1. LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
© 2011 Pearson Education, Inc.
Lectures by
Erin Barley
Kathleen Fitzpatrick
From Gene to Protein
Chapter 17

2. Overview: The Flow of Genetic Information
• The information content of DNA is in the form of
specific sequences of nucleotides
• The DNA inherited by an organism leads to
specific traits by dictating the synthesis of proteins
• Proteins are the links between genotype and
phenotype
• Gene expression, the process by which DNA
directs protein synthesis, includes two stages:
transcription and translation
© 2011 Pearson Education, Inc.

3. Figure 17.1

4. Concept 17.1: Genes specify proteins via
transcription and translation
• How was the fundamental relationship between
genes and proteins discovered?
© 2011 Pearson Education, Inc.

5. Evidence from the Study of Metabolic
Defects
• In 1902, British physician Archibald Garrod first
suggested that genes dictate phenotypes through
enzymes that catalyze specific chemical reactions
• He thought symptoms of an inherited disease
reflect an inability to synthesize a certain enzyme
• Linking genes to enzymes required understanding
that cells synthesize and degrade molecules in a
series of steps, a metabolic pathway
© 2011 Pearson Education, Inc.

6. Nutritional Mutants in Neurospora:
Scientific Inquiry
• George Beadle and Edward Tatum exposed bread
mold to X-rays, creating mutants that were unable
to survive on minimal media
• Using crosses, they and their coworkers identified
three classes of arginine-deficient mutants, each
lacking a different enzyme necessary for
synthesizing arginine
• They developed a one gene–one enzyme
hypothesis, which states that each gene dictates
production of a specific enzyme
© 2011 Pearson Education, Inc.

7. Figure 17.2
Minimal medium
No growth:
Mutant cells
cannot grow
and divide
Growth:
Wild-type
cells growing
and dividing
EXPERIMENT RESULTS
CONCLUSION
Classes of Neurospora crassa
Wild type Class I mutants Class II mutants Class III mutants
Minimal
medium
(MM)
(control)
MM +
ornithine
MM +
citrulline
Condition
MM +
arginine
(control)
Summary
of results
Can grow with
or without any
supplements
Can grow on
ornithine,
citrulline, or
arginine
Can grow only
on citrulline or
arginine
Require arginine
to grow
Wild type
Class I mutants
(mutation in
gene A)
Class II mutants
(mutation in
gene B)
Class III mutants
(mutation in
gene C)
Gene
(codes for
enzyme)
Gene A
Gene B
Gene C
Precursor Precursor Precursor Precursor
Enzyme A Enzyme A Enzyme A Enzyme A
Enzyme B Enzyme B Enzyme B Enzyme B
Enzyme C Enzyme C Enzyme C Enzyme C
Ornithine Ornithine Ornithine Ornithine
Citrulline Citrulline Citrulline Citrulline
Arginine Arginine Arginine Arginine

8. Figure 17.2a
Minimal medium
No growth:
Mutant cells
cannot grow
and divide
Growth:
Wild-type
cells growing
and dividing
EXPERIMENT

9. Figure 17.2b
RESULTS
Classes of Neurospora crassa
Wild type Class I mutants Class II mutants Class III mutants
Minimal
medium
(MM)
(control)
MM +
ornithine
MM +
citrulline
Condition
MM +
arginine
(control)
Summary
of results
Can grow with
or without any
supplements
Can grow on
ornithine,
citrulline, or
arginine
Can grow only
on citrulline or
arginine
Require arginine
to grow
Growth
No
growth

10. Figure 17.2c
CONCLUSION
Wild type
Class I mutants
(mutation in
gene A)
Class II mutants
(mutation in
gene B)
Class III mutants
(mutation in
gene C)
Gene
(codes for
enzyme)
Gene A
Gene B
Gene C
Precursor Precursor Precursor Precursor
Enzyme A Enzyme A Enzyme A Enzyme A
Enzyme B Enzyme B Enzyme B Enzyme B
Ornithine Ornithine Ornithine Ornithine
Enzyme C Enzyme C Enzyme CEnzyme C
Citrulline Citrulline Citrulline Citrulline
Arginine Arginine Arginine Arginine

11. The Products of Gene Expression:
A Developing Story
• Some proteins aren’t enzymes, so researchers
later revised the hypothesis: one gene–one
protein
• Many proteins are composed of several
polypeptides, each of which has its own gene
• Therefore, Beadle and Tatum’s hypothesis is
now restated as the one gene–one polypeptide
hypothesis
• Note that it is common to refer to gene products
as proteins rather than polypeptides
© 2011 Pearson Education, Inc.

12. Basic Principles of Transcription and
Translation
• RNA is the bridge between genes and the
proteins for which they code
• Transcription is the synthesis of RNA using
information in DNA
• Transcription produces messenger RNA
(mRNA)
• Translation is the synthesis of a polypeptide,
using information in the mRNA
• Ribosomes are the sites of translation
© 2011 Pearson Education, Inc.

