What Is The Genetic Code?
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11.3 What Is The Genetic Code?
 How does a cell recognize where codons
start and stop, and where the code for an
entire proteins starts and stops?
• Most codons specify a specific amino acid in a
protein sequence, but others are punctuation
marks that indicate the end of one protein
sequence and the start of another.
• All proteins begin with the start codon AUG
(methionine), and all end with UAG, UAA, or
UGA, called stop codons.
• Almost all amino acids are coded for by more
than one codon (e.g., six codons code for
leucine).
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription copies the genetic information
of DNA into RNA in the nucleus of
eukaryotic cells.
• Transcription is made up of three different
processes:
• Initiation: the promotor region at the
beginning of a gene starts transcription
• Elongation: the main body of a gene is
where the RNA strand is elongated
• Termination: the termination signal at end of
a gene is where RNA synthesis stops
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription begins when RNA polymerase
binds to the promotor of a gene.
• RNA polymerase catalyzes the transcription of
DNA to RNA.
• RNA polymerase first finds the promoter
region (a non-transcribed sequence of DNA
bases) that marks the start of a gene, and
then binds to it, opening up the DNA as it
does.
• Transcription of the gene begins after the
promoter is bound to RNA polymerase.
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Initiation
DNA
gene 1
gene 2
gene 3
RNA
polymerase
DNA
promoter
Initiation: RNA polymerase binds to the promoter region of DNA near
the beginning of a gene, separating the double helix near the promoter.
Fig. 11-3(1)
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Elongation generates a growing strand of
RNA.
• RNA polymerase adds complementary bases
to those in the DNA template strand, to make
a growing RNA strand that has uracil rather
than thymine complementary to adenine.
• The two strands of DNA re-form the original
double helix.
• One end of the growing RNA strand drifts
away from the DNA molecule, while the other
remains attached to the DNA template strand
by the RNA polymerase.
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Elongation
RNA
DNA template strand
Elongation: RNA polymerase travels along the DNA template strand (blue), unwinding
the DNA double helix and synthesizing RNA by catalyzing the addition of ribose nucleotides
into an RNA molecule (red). The nucleotides in the RNA are complementary to the template
strand of the DNA.
Fig. 11-3(2)
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 RNA transcription in action
gene
growing end of
RNA
gene
molecules
DNA
beginning
of gene
Fig. 11-4
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription stops when RNA polymerase
reaches the termination signal.
• RNA polymerase continues along the DNA
template strand until it comes to the
termination signal (a specific sequence of
DNA bases).
• At the termination signal, RNA polymerase
drops off the DNA and releases the completed
RNA molecule.
• The enzyme is ready to bind to another
promoter, to start the process over.
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Termination
termination signal
Termination: At the end of the gene, RNA polymerase encounters a DNA
sequence called a termination signal. RNA polymerase detaches from the
DNA and releases the RNA molecule.
Fig. 11-3(3)
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Conclusion of transcription
RNA
Conclusion of transcription: After termination, the DNA completely rewinds into a double
helix. The RNA molecule is free to move from the nucleus to the cytoplasm for translation,
and RNA polymerase may move to another gene and begin transcription once again.
Fig. 11-3(4)
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
PLAY
Animation—Transcription
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription is selective.
• Some genes are transcribed in all cells
because they encode essential proteins, like
the electron transport chain of mitochondria.
• Other genes are transcribed only in specific
types of cells.
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11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription is selective (continued).
• How do cells regulate which genes are
transcribed?
• Proteins bind to “control regions” near gene
promotors and block or enhance the binding of
RNA polymerase.
• By this means, the amount of a specific
protein encoded by a specific gene in a cell
can be controlled.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 mRNA, with a specific base sequence, is
used during translation to direct the
synthesis of a protein with the amino acid
sequence encoded by the mRNA.
• Decoding the base sequence of mRNA is the
job of tRNA and ribosomes in the cytoplasm.
• The ability of tRNA to deliver the correct
amino acid to the ribosomes depends on base
pairing between each codon of mRNA and a
set of three complementary bases in tRNA,
called the anticodon.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Like transcription, translation has three
steps:
• Initiation of protein synthesis
• Elongation of the protein chain
• Termination of translation
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Initiation: translation begins when tRNA and
mRNA bind to a ribosome
• The first amino acid in all proteins is a methionine
(AUG codon).
• An initiation complex—a small ribosomal subunit, a
methonine tRNA, and a methionine amino acid—
binds to an AUG codon in an mRNA molecule.
• The large subunit of the ribosome joins the complex
to complete the assembly of the ribosome.
