Biology in Focus - Chapter 14

CAMPBELL BIOLOGY IN FOCUS
© 2014 Pearson Education, Inc.
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
14
Gene Expression:
From Gene to Protein

Overview: The Flow of Genetic Information
 The information content of genes is in the form of
specific sequences of nucleotides in DNA
 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

Figure 14.1

Concept 14.1: Genes specify proteins via
 How was the fundamental relationship between
genes and proteins discovered?

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
 Cells synthesize and degrade molecules in a series
of steps, a metabolic pathway

Nutritional Mutants in Neurospora: Scientific
Inquiry
 George Beadle and Edward Tatum disabled genes
in bread mold one by one and looked for phenotypic
changes
 They studied the haploid bread mold because it
would be easier to detect recessive mutations
 They studied mutations that altered the ability of the
fungus to grow on minimal medium

Figure 14.2
Neurospora
cells
Each surviving
cell forms a
colony of
genetically
identical cells.
No
growth
Growth
Mutant cells placed
in a series of vials,
each containing
minimal medium
plus one additional
nutrient.
Surviving cells
tested for inability
to grow on
minimal medium.
Individual Neurospora
cells placed on complete
growth medium. Growth
Cells subjected
to X-rays.
Control: Wild-type
cells in minimal
medium
2
1 3
4
5

Figure 14.2a
Neurospora
cells
Each surviving
cell forms a
colony of
genetically
identical cells.
Individual Neurospora
cells placed on complete
growth medium.
Cells subjected
to X-rays.
2
1 3

Figure 14.2b
No
growth
Growth
Mutant cells placed
in a series of vials,
each containing
minimal medium
plus one additional
nutrient.
Surviving cells
tested for inability
to grow on
minimal medium.
Growth
Control: Wild-type
cells in minimal
medium
4
5

 The researchers amassed a valuable collection of
Neurospora mutant strains, catalogued by their
defects
 For example, one set of mutants all required
arginine for growth
 It was determined that different classes of these
mutants were blocked at a different step in the
biochemical pathway for arginine biosynthesis

Figure 14.3
Precursor
Gene A
Ornithine Citrulline Arginine
Gene B Gene C
Enzyme
A
Enzyme
B
Enzyme
C

The Products of Gene Expression: A Developing
Story
 Some proteins are not enzymes, so researchers
later revised the one gene–one enzyme 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
 It is common to refer to gene products as proteins
rather than polypeptides

Basic Principles of Transcription and Translation
 RNA is the bridge between DNA and protein
synthesis
 RNA is chemically similar to DNA, but RNA has a
ribose sugar and the base uracil (U) rather than
thymine (T)
 RNA is usually single-stranded
 Getting from DNA to protein requires two stages:

 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

 In prokaryotes, translation of mRNA can begin
before transcription has finished
 In eukaryotes, the nuclear envelope separates
transcription from translation
 Eukaryotic RNA transcripts are modified through
RNA processing to yield the finished mRNA
 Eukaryotic mRNA must be transported out of the
nucleus to be translated

Figure 14.4
Nuclear
envelope
Pre-mRNA
mRNA
DNA
RNA PROCESSING
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
mRNA
DNA
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
(a) Bacterial cell (b) Eukaryotic cell

Figure 14.4a-1
mRNA
DNA
TRANSCRIPTION
(a) Bacterial cell

Figure 14.4a-2
mRNA
DNA
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
(a) Bacterial cell

Figure 14.4b-1
Nuclear
envelope
Pre-mRNA
DNA
TRANSCRIPTION
(b) Eukaryotic cell

Figure 14.4b-2
Nuclear
envelope
Pre-mRNA
mRNA
DNA
RNA PROCESSING
TRANSCRIPTION
(b) Eukaryotic cell

Figure 14.4b-3
Nuclear
envelope
Pre-mRNA
mRNA
DNA
RNA PROCESSING
TRANSCRIPTION
TRANSLATION
Polypeptide
Ribosome
(b) Eukaryotic cell

 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

Figure 14.UN01
DNA RNA Protein

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?