13. • In prokaryotes, translation of mRNA can begin
before transcription has finished
• In a eukaryotic cell, the nuclear envelope
separates transcription from translation
• Eukaryotic RNA transcripts are modified through
RNA processing to yield the finished mRNA
© 2011 Pearson Education, Inc.

14. • A primary transcript is the initial RNA
transcript from any gene prior to processing
• The central dogma is the concept that cells are
governed by a cellular chain of command:
DNA → RNA → protein
© 2011 Pearson Education, Inc.

15. Figure 17.UN01
DNA RNA Protein

16. Figure 17.3
DNA
mRNA
Ribosome
Polypeptide
TRANSCRIPTION
TRANSLATION
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
DNA
mRNA
Pre-mRNA
RNA PROCESSING
(a) Bacterial cell (b) Eukaryotic cell
Nuclear
envelope

17. Figure 17.3a-1
TRANSCRIPTION
DNA
mRNA
(a) Bacterial cell

18. Figure 17.3a-2
TRANSCRIPTION
DNA
mRNA
(a) Bacterial cell
TRANSLATION
Ribosome
Polypeptide

19. Figure 17.3b-1
Nuclear
envelope
DNA
Pre-mRNA
(b) Eukaryotic cell
TRANSCRIPTION

20. Figure 17.3b-2
RNA PROCESSING
Nuclear
envelope
DNA
Pre-mRNA
(b) Eukaryotic cell
mRNA
TRANSCRIPTION

21. Figure 17.3b-3
RNA PROCESSING
Nuclear
envelope
DNA
Pre-mRNA
(b) Eukaryotic cell
mRNA
TRANSCRIPTION
TRANSLATION Ribosome
Polypeptide

22. The Genetic Code
• How are the instructions for assembling amino
acids into proteins encoded into DNA?
• There are 20 amino acids, but there are only four
nucleotide bases in DNA
• How many nucleotides correspond to an amino
acid?
© 2011 Pearson Education, Inc.

23. Codons: Triplets of Nucleotides
• The flow of information from gene to protein is
based on a triplet code: a series of
nonoverlapping, three-nucleotide words
• The words of a gene are transcribed into
complementary nonoverlapping three-nucleotide
words of mRNA
• These words are then translated into a chain of
amino acids, forming a polypeptide
© 2011 Pearson Education, Inc.

24. Figure 17.4
DNA
template
strand
TRANSCRIPTION
mRNA
TRANSLATION
Protein
Amino acid
Codon
Trp Phe Gly
5′
5′
Ser
U U U U U
3′
3′
5′3′
G
G
G G C C
T
C
A
A
AAAAA
T T T T
T
G
G G G
C C C G G
DNA
molecule
Gene 1
Gene 2
Gene 3
C C

25. • During transcription, one of the two DNA strands,
called the template strand, provides a template
for ordering the sequence of complementary
nucleotides in an RNA transcript
• The template strand is always the same strand for
a given gene
• During translation, the mRNA base triplets, called
codons, are read in the 5′ to 3′ direction
© 2011 Pearson Education, Inc.

26. • Codons along an mRNA molecule are read by
translation machinery in the 5′ to 3′ direction
• Each codon specifies the amino acid (one of 20)
to be placed at the corresponding position along
a polypeptide
© 2011 Pearson Education, Inc.

27. Cracking the Code
• All 64 codons were deciphered by the mid-1960s
• Of the 64 triplets, 61 code for amino acids; 3
triplets are “stop” signals to end translation
• The genetic code is redundant (more than one
codon may specify a particular amino acid) but
not ambiguous; no codon specifies more than
one amino acid
• Codons must be read in the correct reading
frame (correct groupings) in order for the
specified polypeptide to be produced
© 2011 Pearson Education, Inc.

28. Figure 17.5
Second mRNA base
FirstmRNAbase(5′endofcodon)
ThirdmRNAbase(3′endofcodon)
UUU
UUC
UUA
CUU
CUC
CUA
CUG
Phe
Leu
Leu
Ile
UCU
UCC
UCA
UCG
Ser
CCU
CCC
CCA
CCG
UAU
UAC
Tyr
Pro
Thr
UAA Stop
UAG Stop
UGA Stop
UGU
UGC
Cys
UGG Trp
GC
U
U
C
A
U
U
C
C
C
A
U
A
A
A
G
G
His
Gln
Asn
Lys
Asp
CAU CGU
CAC
CAA
CAG
CGC
CGA
CGG
G
AUU
AUC
AUA
ACU
ACC
ACA
AAU
AAC
AAA
AGU
AGC
AGA
Arg
Ser
Arg
Gly
ACGAUG AAG AGG
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
GAU
GAC
GAA
GAG
Val Ala
GGU
GGC
GGA
GGG
Glu
Gly
G
U
C
A
Met or
start
UUG
G

29. Evolution of the Genetic Code
• The genetic code is nearly universal, shared by
the simplest bacteria to the most complex animals
• Genes can be transcribed and translated after
being transplanted from one species to another
© 2011 Pearson Education, Inc.

