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Chapter 17
From Gene to Protein
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: The Flow of Genetic Information
• The information content of DNA
– Is in the form of specific sequences of
nucleotides along the DNA strands
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The DNA inherited by an organism
– Leads to specific traits by dictating the
synthesis of proteins
• The process by which DNA directs protein
synthesis is called gene expression which
– Includes two stages, called transcription and
translation
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• The ribosome
– Is part of the cellular machinery for translation,
polypeptide synthesis (protein synthesis)
Figure 17.1
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• Concept 17.1: Genes specify proteins via
transcription and translation
• What is the evidence for such a theory? See
the next slide
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Evidence from the Study of Metabolic Defects
• In 1909, British physician Archibald Garrod
– Postulated that genes dictate phenotypes
through enzymes that catalyze specific
chemical reactions in the cell. i.e symptoms of
an inherited disease reflects person’s inability
to synthesize a particular enzyme.
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Nutritional Mutants in Neurospora: Scientific Inquiry
• Later Beadle and Tatum working on a mold that
was easily mutated provide a strong support for
the one gene one enzyme hypothesis. HOW?
• They cause bread mold to mutate with X-rays
creating mutants that could not survive on
minimal medium
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Beadle and Tatum Experiment
• Using genetic crosses
– They determined that their mutants fell into three
classes, each mutated in a different gene
EXPERIMENT
RESULTS
Working with the mold Neurospora crassa, Beadle and Tatum had isolated mutants requiring arginine
in their growth medium and had shown genetically that these mutants fell into three classes, each
defective in a different gene.
The wild-type strain required only the minimal medium for growth. The three classes of mutants
had different growth requirements
Class I Class II Class III
Wild type Mutants Mutants Mutants
Minimal medium
(MM) (control)
MM +Ornithine
MM + Citrulline
MM +Arginine
(control)
Figure 17.2
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CONCLUSION
From the growth patterns of the mutants, Beadle and Tatum deduced that each mutant was
unable to carry out one step in the pathway for synthesizing arginine, presumably because it
lacked the necessary enzyme. Because each of their mutants was mutated in a single gene,
they concluded that each mutated gene must normally dictate the production of one enzyme.
Their results supported the one gene–one enzyme hypothesis and also confirmed the arginine
pathway.
(Notice that a mutant can grow only if supplied with a compound made after the defective
step.)
Class I
Class II
Class III
Mutants
Mutants
Mutants
(mutation
(mutation
(mutation
in
gene
A)
in
gene
B)
in gene C)
Wild type
Precursor
Gene A
Enzyme
A
Precursor
Precursor
Precursor
A
A
A
Ornithine
Gene B
Enzyme
B
Ornithine
Enzyme
C
Ornithine
B
B
B
Citrulline
Citrulline
Citrulline
C
C
C
Arginine
Arginine
Arginine
Citrulline
Gene C
Ornithine
Arginine
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• Beadle and Tatum developed the “one gene–
one enzyme hypothesis”
– Which states that the function of a gene is to
dictate the production of a specific enzyme
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The Products of Gene Expression: A Developing Story
• As researchers learned more about proteins
– The made minor revision to the one gene–one
enzyme hypothesis
• Genes code for polypeptide chains or for RNA
molecules, one gene one peptide would be
more appropriate that one gene one enzyme.
• This revision was necessary as not all proteins
are enzyme, some are hormones like insulin.
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Basic Principals of Transcription and Translation
• Genes provide the instructions indirectly for
protein synthesis.
• The DNA contains A, G, C and T nucleotides
while RNA has A, G, C and U instead of T.
• RNA is always single strand molecule.
• Genes are typically hundreds of thousands of
nucleotides long, each having a specific
sequence of bases.
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Basic Principles of Transcription and Translation
• Transcription
– Is the synthesis of RNA under the direction of
DNA
– Produces messenger RNA (mRNA) which is
the faithful transcript of the protein
• Translation
– Is the actual synthesis of a polypeptide, which
occurs under the direction of mRNA
– Occurs on ribosomes which facilitate the
orderly linkage of amino acids into polypeptide
chains.