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

Figure 14.5
DNA
template
strand
Protein
mRNA
3′
Trp
TRANSCRIPTION
TRANSLATION
Amino acid
Codon
5′
3′5′
3′
5′
Phe Gly Ser
GU G U UU G G UC C A
CA C A AA C C AG G T
GT G T TT G G TC C A

 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
any given gene

 During translation, the mRNA base triplets, called
codons, are read 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

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 it is 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
 Codons are read one at a time in a nonoverlapping
fashion

Figure 14.6
UUU
Second mRNA base
UUC
UUA
UUG
UCU
UCC
UCA
UCG
UAU
UAC
UAA
UAG
UGU
UGC
UGA
UGG
CUU
CUC
CUA
CUG
CCU
CCC
CCA
CCG
CAU
CAC
CAA
CAG
CGU
CGC
CGA
CGG
AUU
AUC
AUA
AUG
ACU
ACC
ACA
ACG
AAU
AAC
AAA
AAG
AGU
AGC
AGA
AGG
GUU
GUC
GUA
GUG
GCU
GCC
GCA
GCG
GAU
GAC
GAA
GAG
GGU
GGC
GGA
GGG
FirstmRNAbase(5′endofcodon)
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
U C A G
Phe
Leu
Ser
Tyr Cys
Trp
Met or
start
Stop
Stop Stop
Arg
Gln
His
ProLeu
Val Ala
Asp
Glu
Gly
IIe
Thr
Lys
Asn
Arg
Ser
ThirdmRNAbase(3′endofcodon)

Evolution of the Genetic Code
 The genetic code is nearly universal, shared by the
simplest bacteria and the most complex animals
 Genes can be transcribed and translated after being
transplanted from one species to another

Figure 14.7
(a) Tobacco plant expressing
a firefly gene
(b) Pig expressing a jellyfish
gene

Figure 14.7a
(a) Tobacco plant expressing
a firefly gene

Figure 14.7b
(b) Pig expressing a jellyfish
gene

Concept 14.2: Transcription is the DNA-directed
synthesis of RNA: a closer look
 Transcription is the first stage of gene expression

Molecular Components of Transcription
 RNA synthesis is catalyzed by RNA polymerase,
which pries the DNA strands apart and joins
together the RNA nucleotides
 RNA polymerases assemble polynucleotides in the
5′ to 3′ direction
 However, RNA polymerases can start a chain
without a primer
Animation: Transcription Introduction

Figure 14.8-1
Transcription unit
RNA polymerase
Promoter
Template strand of DNA
Start point
RNA
transcript
Unwound
DNA
Initiation
3′
5′
3′
5′ 3′
5′
3′
5′
1

Figure 14.8-2
Transcription unit
RNA polymerase
Promoter
Start point
RNA
transcript
Unwound
DNA
Rewound
DNA
RNA
transcript
Direction of
transcription
(“downstream”)
Initiation
Elongation
3′
5′
3′
5′
3′
5′ 3′
5′
3′
5′
3′
5′
3′
5′
2
1

Figure 14.8-3
Transcription unit
RNA polymerase
Promoter
Start point
Termination
Completed RNA transcript
RNA
transcript
Unwound
DNA
Rewound
DNA
RNA
transcript
Direction of
transcription
(“downstream”)
Initiation
Elongation
3′
5′
3′
5′
3′
5′ 3′
5′
3′
5′
3′5′
3′
5′
3′
5′
3′
5′
3′
5′
3
2
1

 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

Synthesis of an RNA Transcript
 The three stages of transcription
 Initiation
 Elongation
 Termination

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

Figure 14.UN02
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
TRANSLATION
TRANSCRIPTION
RNA PROCESSING

Figure 14.9
Transcription
factors
TATA box
Promoter
Nontemplate strand
Start point
Transcription
initiation
complex forms.
Transcription initiation complex
DNA
RNA transcript
A eukaryotic
promoter
Several transcription
factors bind to DNA.
3′
5′
5′ 3′
3′
5′
3′
5′
3′
5′
3
2
1
Template
strand
Transcription
factors
RNA polymerase II
3′
5′
3′
5′ T A T A A A A
AT A T T T T

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

Figure 14.10
Nontemplate
strand of DNA
Direction of transcription
RNA
polymerase
3′
5′3′
5′
RNA nucleotides
Template
strand of DNA
Newly made
RNA
3′ end
5′
U
C
U
G
A
A
A
A
AA
A
A
A
A
T T T
T
T
T
T
CC
C
CCC C
G
G
G
U