30. Figure 17.6
(a) Tobacco plant expressing
a firefly gene gene
(b) Pig expressing a jellyfish

31. Figure 17.6a
(a) Tobacco plant expressing
a firefly gene

32. Figure 17.6b
(b) Pig expressing a jellyfish
gene

33. Concept 17.2: Transcription is the DNA-
directed synthesis of RNA: a closer look
• Transcription is the first stage of gene expression
© 2011 Pearson Education, Inc.

34. Molecular Components of Transcription
• RNA synthesis is catalyzed by RNA polymerase,
which pries the DNA strands apart and hooks
together the RNA nucleotides
• The RNA is complementary to the DNA template
strand
• RNA synthesis follows the same base-pairing
rules as DNA, except that uracil substitutes for
thymine
© 2011 Pearson Education, Inc.

35. • The DNA sequence where RNA polymerase
attaches is called the promoter; in bacteria, the
sequence signaling the end of transcription is
called the terminator
• The stretch of DNA that is transcribed is called a
transcription unit
© 2011 Pearson Education, Inc.
Animation: Transcription

36. Figure 17.7-1 Promoter
RNA polymerase
Start point
DNA
5′
3′
Transcription unit
3′
5′

37. Figure 17.7-2 Promoter
RNA polymerase
Start point
DNA
5′
3′
Transcription unit
3′
5′
Initiation
5′
3′
3′
5′
Nontemplate strand of DNA
Template strand of DNA
RNA
transcript
Unwound
DNA
1

38. Figure 17.7-3 Promoter
RNA polymerase
Start point
DNA
5′
3′
Transcription unit
3′
5′
Elongation
5′
3′
3′
5′
Nontemplate strand of DNA
Template strand of DNA
RNA
transcript
Unwound
DNA
2
3′
5′3′
5′
3′
Rewound
DNA
RNA
transcript
5′
Initiation1

39. Figure 17.7-4 Promoter
RNA polymerase
Start point
DNA
5′
3′
Transcription unit
3′
5′
Elongation
5′
3′
3′
5′
Nontemplate strand of DNA
Template strand of DNA
RNA
transcript
Unwound
DNA
2
3′
5′3′
5′
3′
Rewound
DNA
RNA
transcript
5′
Termination3
3′
5′
5′
Completed RNA transcript
Direction of transcription (“downstream”)
5′
3′
3′
Initiation1

40. Synthesis of an RNA Transcript
• The three stages of transcription
– Initiation
– Elongation
– Termination
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41. RNA Polymerase Binding and Initiation of
Transcription
• Promoters signal the transcriptional start point
and usually extend several dozen nucleotide pairs
upstream of the start point
• Transcription factors mediate the binding of
RNA polymerase and the initiation of transcription
• The completed assembly of transcription factors
and RNA polymerase II bound to a promoter is
called a transcription initiation complex
• A promoter called a TATA box is crucial in
forming the initiation complex in eukaryotes
© 2011 Pearson Education, Inc.

42. Figure 17.8
Transcription initiation
complex forms
3
DNA
Promoter
Nontemplate strand
5′
3′
5′
3′
5′
3′
Transcription
factors
RNA polymerase II
Transcription factors
5′
3′
5′
3′
5′
3′
RNA transcript
Transcription initiation complex
5′
3′
TATA box
T
T T T T T
A AA AA
A A
T
Several transcription
factors bind to DNA
2
A eukaryotic promoter1
Start point Template strand

43. Elongation of the RNA Strand
• As RNA polymerase moves along the DNA, it
untwists the double helix, 10 to 20 bases at a
time
• Transcription progresses at a rate of 40
nucleotides per second in eukaryotes
• A gene can be transcribed simultaneously by
several RNA polymerases
• Nucleotides are added to the 3′ end of the
growing RNA molecule
© 2011 Pearson Education, Inc.

44. Nontemplate
strand of DNA
RNA nucleotides
RNA
polymerase
Template
strand of DNA
3′
3′5′
5′
5′
3′
Newly made
RNA
Direction of transcription
A
A A A
A
A
A
T
T
T
T
TTT G
G
G
C
C C
C
C
G
C CC A A
A
U
U
U
end
Figure 17.9

45. Termination of Transcription
• The mechanisms of termination are different in
bacteria and eukaryotes
• In bacteria, the polymerase stops transcription at
the end of the terminator and the mRNA can be
translated without further modification
• In eukaryotes, RNA polymerase II transcribes the
polyadenylation signal sequence; the RNA
transcript is released 10–35 nucleotides past this
polyadenylation sequence
© 2011 Pearson Education, Inc.