• The methionine tRNA binds to the first binding site on
the large ribosome subunit.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Initiation
Initiation:
second tRNA binding site
amino acid
met
met
tRNA
initiation
complex
methionine
tRNA
U A C
small
ribosomal
subunit
A tRNA with an attached
methionine amino acid binds
to a small ribosomal subunit,
forming an initiation complex.
catalytic site
anticodon
U A C
first tRNA
binding
site
mRNA
GC A U G G U U C A
U A C
large
ribosomal
subunit
GC A U G G U U C A
start codon
The initiation complex binds to an
mRNA molecule. The methionine (met)
tRNA anticodon (UAC) base-pairs with
the start codon (AUG) of the mRNA.
The large ribosomal subunit
binds to the small subunit. The
methionine tRNA binds to the first
tRNA site on the large subunit.
Fig. 11-5(1,2,3)
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Elongation: amino acids are added one at a
time to the growing protein chain
• Assembled ribosomes have two binding sites
and a catalytic site.
• The first binding site has methionine and its
rRNA attached.
• The second binding site accepts another tRNA
with an anticodon complementary to the
codon on the mRNA associated with the
second binding site.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Elongation
Elongation:
catalytic site
peptide
bond
U A C C A A
G C A U G G U U C A
The second codon of mRNA
(GUU) base-pairs with the
anticodon (CAA) of a second
tRNA carrying the amino acid
valine (val). This tRNA binds to
the second tRNA site on the
large subunit.
U A C C A A
G C A U G G U U C A
The catalytic site on the
large subunit catalyzes the
formation of a peptide bond
linking the amino acids
methionine and valine. The
two amino acids are now
attached to the tRNA in the
second binding site.
initiator
tRNA detaches
C
A A
G C A U G G U U C A U A G
ribosome moves one codon to the right
The “empty” tRNA is released
and the ribosome moves down the
mRNA, one codon to the right. The
tRNA that is attached to the two
amino acids is now in the first tRNA
binding site and the second tRNA
binding site is empty.
Fig. 11-5(4,5,6)
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Elongation (continued)
• The catalytic site forms a peptide bond between the
two amino acids.
• The ribosome moves to the next codon on mRNA and
shifts the growing amino acid chain from the second
to the first binding site.
• The third amino acid is then added to the chain.
• The ribosome moves along mRNA, adding one amino
acid to the next.
• The process repeats over and over as the ribosome
moves along the mRNA, one codon at a time.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Elongation (continued)
C A A G U A
G C A U G G U U C A U A G
The third codon of mRNA (CAU)
base-pairs with the anticodon (GUA) of
a tRNA carrying the amino acid histidine
(his). This tRNA enters the second tRNA
binding site on the large subunit.
C A A G U A
G C A U G G U U C A U A G
The catalytic site forms a peptide bond
between valine and histidine, leaving the peptide
attached to the tRNA in the second binding site.
The tRNA in the first site leaves, and the
ribosome moves one codon over on the mRNA.
Fig. 11-5(7,8)
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Termination: a stop codon signals the end of
translation
• Ribosome encounters a stop codon in the
mRNA sequence that signals that protein
synthesis is complete.
• Stop codons do not bind tRNA, but rather,
they bind proteins that cause the ribosome to
release the complete amino acid chain.
• The large and small subunits of the ribosome
also come apart once the stop codon is
reached.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Termination
Termination:
completed
peptide
stop codon
CGA A UC U AG UAA
This process repeats until
a stop codon is reached; the
mRNA and the completed
peptide are released from the
ribosome, and the subunits
separate.
Fig. 11-5(9)
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
PLAY
Animation—Translation
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Summing up: transcription and translation
• With a few exceptions, each gene codes for a
single protein.
• Transcription of a protein-coding gene
produces an mRNA that is complementary to
the template strand of the DNA for the gene.
• Enzymes in the cytoplasm attach the
appropriate amino acid to each tRNA.
• The mRNA moves from the nucleus to the
cytoplasm.
• tRNAs carry their attached amino acids to the
ribosome.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Summing up: transcription and translation
(continued)
• At the ribosome, the bases in tRNA
anticodons bind to the complementary bases
in mRNA codons.
• The amino acids attached to the tRNAs line up
in the sequence specified by the codons.
• The ribosome joins the amino acids together
with peptide bonds to form a protein.
• When a stop codon is reached, the finished
protein is released from the ribosome.
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11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Complementary
base pairing is
critical to decoding
genetic information.
gene
(a) DNA
A
T
G G
G
A
G
T
T
T
A
C C
C
T
C
A
A
etc.
G
U
U
etc.
complementary
DNA strand
template DNA
strand
etc.
codons
(b) mRNA
A U
G G
G
A
anticodons
(c) tRNA
U
A
C
C C
U
C
A
A etc.
amino acids
(d) protein
methionine
glycine
valine
etc.