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In prokaryotes
• Transcription and translation occur together at
the same time due to the lack of nucleus.
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
(a)
Figure 17.3a
Prokaryotic cell. In a cell lacking a nucleus, mRNA
produced by transcription is immediately translated
without additional processing.
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In eukaryotes
• RNA transcripts (pre-mRNA, primary) are modified
before becoming true mRNA or the final mRNA.
Nuclear
envelope
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
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
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The Genetic Code
• How many bases correspond to an amino
acid?
• If each nucleotide can be translated to an
amino acid, then only 4 of the 20 amino acids
could be specified.
• If two letters (eg. AG or GT) code for each
amino acid then 42 = 16 amino acids would be
specified.
• Therefore, triplets would be the smallest units
of uniform length that can code for all the
amino acids and that would be 43 = 64
combinations.
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Codons: Triplets of Bases
• For each gene only one strand which is called the
template strand gets transcribed.
• An mRNA strand is therefore a complementary rather
than identical to the DNA strand.
• mRNA triplets are called codons. The sequence of
these codons is translated into a sequence of amino
acids making up a polypeptide chain. These codons
are read in the
5’
3’ direction along the
mRNA.
• The number of nucleotides making up a genetic
message must be three times the number of amino
acids making up a proteins i.e 300 nucleotides makes
a 100 amino acid polypeptide chain.
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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 3
A C C A A A C C
(template)
G A G
T
5
TRANSCRIPTION
mRNA
5
U G G U
U U G G C U C A
3
Codon
TRANSLATION
Protein
Figure 17.4
Trp
Amino acid
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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
ACU
lle ACC
ACA
Met or
AUG start ACG
Thr
AUU
AUC
A
AUA
GUU
G GUC
GUA
GUG
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
For the specified polypeptide to be produced
Codons must be read in the correct reading
frame.
– The message is NOT read in overlapping
words
– All the reading follows the 5’ → 3’ direction in
groups of three; for example the following
sequence UGG UUU GGC UCA gives a
certain peptide. But if it was read in GGU
UUG GCU…. etc it will give completely
different amino acid sequence.
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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
– A very beneficial application of this fact is that
bacteria can be programmed by the insertion
of human genes to synthesize certain human
proteins that are medically or commercially
important.
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• In laboratory experiments
– Genes can be transcribed and translated after
being transplanted from one species to
another
Figure 17.6
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• Concept 17.2: Transcription is the DNAdirected synthesis of RNA: a closer look
• The mRNA is transcribed from the template
strand of a gene.
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Molecular Components of Transcription
• RNA synthesis
– Is catalyzed by RNA polymerase, which pries
(force open) the DNA strands apart and hooks
together the RNA nucleotides
– Follows the same base-pairing rules as DNA,
except that in RNA, uracil substitutes for
thymine
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• The RNA polymerase can add nucleotides only to the 3’
end of the growing polymer therefore the direction of
molecule elongation is from 5’→3’. This is also called
down stream while the other direction is called upstream
• The DNA sequence where the RNA polymerase attaches
and initiates transcription is called promoter.
• The sequence that signals the end of transcription is called
terminator.
• The promoter is said to be upstream from the terminator
and the stretch of the DNA that is transcribed is called the
transcription unit.
• Bacteria have only one type of RNA polymerase while
eukaryotic cells has three types of polymerase named I, II
and III.
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Synthesis of an RNA Transcript
• The stages of transcription are
– Initiation
– Elongation
– Termination
Promoter
Transcription unit
5
3
3
5
Start pointDNA
1 Initiation. After RNA polymerase binds to
RNA polymerase
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
5
3
3
5
Template strand of
Unwound RNA DNA
DNA
transcript
Elongation. The polymerase moves downstream,
2
unwinding the DNA and elongating the RNA
transcript 5  3 . In the wake of
Rewound
transcription, the DNA strands re-form a double helix.
RNA
5
3
3
5
3
5
RNA
transcript
3
Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.