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

Concept 14.3: 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 altered
 Also, usually some interior parts of the molecule
are cut out and the other parts spliced together

Alteration of mRNA Ends
 Each end of a pre-mRNA molecule is modified in a
particular way
 The 5′ end receives a modified G nucleotide 5′ cap
 The 3′ end gets a poly-A tail
 These modifications share several functions
 Facilitating the export of mRNA to the cytoplasm
 Protecting mRNA from hydrolytic enzymes
 Helping ribosomes attach to the 5′ end

Figure 14.UN03
DNA
Pre-mRNA
mRNA
Ribosome
Polypeptide
TRANSLATION
TRANSCRIPTION
RNA PROCESSING

Figure 14.11
Protein-coding segment
Polyadenylation
signal
G P
A modified guanine
nucleotide added to
the 5′ end
50–250 adenine
nucleotides added
to the 3′ end
3′5′
5′ Cap 5′ UTR 3′ UTR Poly-A tail
Start
codon
Stop
codon
P P AAUAAA …AAA AAA

Split Genes and RNA Splicing
 Most eukaryotic mRNAs have long noncoding
stretches of nucleotides that lie between coding
regions
 The noncoding regions are called intervening
sequences, or introns
 The other regions are called exons and are usually
translated into amino acid sequences
 RNA splicing removes introns and joins exons,
creating an mRNA molecule with a continuous
coding sequence

Figure 14.12
Introns cut out and
exons spliced together
31–104
5′ Cap
5′ UTR 3′ UTR
Poly-A tail
Coding
segment
1–146
AAUAAA
105–
146
5′ Cap Poly-A tail
1–30
mRNA
Pre-mRNA
Intron Intron

 Many genes can give rise to two or more different
polypeptides, depending on which segments are
used as exons
 This process is called alternative RNA splicing
 RNA splicing is carried out by spliceosomes
 Spliceosomes consist of proteins and small RNAs

Figure 14.13
Spliceosome
components
5′
Pre-mRNA
5′
mRNA
Intron
Spliceosome
Exon 1
Small RNAs
Exon 2
Exon 2Exon 1
Cut-out
intron

Ribozymes
 Ribozymes are RNA molecules that function as
enzymes
 RNA splicing can occur without proteins, or even
additional RNA molecules
 The introns can catalyze their own splicing

Concept 14.4: Translation is the RNA-directed
synthesis of a polypeptide: a closer look
 Genetic information flows from mRNA to protein
through the process of translation

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

Figure 14.UN04
DNA
mRNA
Ribosome
Polypeptide
TRANSLATION
TRANSCRIPTION

Figure 14.14
5′
tRNA
Polypeptide
Ribosome
Anticodon
mRNA
Codons 3′
tRNA with
amino acid
attached
Amino acids
Gly
Trp
Phe
A A A
A
C
C
C
C
G
U U U G G CU G G

The Structure and Function of Transfer RNA
 Each tRNA can translate a particular mRNA codon
into a given amino acid
 The tRNA contains an amino acid at one end and at
the other end has a nucleotide triplet that can base-
pair with the complementary codon on mRNA

 A tRNA molecule consists of a single RNA strand
that is only about 80 nucleotides long
 tRNA molecules can base-pair with themselves
 Flattened into one plane, a tRNA molecule looks like
a cloverleaf
 In three dimensions, tRNA is roughly L-shaped,
where one end of the L contains the anticodon that
base-pairs with an mRNA codon
Video: tRNA Model

Figure 14.15
5′
Anticodon
3′
Amino acid
attachment site
Anticodon
Anticodon
A A G
5′3′
Hydrogen
bonds
5′
3′
Amino acid
attachment
site
Hydrogen
bonds
A
A
G
A
C
G
C
C
C
C
U
A
C G
U
U
A
A A
A
C
G U
A
C
G U
A
C
G
U
A
C
G
U
*
C
G
*
G
G
U
A
A
A
A
C C
C
C
C
G
G
GG
G
U
U
U
U
G
G
G
G
A
A
*
*
*
*
*
*
*
*
*
(b) Three-dimensional
structure
(c) Symbol used
in this book(a) Two-dimensional structure
*

