BIO 212

1. Chapter 17
From Gene to Protein
Transcription and Translation

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
• Gene expression, the process by which DNA
directs protein synthesis, includes two stages:
transcription and translation
• The ribosome is part of the cellular machinery
for translation, polypeptide synthesis

3. 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

4. Nutritional Mutants in Neurospora:
Scientific Inquiry
• Beadle and Tatum exposed bread mold to X-rays,
creating mutants that were unable to survive on
minimal medium as a result of inability to
synthesize certain molecules
• Using crosses, they 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

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

6. 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 one gene–one polypeptide

7. Basic Principles of Transcription and
Translation
• Transcription is the synthesis of RNA under the
direction of DNA
• Transcription produces messenger RNA (mRNA)
• Translation is the synthesis of a polypeptide,
which occurs under the direction of mRNA
• Ribosomes are the sites of translation

8. • In prokaryotes, mRNA produced by transcription
is immediately translated without more
processing--coupled
• In a eukaryotic cell, the nuclear envelope
separates transcription from translation
• Eukaryotic RNA transcripts are modified through
RNA processing to yield finished mRNA
• Cells are governed by a cellular chain of
command: DNA → RNA → protein

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

10. 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
• So how many bases correspond to an amino acid?

11. Codons: Triplets of Bases
• The flow of information from gene to protein is
based on a triplet code: a series of
nonoverlapping, three-nucleotide words
• These triplets are the smallest units of uniform
length that can code for all the amino acids
• Example: AGT at a particular position on a DNA
strand results in the placement of the amino
acid serine at the corresponding position of the
polypeptide to be produced

12. • During transcription, a DNA strand called the
template strand provides a template for
ordering the sequence of nucleotides in an RNA
transcript
• During translation, the mRNA base triplets,
called codons, are read in the 5′ to 3′ direction
• Each codon specifies the amino acid to be
placed at the corresponding position along a
polypeptide

13. Figure 17.4
A C C A A A C C G A G T
ACTTTT CGGGGT
U G G U U U G G C CU A
SerGlyPheTrp
Codon
TRANSLATION
TRANSCRIPTION
Protein
mRNA 5′
5′
3′
Amino acid
DNA
template
strand
5′
3′
3′

14. Cracking the Code
• All 64 codons were deciphered by the mid-1960s
• The genetic code is redundant 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

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

16. 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
Pig expressing a
jellyfish
gene
(b)Tobacco plant
expressing
a firefly gene
(a)

17. Molecular Components of
Transcription
• RNA synthesis is catalyzed by RNA polymerase,
which pries the DNA strands apart and hooks
together the RNA nucleotides
• RNA synthesis follows the same base-pairing rules
as DNA, except uracil substitutes for thymine
• The DNA sequence where RNA polymerase
attaches is called the promoter; in prokaryotes,
the sequence signaling the end of transcription
is called the terminator
• The stretch of DNA that is transcribed is called a
transcription unit

18. Synthesis of an RNA Transcript
• The three stages of transcription:
– Initiation
– Elongation
– Termination

19. Figure 17.7-3 Promoter Transcription unit
RNA polymerase
Start point
1
Template strand of DNARNA
transcript
Unwound
DNA
Rewound
DNA
RNA
transcript
Direction of
transcription
(“downstream”)
Completed RNA transcript
Initiation
Elongation
Termination
2
3
5′
3′
5′
5′
3′
5′
3′
5′
3′
5′
5′
3′
3′
5′
3′
3′
5′
3′
5′
3′

20. Figure 17.9 Nontemplate
strand of DNA
5′
3′
3′ end
5′
3′
5′
Direction of transcription
RNA
polymerase
Template
strand of DNA
Newly made
RNA
RNA nucleotides
A
A
A
A
A AA
A
A
C
C
G G T T
T
C
C CU
U
C CT
T
C
A
T
T
G
G
U

21. RNA Polymerase Binding and Initiation
of Transcription
• Promoters signal the initiation of RNA synthesis
• Transcription factors mediate the binding of RNA
polymerase and initiation of transcription in
eukaryotes
• 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

22. Promoter
Nontemplate strand
15′
3′
5′
3′
Start point
RNA polymerase II
Template
strand
TATA box
Transcription
factors
DNA
3′
5′
3′
5′
3′
5′
2
3
Transcription factors
RNA transcript
Transcription initiation complex
3′5′
5′
3′
A eukaryotic
promoter
Several
transcription
factors bind
to DNA.
Transcription
initiation
complex
forms.
T A T AAAA
TA AT T T T

