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From Gene to Protein
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
DNA RNA Protein synthesis
• The DNA inherited by an organism
– Leads to specific traits by dictating the
synthesis of proteins
• The process by which DNA directs protein
synthesis, gene expression
– Includes two stages, called transcription and
translation
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The ribosome
– Is part of the cellular machinery for translation
Figure 17.1
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Concept 17.1: Genes specify proteins via
transcription and translation
DNA Replication- Making a copy of DNA from a
DNA template strand.
Transcription- Making an RNA molecule from a
DNA template strand.
Translation- Making a protein from a mRNA
template strand.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Basic Principles of Transcription and Translation
• Transcription
– Is the synthesis of RNA under the direction of
DNA
– Produces messenger RNA (mRNA)
• Translation
– Is the actual synthesis of a polypeptide, which
occurs under the direction of mRNA
– Occurs on ribosomes
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In prokaryotes
– Transcription and translation occur together
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
(a) Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
Figure 17.3a
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In eukaryotes
– RNA transcripts are modified before becoming
true mRNA
Nuclear
envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
Figure 17.3b
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(b) Eukaryotic cell. The nucleus provides a separate
compartment for transcription. The original RNA
transcript, called pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
Cells are governed by a cellular chain of command
– DNA RNA protein
Processes:
Transcription  Translation
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The Genetic Code
• How many bases correspond to an amino
acid?
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Codons: Triplets of Bases
• Genetic information
– Is encoded as a sequence of nonoverlapping
base triplets, or codons
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During transcription
– The gene determines the sequence of bases
along the length of an mRNA molecule
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand
(template)
3
A
C
C
A
A A C C
U
G
G
U
U U G
G
A G
T
5
TRANSCRIPTION
mRNA
5
G
C U
C
3
Codon
TRANSLATION
Protein
Figure 17.4
Trp
Amino acid
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A
Phe
Gly
Ser
Cracking the Code
• A codon in messenger RNA
Figure 17.5
Second mRNA base
U
C
A
UAU
UUU
UCU
Tyr
Phe
UAC
UUC
UCC
U
UUA
UCA Ser UAA Stop
UAG Stop
UUG Leu UCG
CUU
CUC
C
CUA
CUG
CCU
CCC
Leu CCA
CCG
Pro
AUU
AUC
A
AUA
AUG
ACU
ACC
ACA
ACG
Thr
GUU
G GUC
GUA
GUG
lle
Met or
start
GCU
GCC
Val
GCA
GCG
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Ala
G
U
UGU
Cys
UGC
C
UGA Stop A
UGG Trp G
U
CAU
CGU
His
CAC
CGC
C
Arg
CAA
CGA
A
Gln
CAG
CGG
G
U
AAU
AGU
Asn
AAC
AGC Ser C
A
AAA
AGA
Lys
Arg
G
AAG
AGG
U
GAU
GGU
C
GAC Asp GGC
Gly
GAA
GGA
A
Glu
GAG
GGG
G
Third mRNA base (3 end)
First mRNA base (5 end)
– Is either translated into an amino acid or serves as
a translational stop signal
Codons must be read in the correct reading frame
– For the specified polypeptide to be produced
Consider this sentence:
“The red dog ate the cat”
Grouping in the wrong order reads as:
“her edd oga tet hec at.”
**The reading frame is important as a genetic
message that tells the cell’s protein synthesizing
machinery the EXACT message.
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Evolution of the Genetic Code
• The genetic code is nearly universal
– Shared by organisms from the simplest
bacteria to the most complex animals
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In laboratory experiments
– Genes can be transcribed and translated after
being transplanted from one species to another.
– The RNA codon CCG, for instance, translates to
the amino acid proline in all organisms whose
genetic code has been examined.
Figure 17.6
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Check it!
• http://www.technologyreview.com/view/413616/
glowing-monkeys-inherit-jellyfish-genes/
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A baby marmoset!
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Another baby marmoset!
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The ribosomes
– Are constructed of proteins and RNA
molecules named ribosomal RNA or rRNA
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
Exit tunnel
Growing
polypeptide
tRNA
molecules
Large
subunit
E
P A
Small
subunit
5
mRNA
Figure 17.16a
3
(a) Computer model of functioning ribosome. This is a model of a bacterial
ribosome, showing its overall shape. The eukaryotic ribosome is roughly
similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules
and proteins.
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Building a Polypeptide
• We can divide translation into three stages
– Initiation
– Elongation
– Termination
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Ribosome Association and Initiation of Translation
• The initiation stage of translation
– Brings together mRNA, tRNA bearing the first
amino acid of the polypeptide, and two
subunits of a ribosome
P site
3 U A C 5
5 A U G 3
Initiator tRNA
Large
ribosomal
subunit
GTP
GDP
E
A
mRNA
5
Start codon
mRNA binding site
Figure 17.17
3
Small
ribosomal
subunit
1 A small ribosomal subunit binds to a molecule of
mRNA. In a prokaryotic cell, the mRNA binding site
on this subunit recognizes a specific nucleotide
sequence on the mRNA just upstream of the start
codon. An initiator tRNA, with the anticodon UAC,
base-pairs with the start codon, AUG. This tRNA
carries the amino acid methionine (Met).