5
3
3
5
5
Figure 17.7
Completed RNA
transcript
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3
Non-template
strand of DNA
Elongation
RNA nucleotides
RNA
polymerase
A
T
C
3
C
A
A
3 end
U
5
A
E
G
C
A
T
A
G
G
T
T
Direction of transcription
(“downstream”)
5
Newly made
RNA
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Template
strand of DNA
RNA Polymerase Binding and Initiation of Transcription
•
Promoters signal the initiation of RNA synthesis and they are located
upstream from transcription point and it serve as the binding site for the
RNA polymerase.
•
Transcription factors
–
Help eukaryotic RNA polymerase recognize promoter sequences
– The complete assembly of transcription factors and
RNA polymerase bound to the promoter is called
transcription initiation complex.
– Figure 17-8 shows the role of this complex and a
crucial promoter DNA sequence called the TATA box
in forming the initiation complex.
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Initiation of transcription at the eukaryotic promoter
TRANSCRIPTION
Eukaryotic promoters;
commonly include a TATA box
1
DNA
Pre-mRNA
RNA PROCESSING
mRNA
Ribosome
TRANSLATION
Polypeptide
Promoter
5
3
3
5
T A T A A A A
A T A T
T T
T
TATA box
Start point
2
Template
DNA strand
Several transcription
factors
Transcription
factors
5
3
3
5
3
Additional transcription
factors
RNA polymerase II
5
3
Transcription factors
3
5
5
RNA transcript
Figure 17.8
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Transcription initiation complex
Elongation of the RNA Strand
• As RNA polymerase moves along the DNA
– It continues to untwist the double helix, exposing
about 10 to 20 DNA bases at a time for pairing with
RNA nucleotides
– The enzyme adds nucleotides to the 3’ end of the
growing RNA
– As the RNA strand is being extended, the DNA double
helix re-wind and the new RNA template peels a way
from its DNA template
– Transcription progresses at a rate of about 60
nucleotides per second.
– A single gene can be transcribed simultaneously by
several molecules of RNA polymerase following each
other.
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Termination of Transcription
• The mechanisms of termination
– Are different in prokaryotes and eukaryotes
– Transcription proceeds until after the RNA
polymerase transcribes a terminator sequence
(AAUAAA) in the DNA.
– The cleavage site of the RNA is also serve as
the site for the addition of the poly A tail.
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• Concept 17.3: Eukaryotic cells modify RNA
after transcription
• Enzymes in the eukaryotic nucleus
– Modify pre-mRNA in specific ways before the
genetic messages are dispatched to the
cytoplasm
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Alteration of mRNA Ends
• Each end of a pre-mRNA molecule is modified
in a particular way
– The 5 end receives a modified nucleotide G
(Guanine) cap.
– The 3 end gets a poly-A tail
A modified guanine nucleotide
added to the 5 end
TRANSCRIPTION
RNA PROCESSING
50 to 250 adenine nucleotides
added to the 3 end
DNA
Pre-mRNA
5
mRNA
Protein-coding segment
Polyadenylation signal
3
G P P P
AAUAAA
AAA…AAA
Ribosome
TRANSLATION
5 Cap
5 UTR
Start codon Stop codon
Polypeptide
Figure 17.9
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3 UTR
Poly-A tail
– This 5’ cap has two functions;
• Helps protect RNA form degradation by hydrolytic
enzymes
• After mRNA reaches cytoplasm, 5’ cap functions as
a sign for the ribosome to attach in a specific place
– The 3’ end is also modified, the enzyme makes a poly
A tail (A) consisting of 50-250 nucleotides
• Poly A tail inhibits degradation of the RNA
• Helps ribosomes attach to RNA
• Facilitate export of mRNA from nucleus
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Split Genes and RNA Splicing
• RNA splicing
–
Removes introns (intervening or none-coding and joins exons
(coding sequences). Both DNA and RNA have coding and nonecoding sequences.
–
DNA that codes for a polypeptide is not necessary continuous.