46. Concept 17.3: Eukaryotic cells modify RNA
after transcription
• Enzymes in the eukaryotic nucleus modify pre-
mRNA (RNA processing) before the genetic
messages are dispatched to the cytoplasm
• During RNA processing, both ends of the primary
transcript are usually altered
• Also, usually some interior parts of the molecule
are cut out, and the other parts spliced together
© 2011 Pearson Education, Inc.

47. Alteration of mRNA Ends
• Each end of a pre-mRNA molecule is modified
in a particular way
– The 5′ end receives a modified nucleotide 5′
cap
– The 3′ end gets a poly-A tail
• These modifications share several functions
– They seem to facilitate the export of mRNA to
the cytoplasm
– They protect mRNA from hydrolytic enzymes
– They help ribosomes attach to the 5′ end
© 2011 Pearson Education, Inc.

48. Figure 17.10
Protein-coding
segment
Polyadenylation
signal
5′ 3′
3′5′ 5′Cap UTR
Start
codon
G P P P
Stop
codon
UTR
AAUAAA
Poly-A tail
AAA AAA…

49. Split Genes and RNA Splicing
• Most eukaryotic genes and their RNA transcripts
have long noncoding stretches of nucleotides that
lie between coding regions
• These noncoding regions are called intervening
sequences, or introns
• The other regions are called exons because they
are eventually expressed, usually translated into
amino acid sequences
• RNA splicing removes introns and joins exons,
creating an mRNA molecule with a continuous
coding sequence
© 2011 Pearson Education, Inc.

50. Figure 17.11
5′ Exon Intron Exon
5′ CapPre-mRNA
Codon
numbers
1−30 31−104
mRNA 5′Cap
5′
Intron Exon
3′ UTR
Introns cut out and
exons spliced together
3′
105−
146
Poly-A tail
Coding
segment
Poly-A tail
UTR
1−146

51. • In some cases, RNA splicing is carried out by
spliceosomes
• Spliceosomes consist of a variety of proteins
and several small nuclear ribonucleoproteins
(snRNPs) that recognize the splice sites
© 2011 Pearson Education, Inc.

52. Figure 17.12-1
RNA transcript (pre-mRNA)
5′
Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other
proteins

53. Figure 17.12-2
RNA transcript (pre-mRNA)
5′
Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other
proteins
Spliceosome
5′

54. Figure 17.12-3
RNA transcript (pre-mRNA)
5′
Exon 1
Protein
snRNA
snRNPs
Intron Exon 2
Other
proteins
Spliceosome
5′
Spliceosome
components
Cut-out
intronmRNA
5′
Exon 1 Exon 2

55. Ribozymes
• Ribozymes are catalytic RNA molecules that
function as enzymes and can splice RNA
• The discovery of ribozymes rendered obsolete
the belief that all biological catalysts were
proteins
© 2011 Pearson Education, Inc.

56. • Three properties of RNA enable it to function as
an enzyme
– It can form a three-dimensional structure because
of its ability to base-pair with itself
– Some bases in RNA contain functional groups
that may participate in catalysis
– RNA may hydrogen-bond with other nucleic acid
molecules
© 2011 Pearson Education, Inc.

57. The Functional and Evolutionary Importance
of Introns
• Some introns contain sequences that may
regulate gene expression
• Some genes can encode more than one kind of
polypeptide, depending on which segments are
treated as exons during splicing
• This is called alternative RNA splicing
• Consequently, the number of different proteins an
organism can produce is much greater than its
number of genes
© 2011 Pearson Education, Inc.

58. • Proteins often have a modular architecture
consisting of discrete regions called domains
• In many cases, different exons code for the
different domains in a protein
• Exon shuffling may result in the evolution of new
proteins
© 2011 Pearson Education, Inc.

59. Gene
DNA
Exon 1 Exon 2 Exon 3Intron Intron
Transcription
RNA processing
Translation
Domain 3
Domain 2
Domain 1
Polypeptide
Figure 17.13

60. Concept 17.4: Translation is the RNA-
directed synthesis of a polypeptide: a
closer look
• Genetic information flows from mRNA to protein
through the process of translation
© 2011 Pearson Education, Inc.

61. Molecular Components of Translation
• A cell translates an mRNA message into protein
with the help of transfer RNA (tRNA)
• tRNAs transfer amino acids to the growing
polypeptide in a ribosome
• Translation is a complex process in terms of its
biochemistry and mechanics
© 2011 Pearson Education, Inc.

