Download Ch11_lecture students

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.
• Proteins form cellular structures and the enzymes that
catalyze cellular chemical reactions.
 Proteins are synthesized through the processes of
_____________ and ______________.
• DNA does not directly guide protein synthesis, but
rather, an intermediary—ribonucleic acid (RNA)—carries
information from the _________ to the ___________.
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
 RNA is different than DNA:
•
•
•
•
RNA is ________ stranded; DNA is ________ stranded.
RNA has the sugar ________; DNA has ____________.
RNA contains the base ________; DNA has _________.
There are 3 types of RNA:
• Messenger RNA, Transfer RNA, Ribosomal RNA
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
Copyright © 2009 Pearson Education Inc.
11.1 How Is The Information In DNA Used
In A Cell?
 Protein synthesis occurs in two
steps, called transcription and
translation.
 Transcription: the information
contained in the DNA of a
specific gene is copied into
Messenger RNA (mRNA)
gene
DNA
(nucleus)
(a) Transcription
messenger RNA
• occurs in the cytoplasm
Copyright © 2009 Pearson Education Inc.
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
• occurs in the nucleus.
 Translation: ribosomes convert
the base sequence in mRNA to
the amino acid sequence of a
protein
(cytoplasm)
(b) Translation
ribosome
protein
Fig. 11-1
 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 _________.
 In the cytoplasm, mRNA binds to ribosomes, which
synthesize a protein specified by the mRNA base
sequence; DNA remains in the nucleus.
Copyright © 2009 Pearson Education Inc.
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
Copyright © 2009 Pearson Education Inc.
11.2 What Are The Functions Of RNA?
 Ribosomal RNA and proteins form ____________.
• Each ribosome consists of two subunits—one small and one
large.
• The __________ subunit has binding sites for mRNA, a “start”
tRNA, and other proteins that cooperate to read mRNA to start
protein synthesis.
• The __________ 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.
Copyright © 2009 Pearson Education Inc.
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
Copyright © 2009 Pearson Education Inc.
11.2 What Are The Functions Of RNA?
 Transfer RNA molecules carry _________ _______
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) 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.
Copyright © 2009 Pearson Education Inc.
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
Copyright © 2009 Pearson Education Inc.
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 _______.
• There are 64 possible combinations of codons,
which is more than enough to code for the 20
amino acids in proteins.
Copyright © 2009 Pearson Education Inc.
11.3 What Is The Genetic Code?
Copyright © 2009 Pearson Education Inc.
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 _______ (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).
Copyright © 2009 Pearson Education Inc.
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:
• __________: the promotor region at the beginning of a gene
starts transcription
• __________: the main body of a gene is where the RNA
strand is elongated
• __________: the termination signal at end of a gene is where
RNA synthesis stops
Copyright © 2009 Pearson Education Inc.
11.4 How Is The Information In A Gene
Transcribed Into RNA?
 Transcription begins when RNA polymerase binds to the
_________ of a gene.
• RNA ___________ catalyzes the transcription of DNA to
RNA.
• RNA polymerase first finds the promoter region (a nontranscribed 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.
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
11.4 How Is The Information In A Gene
Transcribed Into RNA?
 ____________ 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 __________ 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.
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
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
Copyright © 2009 Pearson Education Inc.
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.
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
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.
• 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.
Copyright © 2009 Pearson Education Inc.
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 ________ with the
amino acid sequence encoded by the mRNA.
• Decoding the base sequence of mRNA is the job of
tRNA and ribosomes in the ______________.
• 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 _______________.
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Like transcription, translation has three steps:
• ___________ of protein synthesis
• ___________ of the protein chain
• ___________ of translation
Copyright © 2009 Pearson Education Inc.
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 (______
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.
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
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 _____ binding sites and a
___________ 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.
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Elongation (continued)
• The catalytic site forms a ________ 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.
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
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.
Copyright © 2009 Pearson Education Inc.
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)
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
 Summary of 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.
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
• 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.
Copyright © 2009 Pearson Education Inc.
11.5 How Is The Information In Messenger
RNA Translated Into Protein?
gene
 Complementary base
pairing is critical to
decoding genetic
information.
(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
Copyright © 2009 Pearson Education Inc.
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 __________.
• 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.
• An ___________ mutation occurs when one or more new
nucleotide pairs are inserted into a gene.
• A ____________ mutation occurs when one or more
nucleotide pairs are removed from a gene.
Copyright © 2009 Pearson Education Inc.
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.
Copyright © 2009 Pearson Education Inc.
11.6 How Do Mutations Affect Gene
Function?
Copyright © 2009 Pearson Education Inc.
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 __________ ____________, and is the
cause
Copyright major
© 2009 Pearson
Education Inc.of evolutionary change.
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.
Copyright © 2009 Pearson Education Inc.
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 _________, while muscle
cells make the proteins actin and myosin but do not make
keratin
.
• A human male does not express a _________ 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.
Copyright © 2009 Pearson Education Inc.
11.7 Are All Genes Expressed?
 Environmental cues influence gene expression.
• Changes in an organism’s environment help determine
which genes are transcribed.
• _________ 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.
Copyright © 2009 Pearson Education Inc.
11.8 How Is Gene Expression Regulated?
 A cell may regulate gene expression in many different
ways.
• It may alter the ______ of transcription of mRNA.
• It may affect how long a given mRNA molecule lasts
before being broken down.
• It may affect how ______ the mRNA is translated into
protein.
• It may affect how long the protein lasts, or how fast a
protein enzyme catalyzes a reaction.
Copyright © 2009 Pearson Education Inc.
11.8 How Is Gene Expression Regulated?
 Regulatory proteins that bind to promoters alter the
transcription of genes.
• Many __________ hormones act in this way.
• In birds, _________ 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.
• 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 ________ protein needed to make
Copyright eggs.
© 2009 Pearson Education Inc.
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.
Copyright © 2009 Pearson Education Inc.
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 _____, which condenses the chromosome and
prevents gene transcription.
Copyright © 2009 Pearson Education Inc.
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
Copyright © 2009 Pearson Education Inc.