Figure 14.15a
5′
3′
Anticodon
Amino acid
attachment
site
Hydrogen
bonds
A
A
G
A
C
G
C
C
C
C
U
A
C G
U
U
A
A A
A
C
G U
A
C
G U
A
C
G
U
A
C
G
U
*
C
G
*
G
G
U
A
A
A
A
C C
C
C
C
G
G
GG
G
U
U
U
U
G
G
G
G
A
A
*
*
*
*
*
*
*
*
*
(a) Two-dimensional structure
*

Figure 14.15b
Anticodon
Amino acid
attachment site
Anticodon
A A G
5′3′
Hydrogen
bonds
5′
3′
(b) Three-dimensional
structure
(c) Symbol used
in this book

 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

Figure 14.16-1
Tyrosyl-tRNA
synthetase
Tyrosine (Tyr)
(amino acid)
Amino acid
and tRNA
enter active
site.
Tyr-tRNA
Complementary
tRNA anticodon
1
UA A

Figure 14.16-2
Tyrosyl-tRNA
synthetase
Tyrosine (Tyr)
(amino acid)
Amino acid
and tRNA
enter active
site.
Tyr-tRNA
Complementary
tRNA anticodon
Using ATP,
synthetase
catalyzes
covalent
bonding.
AMP + 2
ATP
2
1
UA A
P i

Figure 14.16-3
Tyrosyl-tRNA
synthetase
Tyrosine (Tyr)
(amino acid)
Amino acid
and tRNA
enter active
site.
Tyr-tRNA
Complementary
tRNA anticodon
Aminoacyl
tRNA
released.
Using ATP,
synthetase
catalyzes
covalent
bonding.
AMP + 2
ATP
2
3
1
UA A
P i

Ribosomes
 Ribosomes facilitate specific coupling of tRNA
anticodons with mRNA codons during protein
synthesis
 The large and small ribosomal are made of
proteins and ribosomal RNAs (rRNAs)
 In bacterial and eukaryotic ribosomes the large and
small subunits join to form a ribosome only when
attached to an mRNA molecule

Figure 14.17
PE A
tRNA
molecules
A
Large
subunit
Small
subunit
Growing polypeptide Exit tunnel
E P
mRNA
5′
3′
Growing polypeptide
(a) Computer model of functioning ribosome
tRNA
5′
3′
E
mRNA
(c) Schematic model with mRNA and tRNA
Codons
Amino end Next amino acid
to be added to
polypeptide
chain
Large
subunit
Small
subunit
A site (Aminoacyl-
tRNA binding site)
P site
(Peptidyl-tRNA
binding site)
Exit tunnel
E site
(Exit site)
mRNA
binding site
(b) Schematic model showing binding sites

Figure 14.17a
tRNA
molecules
A
Large
subunit
Small
subunit
Growing polypeptide Exit tunnel
E P
mRNA
5′
3′
(a) Computer model of functioning ribosome

Figure 14.17b
PE A Large
subunit
Small
subunit
P site
(Peptidyl-tRNA
binding site)
Exit tunnel
E site
(Exit site)
mRNA
binding site
(b) Schematic model showing binding sites
A site (Aminoacyl-
tRNA binding site)

Figure 14.17c
Growing polypeptide
tRNA
5′
3′
E
mRNA
(c) Schematic model with mRNA and tRNA
Codons
Amino end Next amino acid
to be added to
polypeptide
chain

 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

Building a Polypeptide
 The three stages of translation
 Initiation
 Elongation
 Termination
 All three stages require protein “factors” that aid in
the translation process

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
 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)
Animation: Translation Introduction

Figure 14.18
A
Small ribosomal subunit binds
to mRNA.
GTP
P site
U A
mRNA
5′
3′
Met
P i
mRNA binding site
Start codon
Small
ribosomal
subunit
Initiator
tRNA
5′
3′
5′
3′
Large ribosomal subunit
completes the initiation complex.
U G
C
5′
3′
Translation initiation complex
Large
ribosomal
subunit
AE
Met
GDP
+
1 2

 The start codon is important because it establishes
the reading frame for the mRNA
 The addition of the large ribosomal subunit is last
and completes the formation of the translation
initiation complex
 Proteins called initiation factors bring all these
components together

Elongation of the Polypeptide Chain
 During elongation, amino acids are added one by
one to the previous 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