62. Figure 17.14
Polypeptide
Ribosome
Trp
Phe Gly
tRNA with
amino acid
attached
Amino
acids
tRNA
Anticodon
Codons
U U U UG G G G C
A
C
C
C
C
G
A A A
C
G
C
G
5′ 3′
mRNA

63. The Structure and Function of Transfer RNA
• Molecules of tRNA are not identical
– Each carries a specific amino acid on one end
– Each has an anticodon on the other end; the
anticodon base-pairs with a complementary
codon on mRNA
© 2011 Pearson Education, Inc.
BioFlix: Protein Synthesis

64. • A tRNA molecule consists of a single RNA
strand that is only about 80 nucleotides long
• Flattened into one plane to reveal its base
pairing, a tRNA molecule looks like a cloverleaf
© 2011 Pearson Education, Inc.

65. Figure 17.15
Amino acid
attachment
site
3′
5′
Hydrogen
bonds
Anticodon
(a) Two-dimensional structure (b) Three-dimensional structure
(c) Symbol used
in this book
Anticodon Anticodon
3′ 5′
Hydrogen
bonds
Amino acid
attachment
site5′
3′
A A G

66. Figure 17.15a
Amino acid
attachment
site
3′
5′
Hydrogen
bonds
Anticodon
(a) Two-dimensional structure

67. (b) Three-dimensional structure
(c) Symbol used
Anticodon Anticodon
3′ 5′
Hydrogen
bonds
Amino acid
attachment
site5′
3′
in this book
A A G
Figure 17.15b

68. • Because of hydrogen bonds, tRNA actually
twists and folds into a three-dimensional
molecule
• tRNA is roughly L-shaped
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69. • Accurate translation requires two steps
– First: a correct match between a tRNA and an
amino acid, done by the enzyme aminoacyl-tRNA
synthetase
– Second: a correct match between the tRNA
anticodon and an mRNA codon
• Flexible pairing at the third base of a codon is
called wobble and allows some tRNAs to bind to
more than one codon
© 2011 Pearson Education, Inc.

70. Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
Figure 17.16-1

71. Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
P
Pi
i
i
Adenosine
Figure 17.16-2

72. Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
P
Pi
i
i
Adenosine
tRNA
AdenosineP
tRNA
AMP
Computer model
Amino
acid
Aminoacyl-tRNA
synthetase
Figure 17.16-3

73. Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P P P Adenosine
ATP
P
P
P
P
Pi
i
i
Adenosine
tRNA
AdenosineP
tRNA
AMP
Computer model
Amino
acid
Aminoacyl-tRNA
synthetase
Aminoacyl tRNA
(“charged tRNA”)
Figure 17.16-4

74. Ribosomes
• Ribosomes facilitate specific coupling of tRNA
anticodons with mRNA codons in protein synthesis
• The two ribosomal subunits (large and small) are
made of proteins and ribosomal RNA (rRNA)
• Bacterial and eukaryotic ribosomes are somewhat
similar but have significant differences: some
antibiotic drugs specifically target bacterial
ribosomes without harming eukaryotic ribosomes
© 2011 Pearson Education, Inc.

75. tRNA
molecules
Growing
polypeptide Exit tunnel
E P
A
Large
subunit
Small
subunit
mRNA
5′
3′
(a) Computer model of functioning ribosome
Exit tunnel Amino end
A site (Aminoacyl-
tRNA binding site)
Small
subunit
Large
subunit
E P A
mRNA
E
P site (Peptidyl-tRNA
binding site)
mRNA
binding site
(b) Schematic model showing binding sites
E site
(Exit site)
(c) Schematic model with mRNA and tRNA
5′ Codons
3′
tRNA
Growing polypeptide
Next amino
acid to be
added to
polypeptide
chain
Figure 17.17

76. Figure 17.17a
tRNA
molecules
Growing
polypeptide Exit tunnel
E P A
Large
subunit
Small
subunit
mRNA
5′
3′
(a) Computer model of functioning ribosome

77. Figure 17.17b
Exit tunnel
A site (Aminoacyl-
tRNA binding site)
Small
subunit
Large
subunit
P A
P site (Peptidyl-tRNA
binding site)
mRNA
binding site
(b) Schematic model showing binding sites
E site
(Exit site)
E

78. Figure 17.17c
Amino end
mRNA
E
(c) Schematic model with mRNA and tRNA
5′ Codons
3′
tRNA
Growing polypeptide
Next amino
acid to be
added to
polypeptide
chain

79. • A ribosome has three binding sites for tRNA
– The P site holds the tRNA that carries the
growing polypeptide chain
– The A site holds the tRNA that carries the next
amino acid to be added to the chain
– The E site is the exit site, where discharged
tRNAs leave the ribosome
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80. Building a Polypeptide
• The three stages of translation
– Initiation
– Elongation
– Termination
• All three stages require protein “factors” that aid in
the translation process
© 2011 Pearson Education, Inc.

81. Ribosome Association and Initiation of
Translation
• The initiation stage of translation brings together
mRNA, a tRNA with the first amino acid, and the
two ribosomal subunits
• First, a small ribosomal subunit binds with mRNA
and a special initiator tRNA
• Then the small subunit moves along the mRNA
until it reaches the start codon (AUG)
• Proteins called initiation factors bring in the large
subunit that completes the translation initiation
complex
© 2011 Pearson Education, Inc.