23. 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

24. Termination of Transcription
• The mechanisms of termination are different in
prokaryotes and eukaryotes
• In prokaryotes, the polymerase stops
transcription at the end of the terminator
• In eukaryotes, RNA polymerase II transcribes
the polyadenylation signal sequence; the RNA
transcript is released 10–35 nucleotides past
this polyadenylation sequence

25. Concept 17.3: Eukaryotic cells modify
RNA after transcription
• Enzymes in the eukaryotic nucleus modify pre-
mRNA (RNA processing) before the messages
are dispatched to the cytoplasm
• During RNA processing, both ends of the
primary transcript are usually altered (5’ cap
and 3’ polyA tail addition)
• Also, usually some interior parts of the molecule
are cut out, and the other parts spliced together
(RNA splicing)

26. Figure 17.10
A modified guanine
nucleotide added to
the 5′ end
Region that includes
protein-coding segments
5′
5′
Cap
5′
UTR
Start
codon
Stop
codon
G P P P
3′
UTR
3′
AAUAAA AAA AAA
Poly-A tail
Polyadenylation
signal
50–250 adenine
nucleotides added
to the 3′ end
…
These modifications share several functions:
• They seem to facilitate the export of mRNA
• They protect mRNA from hydrolytic
enzymes

27. 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

28. Figure 17.11
Pre-mRNA Intron Intron
Introns cut out
and exons
spliced together
Poly-A tail5′ Cap
5′ Cap Poly-A tail
1–30 31–104 105–146
1–146
3′ UTR5′ UTR
Coding
segment
mRNA
• 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

29. Figure 17.12
Spliceosome Small RNAs
Exon 2
Cut-out
intron
Spliceosome
components
mRNA
Exon 1 Exon 2
Pre-mRNA
Exon 1
Intron
5′
5′

30. Ribozymes
• Ribozymes are catalytic RNA molecules
• Here they function as enzymes that can splice
RNA
• The discovery of ribozymes rendered obsolete
the belief that all biological catalysts were
proteins

31. • 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

32. 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

33. • 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

34. Figure 17.13
Gene
DNA
Transcription
RNA processing
Translation
Domain 3
Exon 1 Exon 3Exon 2 IntronIntron
Domain 2
Domain 1
Polypeptide

35. Molecular Components of Translation
• A cell translates an mRNA message into protein
with the help of transfer RNA (tRNA) &
ribosomes
• tRNAs transfer amino acids to the growing
polypeptide in a ribosome
• 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

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

37. The Structure and Function of Transfer
RNA
• 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
• Amino acid at one end of sock and anticodon at
the other
• Because of hydrogen bonds, tRNA actually
twists and folds into a three-dimensional
molecule
• tRNA is roughly L-shaped
A
C
C

38. Figure 17.15
Amino acid
attachment
site
Amino acid
attachment site
5′
3′
A
C
C
A
C
G
C
U
U
A
G
G
C
G
A
U
U
U
A
A
GAA CC
CU
*
**
C
G
G U UG
C
*
*
*
*C CU
A G
G
G
G
A
GAGC
C
C
*U
* G A
G
G
U
*
*
*
A
A
A
G
C
U
G
A
A
Hydrogen
bonds
Hydrogen
bonds
Anticodon Anticodon
Symbol used
in this book
Three-dimensional
structure
(a) Two-dimensional structure (b) (c)
Anticodon
5′3′
A A G
3′
5′

39. • Accurate translation requires two steps:
– First step: a correct match between a tRNA and an
amino acid, done by the enzyme aminoacyl-tRNA
synthetase
– Second step: 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

40. Figure 17.16-3
Aminoacyl-tRNA
synthetase
1 Amino acid
and tRNA
enter active
site.
Tyr-tRNA
Tyrosine (Tyr)
(amino acid)
Tyrosyl-tRNA
synthetase
Complementary
tRNA anticodon
A AU
tRNA
Amino
acid
Using ATP,
synthetase
catalyzes
covalent
bonding.
23
ATP
AMP + 2 P i
Aminoacyl
tRNA
released.
Computer model

41. 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)

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

43. • 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 uncharged tRNAs
leave the ribosome

44. Building a Polypeptide
• The three stages of translation:
–Initiation
–Elongation
–Termination
–All 3 stages require protein factors and some
require energy in the form of GTP.