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5
3
Translation initiation complex
2 The arrival of a large ribosomal subunit completes
the initiation complex. Proteins called initiation
factors (not shown) are required to bring all the
translation components together. GTP provides
the energy for the assembly. The initiator tRNA is
in the P site; the A site is available to the tRNA
bearing the next amino acid.
Elongation of the Polypeptide Chain
• In the elongation stage of translation
– Amino acids are added one by one to the
preceding amino acid
TRANSCRIPTION
Amino end
of polypeptide
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
mRNA
Ribosome ready for
next aminoacyl tRNA
E
3
P A
site site
5
1 Codon recognition. The anticodon
of an incoming aminoacyl tRNA
base-pairs with the complementary
mRNA codon in the A site. Hydrolysis
of GTP increases the accuracy and
efficiency of this step.
2 GTP
2 GDP
E
E
P
P
A
GDP
Figure 17.18
3 Translocation. The ribosome
translocates the tRNA in the A
site to the P site. The empty tRNA
in the P site is moved to the E site,
where it is released. The mRNA
moves along with its bound tRNAs,
bringing the next codon to be
translated into the A site.
GTP
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E
P
A
A
2 Peptide bond formation. An
rRNA molecule of the large
subunit catalyzes the formation
of a peptide bond between the
new amino acid in the A site and
the carboxyl end of the growing
polypeptide in the P site. This step
attaches the polypeptide to the
tRNA in the A site.
Termination of Translation
• The final stage of translation is termination
– When the ribosome reaches a stop codon in
the mRNA
Release
factor
Free
polypeptide
5
3
3
5
5
3
Stop codon
(UAG, UAA, or UGA)
1 When a ribosome reaches a stop 2 The release factor hydrolyzes 3 The two ribosomal subunits
codon on mRNA, the A site of the
the bond between the tRNA in and the other components of
ribosome accepts a protein called
the P site and the last amino
the assembly dissociate.
a release factor instead of tRNA.
acid of the polypeptide chain.
The polypeptide is thus freed
from the ribosome.
Figure 17.19
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Concept 17.5: RNA plays multiple roles in the cell:
a review
• RNA
– Can hydrogen-bond to other nucleic acid
molecules
– Can assume a specific three-dimensional
shape
– Has functional groups that allow it to act as a
catalyst
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Types of RNA in a Eukaryotic Cell
Know these 3!
Table 17.1
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The change of a single nucleotide in the DNA’s
template strand
– Leads to the production of an abnormal protein
Wild-type hemoglobin DNA
3
Mutant hemoglobin DNA
5
C T
T
In the DNA, the
mutant template
strand has an A where
the wild-type template
has a T.
G U A
The mutant mRNA has
a U instead of an A in
one codon.
3
5
T
C A
mRNA
mRNA
G A
A
5
3
5
3
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
Figure 17.23
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The mutant (sickle-cell)
hemoglobin has a valine
(Val) instead of a glutamic
acid (Glu).
Mutagens
• Spontaneous mutations
– Can occur during DNA replication,
recombination, or repair
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Mutagens
– Are physical or chemical agents that can
cause mutations
– Examples: Environmental factors (toxins),
Cigarette smoke, radioactive element, UV
radiation (from sunlight)
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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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
A summary of transcription and translation in
a eukaryotic cell
DNA
TRANSCRIPTION
1 RNA is transcribed
from a DNA template.
3
RNA
transcript
5
RNA
polymerase
RNA PROCESSING
Exon
2 In eukaryotes, the
RNA transcript (premRNA) is spliced and
modified to produce
mRNA, which moves
from the nucleus to the
cytoplasm.
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
synthetase
NUCLEUS
Amino
acid
tRNA
FORMATION OF
INITIATION COMPLEX
CYTOPLASM 3 After leaving the
nucleus, mRNA attaches
to the ribosome.
AMINO ACID ACTIVATION
4
Each amino acid
attaches to its proper tRNA
with the help of a specific
enzyme and ATP.
mRNA
Growing
polypeptide
Activated
amino acid
Ribosomal
subunits
5
TRANSLATION
5
E
A
AAA
UG GUU UA U G
Codon
Figure 17.26
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Ribosome
A succession of tRNAs
add their amino acids to
Anticodon the polypeptide chain
as the mRNA is moved
through the ribosome
one codon at a time.
(When completed, the
polypeptide is released
from the ribosome.)