82. Figure 17.18
Initiator
tRNA
mRNA
5′
5′
3′
Start codon
Small
ribosomal
subunit
mRNA binding site
3′
Translation initiation complex
5′ 3′
3′ U
U
A
A G
C
P
P site
i
+
GTP GDP
Met Met
Large
ribosomal
subunit
E A
5′

83. Elongation of the Polypeptide Chain
• During the elongation stage, amino acids are
added one by one to the preceding amino acid at
the C-terminus of the growing chain
• Each addition involves proteins called elongation
factors and occurs in three steps: codon
recognition, peptide bond formation, and
translocation
• Translation proceeds along the mRNA in a 5′ to 3′
direction
© 2011 Pearson Education, Inc.

84. Amino end of
polypeptide
mRNA
5′
E
P
site
A
site
3′
Figure 17.19-1

85. Amino end of
polypeptide
mRNA
5′
E
P
site
A
site
3′
E
GTP
GDP + P i
P A
Figure 17.19-2

86. Amino end of
polypeptide
mRNA
5′
E
P
site
A
site
3′
E
GTP
GDP + P i
P A
E
P A
Figure 17.19-3

87. Amino end of
polypeptide
mRNA
5′
E
A
site
3′
E
GTP
GDP + P i
P A
E
P A
GTP
GDP + P i
P A
E
Ribosome ready for
next aminoacyl tRNA
P
site
Figure 17.19-4

88. Termination of Translation
• Termination occurs when a stop codon in the
mRNA reaches the A site of the ribosome
• The A site accepts a protein called a release
factor
• The release factor causes the addition of a
water molecule instead of an amino acid
• This reaction releases the polypeptide, and the
translation assembly then comes apart
© 2011 Pearson Education, Inc.
Animation: Translation

89. Figure 17.20-1
Release
factor
Stop codon
(UAG, UAA, or UGA)
3′
5′

90. Figure 17.20-2
Release
factor
Stop codon
(UAG, UAA, or UGA)
3′
5′
3′
5′
Free
polypeptide
2 GTP
2 GDP + 2 iP

91. Figure 17.20-3
Release
factor
Stop codon
(UAG, UAA, or UGA)
3′
5′
3′
5′
Free
polypeptide
2 GTP
5′
3′
2 GDP + 2 iP

92. Polyribosomes
• A number of ribosomes can translate a single
mRNA simultaneously, forming a polyribosome
(or polysome)
• Polyribosomes enable a cell to make many copies
of a polypeptide very quickly
© 2011 Pearson Education, Inc.

93. Figure 17.21
Completed
polypeptide
Incoming
ribosomal
subunits
Start of
mRNA
(5′ end)
End of
mRNA
(3′ end)(a)
Polyribosome
Ribosomes
mRNA
(b)
0.1 µm
Growing
polypeptides

94. Figure 17.21a
Ribosomes
mRNA
0.1 µm

95. Completing and Targeting the Functional
Protein
• Often translation is not sufficient to make a
functional protein
• Polypeptide chains are modified after translation
or targeted to specific sites in the cell
© 2011 Pearson Education, Inc.

96. Protein Folding and Post-Translational
Modifications
• During and after synthesis, a polypeptide chain
spontaneously coils and folds into its three-
dimensional shape
• Proteins may also require post-translational
modifications before doing their job
• Some polypeptides are activated by enzymes
that cleave them
• Other polypeptides come together to form the
subunits of a protein
© 2011 Pearson Education, Inc.

97. Targeting Polypeptides to Specific Locations
• Two populations of ribosomes are evident in cells:
free ribsomes (in the cytosol) and bound
ribosomes (attached to the ER)
• Free ribosomes mostly synthesize proteins that
function in the cytosol
• Bound ribosomes make proteins of the
endomembrane system and proteins that are
secreted from the cell
• Ribosomes are identical and can switch from free
to bound
© 2011 Pearson Education, Inc.

98. • Polypeptide synthesis always begins in the
cytosol
• Synthesis finishes in the cytosol unless the
polypeptide signals the ribosome to attach to
the ER
• Polypeptides destined for the ER or for
secretion are marked by a signal peptide
© 2011 Pearson Education, Inc.

99. • A signal-recognition particle (SRP) binds to
the signal peptide
• The SRP brings the signal peptide and its
ribosome to the ER
© 2011 Pearson Education, Inc.

100. Figure 17.22
Ribosome
mRNA
Signal
peptide
SRP
1
SRP
receptor
protein
Translocation
complex
ER
LUMEN
2
3
4
5
6
Signal
peptide
removed
CYTOSOL
Protein
ER
membrane

101. Concept 17.5: Mutations of one or a few
nucleotides can affect protein structure and
function
• Mutations are changes in the genetic material
of a cell or virus
• Point mutations are chemical changes in just
one base pair of a gene
• The change of a single nucleotide in a DNA
template strand can lead to the production of an
abnormal protein
© 2011 Pearson Education, Inc.

