Transcript From Gene to Protein

Chapter 17
From Gene to Protein
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
along the DNA strands
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• 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
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Concept 17.1:
• Genes:
–
specify proteins via transcription and
translation
– Specify different types of RNA via transcription
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Basic Principles of Transcription and Translation
• Transcription
– Is the synthesis of RNA under the direction of
DNA. It produces:
• messenger RNA (mRNA)
• ribosomal RNA (rRNA)
• Transfer RNA (tRNA)
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• Translation
– Is the actual synthesis of a polypeptide, which
occurs under the direction of
– Occurs on
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In prokaryotes
• Transcription and translation occur
simultaneously
TRANSCRIPTION
DNA
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
• Transcription and translation occur at different times
• 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 mRNA protein
Or
– DNA rRNA
Or
DNA tRNA
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The Genetic Code
Genetic information
– Is encoded as a sequence of base triplets,
called
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During transcription
• The DNA base sequence of a gene determines the
sequence of bases of an mRNA molecule
Gene 2
DNA
molecule
Gene 1
Gene 3
DNA strand 3
5
A C C A A A C C G A G T
(template)
TRANSCRIPTION
mRNA
5
U G G U U U G G C U C A
Codon
TRANSLATION
Protein
Trp
Amino acid
Phe
Gly
Ser
Figure 17.4
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3
The Genetic 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
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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Concept 17.2: Transcription
• RNA synthesis
– Is catalyzed by
which
opens the DNA strands and connects the RNA
nucleotides
– Follows the same
rules as
DNA, except that in RNA, uracil substitutes for
thymine
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Synthesis of an RNA Transcript
• The stages of transcription are
Promoter
– Initiation
Transcription unit
5
3
3
5
Start point
RNA polymerase
– Elongation
– Termination
DNA
Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand.
1
5
3
Unwound
DNA
3
5
Template strand of
DNA
transcript
2 Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5  3 . In the wake of
transcription, the DNA strands re-form a double helix.
Rewound
RNA
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
RNA Polymerase Binding and Initiation of Transcription
•
signal the initiation site
for transcription
•
– Help eukaryotic RNA polymerase recognize
promoter sequences
1 Eukaryotic promoters
TRANSCRIPTION
DNA
RNA PROCESSING
Pre-mRNA
mRNA
TRANSLATION
Ribosome
Polypeptide
Promoter
5
3
3
5
T A T A A AA
AT A T T T T
TATA box
Start point
Template
DNA strand
Several transcription
factors
2
Transcription
factors
5
3
3 Additional transcription
3
5
factors
RNA polymerase II
5
3
Figure 17.8
Transcription factors
3
5
5
RNA transcript
Transcription initiation complex
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Elongation of the RNA Strand
• As RNA polymerase moves along the DNA
– It continues to untwist the double helix, adding
successive complementary nucleotides, and
extending the RNA molecule
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Elongation
Non-template
strand of DNA
RNA nucleotides
RNA
polymerase
A
T
C
C
A
A
3
3 end
U
5
A
U
C
C
A
T
A
G
G
T
T
Direction of transcription
(“downstream”)
5
Newly made
RNA
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Template
strand of DNA
Concept 17.3:
• Enzymes in the eukaryotic nucleus
– Modify
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
A modified guanine nucleotide
added to the 5 end
TRANSCRIPTION
50 to 250 adenine nucleotides
added to the 3 end
DNA
Pre-mRNA
RNA PROCESSING
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
Split Genes and RNA Splicing
• RNA splicing
– Removes introns and joins exons
TRANSCRIPTION
RNA PROCESSING
DNA
Pre-mRNA
5 Exon Intron
Pre-mRNA 5 Cap
30
31
1
Coding
segment
mRNA
Ribosome
Intron
Exon
Exon
3
Poly-A tail
104
105
146
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
The Functional Importance of Introns
• The presence of introns
– Allows for alternative RNA splicing
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• Proteins often consist of discrete structural and
functional regions called domains
• In many cases different
the different domains in a protein
Gene
DNA
Exon 1 Intron Exon 2
Transcription
RNA processing
Intron Exon 3
Translation
Domain 3
Domain 2
Domain 1
Polypeptide
Figure 17.12
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code for
Concept 17.4:
• Translation is the mRNA-directed synthesis of
a polypeptide
• A cell translates an mRNA message into
protein
– With the help of
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Translation
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
Amino
acids
Polypeptide
tRNA with
amino acid
Ribosome attached
Gly
tRNA
Anticodon
A A A
U G G U U U G G C
Codons
5
mRNA
Figure 17.13
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3
• Molecules of tRNA are not all identical
– Each carries a specific
one end
– Each has an
end
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on
on the other
The Structure and Function of Transfer RNA
• A tRNA molecule
3
A
C
C
A 5
C G
G C
C G
U G
U A
A U
A U
U C
UA
C A C AG
*
G
*
G U G U *
C
C
* *
U C
*
* G AG C
G C
U A
* G
A
A*
C
U
*
A
G
A
A
C
C
Amino acid
attachment site
C U C
G A G
A G *
*
G
A G G
Hydrogen
bonds
Anticodon
Figure 17.14a
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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 (rRNA)
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
Exit tunnel
Growing
polypeptide
tRNA
molecules
Large
subunit
E
P A
Small
subunit
5
mRNA
3
(a)
Figure 17.16a
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The ribosome has three binding sites for tRNA
• The P site
P site (Peptidyl-tRNA
binding site)
• The A site
A site (AminoacyltRNA binding site)
E site
(Exit site)
Large
subunit
E
P
A
• The E site
mRNA
binding site
Figure 17.16b
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
5
3
Small
ribosomal
subunit
3
Translation initiation complex
2
Figure 17.17
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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
1
TRANSCRIPTION
Amino end
of polypeptide
DNA
mRNA
Ribosome
Codon recognition. The anticodon
of an incoming tRNA
base-pairs with the complementary
mRNA codon in the A site.
TRANSLATION
Polypeptide
mRNA
Ribosome ready for
next aminoacyl tRNA
E
3
P A
site site
5
2 GTP
2 GDP
E
E
P
P
A
2
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.
A
GTP
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Peptide bond formation. This step
attaches the polypeptide to the
tRNA in the A site.
E
P
A
Termination of Translation
• The final stage of translation is termination when
the ribosome reaches a
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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Completing and Targeting the Functional Protein
• Polypeptide chains
– Undergo modifications after the translation
process
• Proteins may be modified in ways that affect
their three-dimensional shape
• Other molecules may be attached to
proteins, such as sugars, lipids, phosphates
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Targeting Polypeptides to Specific Locations
• Two populations of ribosomes are evident in
cells
– Free in the cytosol, and bound to ER
• Free ribosomes in the cytosol
–
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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
2
1 Polypeptide
An SRP binds
synthesis begins
to the signal
on a free
peptide.
ribosome in
the cytosol.
3
The SRP binds to a
receptor protein in the ER
membrane.
4
5 A signalcleaving
The polypeptide begins enzyme
translocating across the cuts off the
membrane.
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
ERLUMEN
Translocation
complex
Figure 17.21
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Signal
peptide
removed
ER
membrane
Protein
What is a gene?
• A gene
– Is a region of DNA whose final product is either
a
or an
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
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