5 Exon Intron
TRANSCRIPTION
DNA
Exon
3
Pre-mRNA 5 Cap
Poly-A tail
30
1
RNA PROCESSING
Intron
Exon
31
104
105
146
Pre-mRNA
Coding
segment
mRNA
Ribosome
Introns cut out and
exons spliced
together
TRANSLATION
Polypeptide
mRNA
5 Cap
1
3 UTR
Figure 17.10
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Poly-A tail
146
3 UTR
RNA splicing
• Is carried out by spliceosomes in some cases
RNA transcript (pre-mRNA)
5
Intron
Exon 1
Exon 2
Protein
1
Other proteins
snRNA
snRNPs
Spliceosome
2
5
Spliceosome
components
3
Figure 17.11
5
mRNA
Exon 1
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Exon 2
Cut-out
intron
Ribozymes
– Are catalytic RNA molecules that function as
enzymes and can splice RNA besides the
spliceosomes.
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The Functional and Evolutionary Importance of Introns
– Introns play a regulatory role in the cell
– Splicing may help regulating the passage of
mRNA from nucleus to the cytoplasm.
– Enable a single gene to encode more than one
kind of polypeptide in a process called
alternative RNA splicing
– Split genes may also facilitate the evolution of
new potentially useful proteins
– Exon shuffling could lead to new proteins with
different functions
– Different exons code for the different domains in a
protein
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• Proteins often have a modular architecture
– Consisting of discrete structural and functional
regions called domains
Gene
DNA
Exon 1
Intron
Exon 2
Intron
Exon 3
Transcription
RNA processing
Translation
Domain 3
Domain 2
Domain 1
Figure 17.12
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Polypeptide
THE SYNTHESIS OF PROEINS
• Concept 17.4: Translation is the RNA-directed
synthesis of a polypeptide: a closer look
• In the process of translation, the cell interprets the genetic
message and builds a protein accordingly.
• The message is a series of codons and the interpreter is
the transfer RNA (tRNA).
• The tRNA transfers amino acids from the cytoplasm’s pool
of amino acids to the ribosome.
• The ribosome in turn adds each amino acid to the growing
end of a polypeptide chain.
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Molecular Components of Translation
• A cell translates an mRNA message into
protein
– With the help of transfer RNA (tRNA)
– Molecules of tRNA are NOT identical.
– Each type of tRNA links a particular mRNA
codon with a particular amino acid.
– As tRNA arrives at a ribosome it bears
specific amino acid at one end while the
other end a nucleotide triplet called anticodon which base pair with a complementary
codon on mRNA.
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Translation: the basic concept
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Amino
acids
Polypeptide
Ribosome
tRNA with
amino acid
attached
Gly
tRNA
Anticodon
U
G
A A
U
U
U
G
Codons
5
Figure 17.13
G
A
mRNA
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G
C
3
Structure of tRNA
• Unlike mRNA (hundreds of nucleotides), tRNA
consists of a single RNA strand about 80 nucleotides
(N) long.
• Folds to form a molecule of three dimensional
structure ( L-shaped) with N from one region form Hbonds with complementary bases of other region.
• tRNA looks like a cloverleaf
• The loop of the L-shape contains the anti-codon that
binds to the specific sequence onto the mRNA
molecule in the translation process
• From the other end of the L-shape, tRNA protrudes its
3’ end which is the attachment site for the amino
acids.
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The Structure and Function of Transfer RNA
3
A
C
C
AA
CC
CG
C
U
U
A
A
U C
C A C A G
*
G
G U G U *
C
*
*
U C
* G AG
G
U
Amino acid
attachment site
*
*
A
*
A
5
G
C
G
G
A
U
U
U
A * C U C
C G A G
*
C
C
A
G
A
C
U
G
A
Anticodon
(a)
Two-dimensional structure. The four base-paired regions and three
loops are characteristic of all tRNAs, as is the base sequence of the
amino acid attachment site at the 3 end. The anticodon triplet is
unique to each tRNA type. (The asterisks mark bases that have been
chemically modified, a characteristic of tRNA.)