102. Figure 17.23
Wild-type hemoglobin
Wild-type hemoglobin DNA
3′
3′
3′5′
5′ 3′
3′5′
5′
5′5′3′
mRNA
A AG
C T T
A AG
mRNA
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
A
A
AUG
G
T
T
Sickle-cell hemoglobin
Mutant hemoglobin DNA
C

103. Types of Small-Scale Mutations
• Point mutations within a gene can be divided into
two general categories
– Nucleotide-pair substitutions
– One or more nucleotide-pair insertions or
deletions
© 2011 Pearson Education, Inc.

104. Substitutions
• A nucleotide-pair substitution replaces one
nucleotide and its partner with another pair of
nucleotides
• Silent mutations have no effect on the amino
acid produced by a codon because of
redundancy in the genetic code
• Missense mutations still code for an amino
acid, but not the correct amino acid
• Nonsense mutations change an amino acid
codon into a stop codon, nearly always leading
to a nonfunctional protein
© 2011 Pearson Education, Inc.

105. Wild type
DNA template strand
mRNA5′
5′
3′
Protein
Amino end
A instead of G
(a) Nucleotide-pair substitution
3′
3′
5′
Met Lys Phe Gly Stop
Carboxyl end
T T T T T
TTTTTA A A A A
AAAACC
C
C
A
A A A A A
G G G G
GC C
G GGU U U U UG
(b) Nucleotide-pair insertion or deletion
Extra A
3′
5′
5′
3′
Extra U
5′ 3′
T T T T
T T T T
A
A A A
A
AT G G G G
GAAA
AC
CCCC A
T3′5′
5′ 3′
5′T T T T TAAAACCA AC C
TTTTTA A A A ATG G G G
U instead of C
Stop
UA A A A AG GGU U U U UG
MetLys Phe Gly
Silent (no effect on amino acid sequence)
T instead of C
T T T T TAAAACCA GT C
T A T T TAAAACCA GC C
A instead of G
CA A A A AG AGU U U U UG UA A A AG GGU U U G AC
AA U U A AU UGU G G C UA
GA U A U AA UGU G U U CG
Met Lys Phe Ser
Stop
Stop Met Lys
missing
missing
Frameshift causing immediate nonsense
(1 nucleotide-pair insertion)
Frameshift causing extensive missense
(1 nucleotide-pair deletion)
missing
T T T T TTCAACCA AC G
AGTTTA A A A ATG G G C
Leu Ala
Missense
A instead of T
TTTTTA A A A ACG G A G
A
CA U A A AG GGU U U U UG
TTTTTA T A A ACG G G G
Met
Nonsense
Stop
U instead of A
3′
5′
3′5′
5′
3′
3′
5′
5′
3′
3′5′ 3′
Met Phe Gly
No frameshift, but one amino acid missing
(3 nucleotide-pair deletion)
missing
3′
5′
5′
3′
5′ 3′
U
T CA AA CA TTAC G
TA G T T T G G A ATC
T T C
A A G
Met
3′
T
A
Stop
3′
5′
5′
3′
5′ 3′
Figure 17.24

106. Figure 17.24a
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino end
Stop
Carboxyl end
3′
3′
3′
5′
Met Lys Phe Gly
A instead of G
(a) Nucleotide-pair substitution: silent
StopMet Lys Phe Gly
U instead of C
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
5′
3′
3′
5′A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
G G G G
A
A
A G A A A AG GGU U U U U
T
U 3′5′

107. Figure 17.24b
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino end
Stop
Carboxyl end
3′
3′
3′
5′
Met Lys Phe Gly
T instead of C
(a) Nucleotide-pair substitution: missense
StopMet Lys Phe Ser
A instead of G
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
5′
3′
3′
5′A
A A
A A A A
A AT
T T T T T
T T TT
C C T C
G
G
GA
A G A A A AA GGU U U U U 3′5′
A C
C
G

108. Figure 17.24c
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino end
Stop
Carboxyl end
3′
3′
3′
5′
Met Lys Phe Gly
A instead of T
(a) Nucleotide-pair substitution: nonsense
Met
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
5′
3′
3′
5′A
A
A A A A
A AT
T A T T T
T T TT
C C C
G
G
GA
A G U A A AGGU U U U U 3′5′
C
C
G
T instead of C
C
GT
U instead of A
G
Stop

109. Insertions and Deletions
• Insertions and deletions are additions or losses
of nucleotide pairs in a gene
• These mutations have a disastrous effect on the
resulting protein more often than substitutions do
• Insertion or deletion of nucleotides may alter the
reading frame, producing a frameshift mutation
© 2011 Pearson Education, Inc.