45. Ribosome Association and
Initiation of Translation
• Initiation 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

46. Figure 17.18
1 2
P site
3′
Large
ribosomal
subunit
Translation initiation complex
Large ribosomal subunit
completes the initiation
complex.
Small ribosomal subunit binds
to mRNA.
mRNA binding site
Small
ribosomal
subunit
Initiator
tRNA
Start codon
3′5′
E A
3′
5′
GTP GDP
P i
+
3′ 5′
5′
U A C
GA UMet
Met
mRNA

47. Elongation of the Polypeptide
Chain
• During elongation, amino acids are added one
by one to 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
• Energy expenditure occurs in the first (docking
in A site) and third steps(translocation)
• Translation proceeds along the mRNA in a
5 → 3 direction′ ′

48. Amino end
of polypeptide
Codon
recognition
1
3′
5′
E
P A
sitesite
E
P A
mRNA
GTP
P iGDP +

49. Amino end
of polypeptide
Codon
recognition
1
3′
5′
E
P A
sitesite
E
P A
mRNA
GTP
P i
2
GDP +
E
P A
Peptide bond
formation

50. Amino end
of polypeptide
Codon
recognition
1
3′
5′
E
P A
sitesite
E
P A
mRNA
GTP
P i
23 GTP
P i
GDP +
GDP +
Translocation
E
P A
Peptide bond
formation
E
P A
Ribosome ready for
next aminoacyl tRNA

51. 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 comes apart

52. Figure 17.20-3
31 2
Release
factor
3′
5′5′
3′
Stop codon
(UAG, UAA, or UGA)
Ribosome reaches a
stop codon on mRNA.
Release factor
promotes hydrolysis.
Ribosomal subunits
and other components
dissociate.
Free
polypeptide
3′
5′
2 GTP
2 GDP + 2 P i

53. Polyribosomes
• A number of ribosomes can translate a single
mRNA simultaneously, forming a polyribosome
• Polyribosomes enable a cell to make many
copies of a polypeptide very quickly

54. LE 17-20
Ribosomes
mRNA
0.1 µm
This micrograph shows a large polyribosome in a prokaryotic cell (TEM).
An mRNA molecule is generally translated simultaneously
by several ribosomes in clusters called polyribosomes.
Incoming
ribosomal
subunits
Growing
polypeptides
End of
mRNA
(3′ end)
Start of
mRNA
(5′ end)
Polyribosome
Completed
polypeptides

55. Completing and Targeting the
Functional Protein
• Often translation is not sufficient to make a
functional protein
• Polypeptide chains are modified after
translation
• Completed proteins are targeted to specific
sites in the cell

56. 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 (rough ER)
• Ribosomes are identical and can switch from
free to bound

57. • 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
• A signal-recognition particle (SRP) binds to the
signal peptide
• The SRP brings the signal peptide and its
ribosome to the ER

58. Figure 17.21
Protein
ER
membrane
Signal
peptide
removed
SRP receptor
protein
Translocation complex
ER LUMEN
CYTOSOL
SRP
Signal
peptide
mRNA
Ribosome
Polypeptide
synthesis
begins.
SRP
binds to
signal
peptide.
SRP
binds to
receptor
protein.
SRP
detaches
and
polypeptide
synthesis
resumes.
Signal-
cleaving
enzyme cuts
off signal
peptide.
Completed
polypeptide
folds into
final
conformation.
1 2 3 4 5 6

59. • A bacterial cell ensures a streamlined process
by coupling transcription and translation
• In this case the newly made protein can
quickly diffuse to its site of function

60. RNA polymerase
Polyribosome
DNA
mRNA
0.25 µm
Ribosome
DNA
mRNA (5′ end)
Polyribosome
Polypeptide
(amino end)
Direction of
transcriptionRNA
polymerase

61. • In eukaryotes, the nuclear envelop separates
the processes of transcription and translation
• RNA undergoes processes before leaving
the nucleus

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

63. Concept 17.5: RNA plays multiple
roles in the cell: a review
Type of RNA Functions
Messenger
RNA (mRNA)
Carries information specifying
amino acid sequences of
proteins from DNA to ribosomes
Transfer RNA
(tRNA)
Serves as adapter molecule in
protein synthesis; translates
mRNA codons into amino acids
Ribosomal
RNA (rRNA)
Plays catalytic (ribozyme) roles
and structural roles in ribosomes