Figure 17.14a
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A G *
*
G
A G G
Hydrogen
bonds
• If one tRNA exists for each mRNA codons that
specifies an amino acid, then there should be
about 61 tRNAs, however the actual number is
about 45 as some anti-codons can recognize
two or more different codons.
• This can happen because the base of U in
tRNA can pair with A or G of an mRNA. This
phenomenon is called the wobble.
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5
3
Amino acid
attachment site
Hydrogen
bonds
A AG
3
Anticodon
5
Anticodon
(c)
(b) Three-dimensional structure of tRNA
Symbol used
in this book
Figure 17.14b
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A specific enzyme called an aminoacyl-tRNA synthetase
• Joins each amino acid to the correct tRNA
Aminoacyl-tRNA
synthetase (enzyme)
Amino acid
P
P
P
Adenosine
Active site binds the
amino acid and ATP.
ATP
1
P
Pyrophosphate
P
Pi
Phosphates
Adenosine
2 ATP loses two P groups
and joins amino acid as AMP.
Pi
Pi
tRNA
3 Appropriate
tRNA covalently
Bonds to amino
Acid, displacing
AMP.
P
Adenosine
AMP
4 Activated amino acid
is released by the enzyme.
Figure 17.15
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Aminoacyl tRNA
(an “activated
amino acid”)
Ribosomes
• Ribosomes
– Facilitate the specific coupling of tRNA
anticodons with mRNA codons during protein
synthesis
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The ribosomal subunits
–
Are constructed of proteins and RNA molecules named ribosomal
RNA or rRNA and are found in thousands in most cells.
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
Exit tunnel
Growing
polypeptide
tRNA
molecules
Large
subunit
E
P A
Small
subunit
5
mRNA
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.
Figure 17.16a
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• These subunits are joined together to form a functional
ribosome only when they attach to an mRNA molecule.
• Although prokaryotic and eukaryotic ribosomes are similar,
the eukaryotic one is larger and has minor differences that
are important medically.
• There are some antibiotics can paralyze the prokaryotic
ribosomes while not affecting the eukaryotic one such as
tetracycline and streptomycine.
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Function of ribosome
• To bring mRNA together with amino acid-bearing
tRNAs.
• Each ribosome has three binding sites for tRNA
– P site (peptidyl-tRNA site) holds the tRNA
carrying the growing polypeptide chian
– A site (aminoacyl-tRNA site) hold tRNA
carrying the next amino acid to be added to the
chain
– Discharged tRNA leaves the ribosome from the
E site.
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P site (Peptidyl-tRNA
binding site)
A site (AminoacyltRNA binding site)
E site
(Exit site)
Large
subunit
E
mRNA
binding site
Figure 17.16b
P
A
Small
subunit
(b) Schematic model showing binding sites. A ribosome has an mRNA
binding site and three tRNA binding sites, known as the A, P, and E
sites. This schematic ribosome will appear in later diagrams.
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Amino end
Growing polypeptide
Next amino acid
to be added to
polypeptide chain
tRNA
3
mRNA
5
Codons
(c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its
anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to the
growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added
to the polypeptide chain. Discharged tRNA leaves via the E site.
Figure 17.16c
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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
1
Figure 17.17
3
Small
ribosomal
subunit
5
3
Translation initiation complex
2
A small ribosomal subunit binds to a molecule of
The arrival of a large ribosomal subunit completes
mRNA. In a prokaryotic cell, the mRNA binding site
the initiation complex. Proteins called initiation
on this subunit recognizes a specific nucleotide
factors (not shown) are required to bring all the
sequence on the mRNA just upstream of the start
translation components together. GTP provides
codon. An initiator tRNA, with the anticodon UAC,
the energy for the assembly. The initiator tRNA is
base-pairs with the start codon, AUG. This tRNA
in the P site; the A site is available to the tRNA
carries the amino acid methionine (Met).
bearing the next amino acid.
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Summary of the initiation
• The first ribosomal subunit (small unit) binds to both
mRNA and the tRNA. It attaches to the leader
segment at the 5’ prime (upstream) end of the mRNA.