110. Figure 17.24d
Wild type
DNA template strand
mRNA5′
5′
Protein
Amino end
Stop
Carboxyl end
3′
3′
3′
5′
Met Lys Phe Gly
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
(b) Nucleotide-pair insertion or deletion: frameshift causing
immediate nonsense
Extra A
Extra U
5′
3′
5′
3′
3′
5′
Met
1 nucleotide-pair insertion
Stop
A C A A GT T A TC T A C G
T A T AT G T CT GG A T GA
A G U A U AU GAU G U U C
A T
A
AG

111. Figure 17.24e
DNA template strand
mRNA5′
5′
Protein
Amino end
Stop
Carboxyl end
3′
3′
3′
5′
Met Lys Phe Gly
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
(b) Nucleotide-pair insertion or deletion: frameshift causing
extensive missense
Wild type
missing
missing
A
U
A A AT T TC C A T TC C G
A AT T TG GA A ATCG G
A G A A GU U U C A AG G U 3′
5′
3′
3′
5′
Met Lys Leu Ala
1 nucleotide-pair deletion
5′

112. Figure 17.24f
DNA template strand
mRNA5′
5′
Protein
Amino end
Stop
Carboxyl end
3′
3′
3′
5′
Met Lys Phe Gly
A
A
A A
A A A A
A AT
T T T T T
T T TT
C C C C
C
C
G G G G
G
G
A
A A A AG GGU U U U U
(b) Nucleotide-pair insertion or deletion: no frameshift, but one
amino acid missing
Wild type
AT C A A A A T TC C G
T T C missing
missing
Stop
5′
3′
3′
5′
3′5′
Met Phe Gly
3 nucleotide-pair deletion
A GU C A AG GU U U U
T GA A AT T TT CG G
A A G

113. Mutagens
• Spontaneous mutations can occur during DNA
replication, recombination, or repair
• Mutagens are physical or chemical agents that
can cause mutations
© 2011 Pearson Education, Inc.

114. Concept 17.6: While gene expression differs
among the domains of life, the concept of a
gene is universal
• Archaea are prokaryotes, but share many
features of gene expression with eukaryotes
© 2011 Pearson Education, Inc.

115. Comparing Gene Expression in Bacteria,
Archaea, and Eukarya
• Bacteria and eukarya differ in their RNA
polymerases, termination of transcription, and
ribosomes; archaea tend to resemble eukarya in
these respects
• Bacteria can simultaneously transcribe and
translate the same gene
• In eukarya, transcription and translation are
separated by the nuclear envelope
• In archaea, transcription and translation are
likely coupled
© 2011 Pearson Education, Inc.

116. Figure 17.25
RNA polymerase
DNA
mRNA
Polyribosome
RNA
polymerase DNA
Polyribosome
Polypeptide
(amino end)
mRNA (5′ end)
Ribosome
0.25 µmDirection of
transcription

117. Figure 17.25a
RNA polymerase
DNA
mRNA
Polyribosome
0.25 µm

118. What Is a Gene? Revisiting the Question
• The idea of the gene has evolved through the
history of genetics
• We have considered a gene as
– A discrete unit of inheritance
– A region of specific nucleotide sequence in a
chromosome
– A DNA sequence that codes for a specific
polypeptide chain
© 2011 Pearson Education, Inc.

119. Figure 17.26
TRANSCRIPTION
DNA
RNA
polymerase
Exon
RNA
transcript
RNA
PROCESSING
NUCLEUS
Intron
RNA transcript
(pre-mRNA)
Poly-A
Poly-A
Aminoacyl-
tRNA synthetase
AMINO ACID
ACTIVATION
Amino
acid
tRNA
5′C
ap
Poly-A
3′
Growing
polypeptidemRNA
Aminoacyl
(charged)
tRNA
Anticodon
Ribosomal
subunits
A
AE
TRANSLATION
5′ Cap
CYTOPLASM
P
E
Codon
Ribosome
5′
3′

120. • In summary, a gene can be defined as a region of
DNA that can be expressed to produce a final
functional product, either a polypeptide or an
RNA molecule
© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.

121. Figure 17.UN02
Transcription unit
RNA polymerase
Promoter
RNA transcript
5′
5′
3′
3′ 3′
5′
Template strand
of DNA

122. Figure 17.UN03
Pre-mRNA
mRNA
Poly-A tail5′ Cap

123. Figure 17.UN04
tRNA
Polypeptide
Amino
acid
E A Anti-
codon
Ribosome
mRNA
Codon

124. Figure 17.UN05
Type of RNA Functions
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Primary transcript
Small nuclear RNA (snRNA)
Plays catalytic (ribozyme) roles and
structural roles in ribosomes

125. Figure 17.UN06a

126. Figure 17.UN06b

127. Figure 17.UN06c

128. Figure 17.UN06d

129. Figure 17.UN10