Figure 14.19-1
Amino end
of polypeptide
mRNA
P
site
P
i
5′
3′
E
GTP
+
A
site
GDP
Codon recognition
E
P A
1

Figure 14.19-2
Amino end
of polypeptide
mRNA
P
site
P
i
5′
3′
E
GTP
+
A
site
GDP
Peptide bond
formation
Codon recognition
E
P A
E
P A
2
1

Figure 14.19-3
Amino end
of polypeptide
mRNARibosome ready for
next aminoacyl tRNA P
site
P
i
5′
3′
E
GTP
+
A
site
GDP
Peptide bond
formation
Codon recognition
Translocation
E
P A
E
P A
P
i
GTP
+GDP
E
P A
3
2
1

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

Figure 14.20-1
Ribosome reaches a
stop codon on mRNA.
5′
3′
Release
factor
Stop codon
(UAG, UAA, or UGA)
1

Figure 14.20-2
Free
polypeptide
Ribosome reaches a
stop codon on mRNA.
5′
3′
Release
factor
Stop codon
(UAG, UAA, or UGA)
5′
3′
Release factor
promotes
hydrolysis.
21

Figure 14.20-3
Free
polypeptide
Ribosome reaches a
stop codon on mRNA.
5′
3′
2 GTP
+2 GDP P
i
Release
factor
Stop codon
(UAG, UAA, or UGA)
5′
3′
5′
3′
Ribosomal
subunits and other
components
dissociate.
Release factor
promotes
hydrolysis.
2 31

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

Protein Folding and Post-Translational
Modifications
 During synthesis, a polypeptide chain spontaneously
coils and folds into its three-dimensional shape
 Proteins may also require post-translational
modifications before doing their jobs

Targeting Polypeptides to Specific Locations
 Two populations of ribosomes are evident in cells:
free ribosomes (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

 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

Figure 14.21
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.
Signal
peptide
removed
ER
membrane
Signal
peptide
Protein
SRP receptor
protein
Ribosome
mRNA
CYTOSOL
ER LUMEM
SRP
Translocation complex
1 2 3 4 5 6

Making Multiple Polypeptides in Bacteria and
Eukaryotes
 In bacteria and eukaryotes multiple ribosomes
translate an mRNA at the same time
 Once a ribosome is far enough past the start codon,
another ribosome can attach to the mRNA
 Strings of ribosomes called polyribosomes (or
polysomes) can be seen with an electron
microscope

Figure 14.22
Incoming
ribosomal
subunits
(b) A large polyribosome in a bacterial
cell (TEM)
Ribosomes
mRNA
(a) Several ribosomes simultaneously translating one
mRNA molecule
0.1 µm
Start of
mRNA
(5′ end)
End of mRNA
(3′ end)
Growing
polypeptides
Completed
polypeptide
Polyribosome

Figure 14.22a
Incoming
ribosomal
subunits
(a) Several ribosomes simultaneously translating one
mRNA molecule
Start of
mRNA
(5′ end)
End of mRNA
(3′ end)
Growing
polypeptides
Completed
polypeptide
Polyribosome

Figure 14.22b
(b) A large polyribosome in a bacterial
cell (TEM)
Ribosomes
mRNA
0.1 µm

 Bacteria and eukaryotes can also transcribe
multiple mRNAs form the same gene
 In bacteria, the transcription and translation can
take place simultaneously
 In eukaryotes, the nuclear envelope separates

Figure 14.23
RNA polymerase
mRNA
0.25 µm
DNA
Polyribosome
RNA
polymerase DNA
Polyribosome
Direction of
transcription
Ribosome
mRNA (5′ end)
Polypeptide
(amino end)

Figure 14.23a
RNA polymerase
mRNA
0.25 µm
DNA
Polyribosome

Figure 14.24
A
U A
tRNA
5′ Cap
3′
mRNA
Ribosomal
subunits
Aminoacyl
(charged)
tRNA
P
E
G
5′
3′
Ribosome
Codon
Anticodon
TRANSLATION
Amino
acid
Aminoacyl-tRNA
synthetase
CYTOPLASM
NUCLEUS
AMINO ACID
ACTIVATION
Intron
5′ C
ap
Poly-A
TRANSCRIPTION
RNA
PROCESSING
RNA
transcript
RNA transcript
(pre-mRNA)
RNA
polymerase
Exon
DNA
Poly-A
Poly-A
U GUG U U
A A A
A
C
C U
A
C
AE