– In prokaryotic cells, rRNA binds of the small subunit
base-pairs with specific sequence of Nucleotides
within the mRNA leader
– In eukaryotic cells, the 5’ cap first tells the small
subunit to attach to the 5’ end of mRNA at the down
stream position where the initiation codon which
signals the beginning of the translation.
• A protein called the initiation factor is required to bring
the mRNA, tRNA bearing the first amino acid of the
polypeptide and the two subunits of a ribosome.
• During this process the cell spends energy in the form
of GTP
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Elongation of the Polypeptide Chain
• In the elongation stage of translation
– Amino acids are added one by one to the preceding
amino acid with the help of elongation factors
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.
1
TRANSCRIPTION
Amino end
of polypeptide
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
mRNA
Ribosome ready for
next aminoacyl tRNA
5
E
3
P A
site site
Figure 17.18
GTP
2 GDP
E
E
P
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.
2
P
A
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.
2
GDP
GTP
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E
P
A
A
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
codon on mRNA, the A site of the
ribosome accepts a protein called
a release factor instead of tRNA.
Figure 17.19
2
The release factor adds an H2O
molecule that hydrolyzes
the bond between the tRNA in
the P site and the last amino
acid of the polypeptide chain.
The polypeptide is thus freed
from the ribosome.
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3 The two ribosomal subunits
and the other components of
the assembly dissociate.
Polyribosomes
• A number of ribosomes can translate a single mRNA
molecule simultaneously Forming a polyribosome
Completed
polypeptide
Growing
polypeptides
Incoming
ribosomal
subunits
Start of
End of
mRNA
mRNA
(5 end)
(3 end)
An
mRNA
molecule
is
generally
translated
simultaneously
(a)
by several ribosomes in clusters called polyribosomes.
Ribosomes
mRNA
Figure 17.20a, b
(b)
0.1 µm
This micrograph shows a large polyribosome in a prokaryotic
cell (TEM). In Eukaryotic cells their exist also polyribosomes.
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Completing and Targeting the Functional Protein
• Polypeptide chains Undergo modifications after
the translation process called posttranslational
modification. How does themodification occur?
– Certain amino acids may be chemically
modified by the attachment of sugars, lipids,
phosphate groups or others.
– Enzymes might remove one or few amino
acids from the leading (amino end) of the
polypeptide chain. Example; insulin protein is
produced as pro-insulin which gets
enzymatically modified to be functional.
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Protein Folding and Post-Translational Modifications
• This posttranslational modification of the
proteins will affect their three-dimensional
structure.
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Targeting Polypeptides to Specific Locations
• Two populations of ribosomes are evident in
cells
– Free ribosomes; suspended in cytosol and
function their. Initiate the synthesis of all
proteins
– Bound ribosomes; attached to the cytosolic
part of the ER. They make proteins of the
endomembrane as well as proteins secreted
from the cell such as insulin
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• Proteins destined for the endomembrane
system or for secretion
– Must be transported into the ER
– Have signal peptides to which a signalrecognition particle (SRP) binds, enabling the
translation ribosome to bind to the ER
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• The signal mechanism for targeting proteins to
the ER
1 Polypeptide
synthesis begins
on a free
ribosome in
the cytosol.
2 An SRP binds
to the signal
peptide, halting
synthesis
momentarily.
3 The SRP binds to a
receptor protein in the ER
membrane. This receptor
is part of a protein complex
(a translocation complex)
that has a membrane pore
and a signal-cleaving enzyme.
4 The SRP leaves, and
the polypeptide resumes
growing, meanwhile
translocating across the
membrane. (The signal
peptide stays attached
to the membrane.)
5 The signalcleaving
enzyme
cuts off the
signal peptide.
6 The rest of
the completed
polypeptide leaves
the ribosome and
folds into its final
conformation.