Fig. 11-6
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Protein Synthesis
Suggested Media Enhancement:
Protein Synthesis
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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11.6 How Do Mutations Affect Gene
Function?
 Changes in the sequence of DNA nucleotide
bases as a result of replication errors,
ultraviolet light, chemicals, and many other
environmental factors are called mutations.
• Sometimes during DNA replication, an
incorrect pair of nucleotides is incorporated
into the growing DNA double helix.
• This is called nucleotide substitution, or point
mutation, because the nucleotides in the DNA
sequence are changed.
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11.6 How Do Mutations Affect Gene
Function?
 Mutations (continued)
• A insertion mutation occurs when one or more
new nucleotide pairs are inserted into a gene.
• A deletion mutation occurs when one or more
nucleotide pairs are removed from a gene.
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11.6 How Do Mutations Affect Gene
Function?
 Mutations may have a variety of effects on
protein structure and function.
• The protein may be unchanged.
• The new protein may be functionally
equivalent to the original one.
• Protein function may be changed by an
altered amino acid sequence.
• Protein function may be destroyed by a
premature stop codon.
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11.6 How Do Mutations Affect Gene
Function?
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11.6 How Do Mutations Affect Gene
Function?
 Mutations are the raw material for evolution.
• Mutations are the ultimate source of all genetic
differences among individuals.
• Without mutations, individuals would share the same
DNA sequence.
• Most mutations are harmful; some improve the
individual’s ability to survive and reproduce.
• The mutation may be passed from generation to
generation and become more common over time.
• This process is known as natural selection, and is the
major cause of evolutionary change.
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11.7 Are All Genes Expressed?
 All of the genes in the human genome are
present in each body cell, but individual
cells express only a small fraction of them.
• The particular set of genes that is expressed
depends on the type of cell and the needs of
the organism.
• This regulation of gene expression is crucial
for proper functioning of individual cells and
entire organisms.
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11.7 Are All Genes Expressed?
 Gene expression differs from cell to cell and
over time.
• The set of genes that are expressed depends
on the function of a particular cell.
• Hair cells synthesize the protein keratin, while
muscle cells make the proteins actin and
myosin but do not make keratin.
• A human male does not express a casein
gene, the protein in human milk, but will pass
on the gene for casein synthesis to his
daughter, who will express it if she bears
children.
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11.7 Are All Genes Expressed?
 Environmental cues influence gene
expression.
• Changes in an organism’s environment help
determine which genes are transcribed.
• Longer spring days stimulate the sex organs
of birds to enlarge and produce sex
hormones.
• These hormones cause the birds to produce
eggs and sperm, to mate, and to build nests.
• All these changes result directly or indirectly
from alterations in gene expression.
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11.8 How Is Gene Expression Regulated?
 A cell may regulate gene expression in
many different ways.
• It may alter the rate of transcription of mRNA.
• It may affect how long a given mRNA
molecule lasts before being broken down.
• It may affect how fast the mRNA is translated
into protein.
• It may affect how long the protein lasts, or how
fast a protein enzyme catalyzes a reaction.
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11.8 How Is Gene Expression Regulated?
 Regulatory proteins that bind to promoters
alter the transcription of genes.
• Many steroid hormones act in this way.
• In birds, estrogen enters cells of the female
reproductive system and binds to a receptor
protein during the breeding season.
• The estrogen–receptor combination then
binds to the DNA in a region near the
promotor of an albumen gene.
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11.8 How Is Gene Expression Regulated?
 Regulatory proteins that bind to promoters
alter the transcription of genes (continued).
• This attachment makes it easier for RNA
polymerase to bind to the promotor and to
transcribe large amounts of albumen mRNA,
which is translated into the albumin protein
needed to make eggs.
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11.8 How Is Gene Expression Regulated?
 Some regions of chromosomes are
condensed and not normally transcribed.
• Certain parts of eukaryotic chromosomes are
in a highly condensed, compact state in which
most of the DNA is inaccessible to RNA
polymerase.
• Some of these tightly condensed regions may
contain genes that are not currently being
transcribed, but when those genes are
needed, the portion of the chromosome
containing those genes becomes
“decondensed” so that transcription can occur.
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11.8 How Is Gene Expression Regulated?
 Entire chromosomes may be inactivated
and not transcribed.
• In some cases, almost an entire chromosome
may be condensed, making it largely
inaccessible to RNA polymerase.
• In human females, one of their two X
chromosomes may become inactivated by a
special coating of RNA called Xist, which
condenses the chromosome and prevents
gene transcription.
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11.8 How Is Gene Expression Regulated?
 The condensed X chromosome shows up in
the nucleus as a dark spot called the Barr
body.
Fig. 11-7
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