Figure 14.24a
tRNA
mRNA
5′
3′
Amino
acid
Aminoacyl-tRNA
synthetase
CYTOPLASM
NUCLEUS
AMINO ACID
ACTIVATION
Intron
5′ C
ap
TRANSCRIPTION
RNA
PROCESSING
RNA
transcript
RNA transcript
(pre-mRNA)
RNA
polymerase
Exon
DNA
Poly-A
Poly-A
Aminoacyl
(charged)
tRNA

Figure 14.24b
A
U A
5′ Cap
3′
mRNA
Ribosomal
subunits
Aminoacyl
(charged)
tRNA
P
E
G
Ribosome
Codon
Anticodon
TRANSLATION
5′ C
ap
Poly-A
U GUG U U
A A A
A
C
C U
A
C
AE
Growing polypeptide

Concept 14.5: 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
or a few nucleotide pairs of a gene
 The change of a single nucleotide in a DNA
template strand can lead to the production of an
abnormal protein

Figure 14.25
AG G
Wild-type hemoglobin
mRNA
5′
3′
mRNA
Wild-type hemoglobin DNA
5′
3′5′
3′
TC C
TG G
AC C 5′
3′
AG G5′ 5′3′ UG G 3′
Normal hemoglobin
Sickle-cell hemoglobin
Mutant hemoglobin DNA
Sickle-cell hemoglobin
ValGlu

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

Figure 14.26
mRNA
DNA template strand
Stop
Carboxyl end
Protein
Amino end
Phe GlyMet Lys
A G3′ T CC 5′T T T TA A A AC C
5′ 3′TA A A A AT T T TG G G G C
U5′ 3′G CA U U G GGA A A AU U
Wild type
Phe GlyMet Lys
A A3′ T CC 5′T T T TA A A AC C
5′ 3′TA A A A AT T T TG G G G T
U5′ 3′G UA U U G GGA A A AU U
Phe SerMet Lys
A G3′ T CC 5′T T T TA A A AC T
5′ 3′TA A A A AT T T TG G A G C
U5′ 3′G CA U U A GGA A A AU U
Met
A C3′ T CT 5′A T A TC A A GC A
5′ 3′TA T A T AG T T CG A T G G
U5′ 3′G GA G U U GAU A U AU U
Leu AlaMet Lys
A A3′ T GC 5′T T T
A
A A C TC C
5′ 3′TA A A AGT T AG G G C T
U5′ 3′G UA U G G GGA A A
U
U A
GlyMet Phe
A T3′ T TA 5′A A
T T
C C G
C
C A
5′ 3′TA T T
A
A
G G C
T
G T T A A
U5′ 3′G AA G C U AUU U
A G
G
A
Met
A G3′ T CC 5′A T T TA A A AC C
5′ 3′TA T A A AT T T TG G G G C
U5′ 3′G UA U U G GGU A A AU U
Stop Stop
Stop
Stop
Stop
A instead of G
Silent (no effect on amino acid sequence)
(a) Nucleotide-pair substitution
U instead of C
T instead of C
Missense
A instead of G
A instead of T
Nonsense
U instead of A
Extra A
Frameshift causing immediate nonsense
(1 nucleotide-pair insertion)
(b) Nucleotide-pair insertion or deletion
Extra U
Frameshift causing extensive missense
(1 nucleotide-pair deletion)
missing
missing
missing
missing
No frameshift, but one amino acid missing
(3 nucleotide-pair deletion)

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

Figure 14.26a
DNA template strand
mRNA
Stop
Carboxyl end
Protein
Amino end
Phe GlyMet Lys
3′ 5′
5′ 3′
5′ 3′
Wild type
Phe GlyMet Lys
3′ 5′
5′ 3′
5′ 3′
Stop
A instead of G
Nucleotide-pair substitution: silent
U instead of C
T A T T A A A A T TC C C C G
TA T TA A AAT TG G G CG
UA U UA A AAU UG G G CG
T A T T A A A A T TC C C C A
TA T TA A AAT TG G G TG
UA U UA A AAU UG G G UG