Ribosome
mRNA
Signal
peptide
Signalrecognition
particle
(SRP) SRP
receptor
CYTOSOL protein
Translocation
ERLUMEN complex
Figure 17.21
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Signal
peptide
removed
ER
membrane
Protein
• 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
Table 17.1
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Concept 17.6
• Comparing gene expression in prokaryotes and eukaryotes
reveals key differences
• Prokaryotic cells lack a nuclear envelope thus allowing
translation to begin while transcription is still in progress
RNA polymerase
DNA
mRNA
Polyribosome
RNA
polymerase
Direction of
transcription
DNA
Polyribosome
Polypeptide
(amino end)
Ribosome
mRNA (5 end)
Figure 17.22; coupled transcription and translation in bacteria
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0.25 m
• In a eukaryotic cell
– The nuclear envelope separates transcription
from translation
– Extensive RNA processing occurs in the
nucleus
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• Concept 17.7: Point mutations can affect
protein structure and function
• Mutations
– Are changes in the genetic material of a cell
• Point mutations
– Are changes in just one base pair of a gene.
However, if this mutation occurs in a gamete, it
can be transported to the offspring.
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Point mutations
• 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
3
5
T
C A
mRNA
T
In the DNA, the
mutant template
strand has an A where
the wild-type template
has a T.
mRNA
G A
A
5
G U A
3
5
3
Normal hemoglobin
Sickle-cell hemoglobin
Glu
Val
Figure 17.23
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The mutant mRNA has
a U instead of an A in
one codon.
The mutant (sickle-cell)
hemoglobin has a valine
(Val) instead of a glutamic
acid (Glu).
Types of Point Mutations
• Point mutations within a gene can be divided
into two general categories
– Base-pair substitutions
– Base-pair insertions or deletions
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Base-pair substitution Substitutions
Is the replacement of one nucleotide and its partner with
another pair of nucleotides
– Can cause missense or nonsense
Wild type
A U G
mRNA
5
Protein
Met
A A G U U U G G C U A A
Lys
Phe
Gly
3
Stop
Amino end
Carboxyl end
Base-pair substitution
No effect on amino acid sequence
U instead of C
A U G A A G U U U G G U U A A
Met
Lys
Missense
Phe
Gly
Stop
A instead of G
A U G A A G U U U A G U U A A
Met
Lys
Phe
Ser
Stop
Nonsense
U instead of A
A U G U A G U U U G G C U A A
Figure 17.24
Met
Stop
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Insertions and Deletions
– Are additions or losses of nucleotide pairs in a gene
– May produce frameshift mutations
Wild type
mRNA
5
Protein
A U G A A G U U U G G C U A A
Met
Lys
Gly
Phe
Stop
Amino end
Carboxyl end
Base-pair insertion or deletion
Frameshift causing immediate nonsense
Extra U
A U G U A A G U U U G G C U A
Met
Stop
Frameshift causing
extensive missense
U
Missing
A U G A A G U U G G C U A A
Met
Lys
Leu
Ala
Insertion or deletion of 3 nucleotides:
no frameshift but extra or missing amino acid
A A G
Missing
A U G U U U G G C U A A
Figure 17.25
Met
Phe
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Gly
Stop
3
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
– A number of chemical and physical agents
called mutagens that interact with DNA to
cause mutations. X-Ray, UV light are common
mutagens.
– Most carcinogens are mutagenic conversely
most mutagens are carcinogenic.
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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
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• A summary of transcription and translation in a
eukaryotic cell
DNA
TRANSCRIPTION
1 RNA is transcribed
from a DNA template.
3
5
RNA
transcript
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.
mRNA
AMINO ACID ACTIVATION
4
Each amino acid
attaches to its proper tRNA
with the help of a specific
enzyme and ATP.
Growing
polypeptide
Activated
amino acid
Ribosomal
subunits
5
TRANSLATION
A succession of tRNAs
add their amino acids to
the polypeptide chain
Anticodon
as the mRNA is moved
through the ribosome
one codon at a time.
(When completed, the
polypeptide is released
from the ribosome.)
5
E
A
AAA
UG GU U U A U G
Codon
Figure 17.26
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Ribosome
End of Chapter 17
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