64. Type of RNA Functions
Primary
transcript
Serves as a precursor to mRNA,
rRNA, or tRNA, before being
processed by splicing or
cleavage
Small nuclear
RNA (snRNA)
Plays structural and catalytic
roles in spliceosomes
SRP RNA Is a component of the the signal-
recognition particle (SRP)
Video: protein synthesis

65. Concept 17.7: Point mutations 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 leads to production of an
abnormal protein

66. Figure 17.25
Wild-type β-globin Sickle-cell β-globin
Mutant β-globin DNAWild-type β-globin DNA
mRNA mRNA
Normal hemoglobin Sickle-cell hemoglobin
Val
U GG
A CC
G GT
Glu
G GA
G GA
C CT3′
5′
5′ 5′
5′
5′
3′
3′
3′ 3′
3′
5′

67. Types of Point Mutations
• Point mutations within a gene can be divided
into two general categories
– Base-pair substitutions
– Base-pair insertions or deletions

68. Substitutions
• A base-pair substitution replaces one nucleotide
and its partner with another pair of nucleotides
• Base-pair substitution can cause missense or
nonsense mutations
• Missense mutations still code for an amino acid,
but not necessarily the right amino acid
• Nonsense mutations change an amino acid
codon into a stop codon, nearly always leading
to a nonfunctional protein
• Missense mutations are more common

69. Wild type
Stop
5′
3′
3′
Stop
3′
5′
5′
Met
(b) Nucleotide-pair insertion or deletion
Carboxyl endAmino end
mRNA
Protein
3′
5′
5′
DNA template strand
(a) Nucleotide-pair substitution
A instead of G
U instead of C
5′
3′
3′
Stop
Extra A
Extra U
Frameshift (1 nucleotide-pair insertion)
missing
missing
Frameshift (1 nucleotide-pair deletion)
missing
3′
5′
5′
missing
3′
5′
3′
Met Lys Leu Ala
3′
5′
3′
T A C T T C A A A C C G A T TA
TA G T A A A AC TTTTG G G
G G C U A AAAA U U UUUGG
A U G A A G G G C UUU
U
A A
AATCGGTTT GG A AA
T A C T T C A A C C G A T T
A
Met Lys Phe Gly
T A C C C C GA A A A
A AAAA
A U G
G G G G
T T T T
TTTTT C
G G G C UUUUAA A A
3′
5′
5′
T
A A A A A
T T T TA A A A AACCCC
T T T T TTGGGG
G G G GUUU U A AA A AU
Met Lys Phe Gly Stop
U
Silent
3′
5′
5′
5′
3′
3′
Stop
Stop Stop
3 nucleotide-pair deletionNonsense
A instead of T
U instead of A
5′
5′
5′
3′
3′
3′
Met Lys Phe Ser
Met Met Phe Gly
A
T
A
A
T
A
A
T
A
T instead of C
A instead of G
Missense
U G G G G A AU U U U UU A
A A A
T T TA A A A AGCCCC A
T T T T T TCGGGG
3′
5′
5′
5′
3′
3′
T T
A A
AA
A A G
U U U U UCGGG
T T T T T
AGCC
CGGG
C
CTT
AAAA
T T T TT
A
AA AA C C C GA
A A A ATTTT GGGT C
A A AAUCGGGU U U U A

70. Mutagens
• Spontaneous mutations can occur during DNA
replication, recombination, or repair
• Mutagens are physical or chemical agents that
can cause mutations

71. What is a gene? revisiting the
question
• A gene is a region of DNA whose final product is
either a polypeptide or an RNA molecule

72. LE 17-26
TRANSCRIPTION
RNA PROCESSING
RNA
transcript
5′
Exon
NUCLEUS
FORMATION OF
INITIATION COMPLEX
CYTOPLASM
3′
DNA
RNA
polymerase
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
synthetase
Amino
acid
tRNA
AMINO ACID ACTIVATION
3′
mRNA
A
P
E Ribosomal
subunits
5′
Growing
polypeptide
E A
Activated
amino acid
Anticodon
TRANSLATION
Codon
Ribosome
Review compartments for eukaryotes