 Missense mutations still code for an amino acid,
but not the correct amino acid
 Substitution mutations are usually missense
mutations
 Nonsense mutations change an amino acid codon
into a stop codon, nearly always leading to a
nonfunctional protein
Animation: Protein Synthesis

Figure 14.26b
DNA template strand
mRNA
Stop
Carboxyl end
Protein
Amino end
Phe GlyMet Lys
3′ 5′
5′ 3′
5′ 3′
Wild type
Phe SerMet Lys
3′ 5′
5′ 3′
5′ 3′
Stop
T instead of C
A instead of G
Nucleotide-pair substitution: missense
T A T T A A A A T TC C T C G
TA T TA A AAT TG A G CG
UA U UA A AAU UG A G CG

Figure 14.26c
DNA template strand
mRNA
Stop
Carboxyl end
Protein
Amino end
Phe GlyMet Lys
3′ 5′
5′ 3′
5′ 3′
Wild type
Nucleotide-pair substitution: nonsense
3′ 5′
5′ 3′
5′ 3′
Met
Stop
A instead of T
U instead of A
T A A T A A A A T TC C C C G
TA T TT A AAT TG G G CG
UA U UU A AAU UG G G CG

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 of the genetic message, producing a
frameshift mutation

Figure 14.26d
DNA template strand
mRNA
Stop
Carboxyl end
Protein
Amino end
Phe GlyMet Lys
3′ 5′
5′ 3′
5′ 3′
Wild type
Nucleotide-pair insertion: frameshift causing immediate nonsense
Met
3′ 5′
5′ 3′
5′ 3′
Stop
Extra A
Extra U
A
T
U

Figure 14.26e
DNA template strand
mRNA
Stop
Carboxyl end
Protein
Amino end
Phe GlyMet Lys
3′ 5′
5′ 3′
5′ 3′
Wild type
Nucleotide-pair deletion: frameshift causing extensive missense
Leu AlaMet Lys
3′ 5′
A
5′ 3′
5′ 3′
U
missing
missing
T A T T A A
G
A T TC C C C G
TA T TA A AATG G CG
UA U UA A AAG UG G CG

Figure 14.26f
DNA template strand
mRNA
Stop
Carboxyl end
Protein
Amino end
Phe GlyMet Lys
3′ 5′
5′ 3′
5′ 3′
Wild type
3 nucleotide-pair deletion: no frameshift, but one amino acid
missing
GlyMet Phe
3′ 5′
T T C
5′ 3′
5′ 3′
A GA
Stop
missing
missing
T A A A C C GC A A T T
TA G GT T CT T A AG
UA U U UG G G C U A A

Mutagens
 Spontaneous mutations can occur during DNA
replication, recombination, or repair
 Mutagens are physical or chemical agents that can
cause mutations
 Researchers have developed methods to test the
mutagenic activity of chemicals
 Most cancer-causing chemicals (carcinogens) are
mutagenic, and the converse is also true

What Is a Gene? Revisiting the Question
 The definition of a 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

 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

Figure 14.UN05a
thrA
−18
lacA
lacY
lacZ
lacl
recA
lexA
galR
met J
trpR
3′5′
−17
−16
−15
−14
−13
−12
−11
−10
0
−1
−2
−3
−4
−5
−6
−7
−8
−9
1
2
3
4
5
6
7
8

Figure 14.UN05b
3′5′
−18
−17
−16
−15
−14
−13
−12
−11
−10
0
−1
−2
−3
−4
−5
−6
−7
−8
−9
1
2
3
4
5
6
7
8

Figure 14.UN05c
−18
−17
−16
−15
−14
−13
−12
−11
−10
0
−1
−2
−3
−4
−5
−6
−7
−8
−9
1
2
3
4
5
6
7
8

Figure 14.UN06
3′
5′
3′
5′ 3′
5′
RNA transcript
RNA polymerase
Transcription unit
Promoter
Template strand
of DNA

Figure 14.UN07
5′ Cap
Pre-mRNA
mRNA
Poly-A tail

Figure 14.UN08
tRNA
Polypeptide
Ribosome
Amino
acid
Anti-
codon
Codon
mRNA
E A

Figure 14.UN09

Biology in Focus - Chapter 14

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