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Transfer RNA
• Consists of a single RNA strand that is only about 80
nucleotides long
• Each carries a specific amino acid on one end and has an
anticodon on the other end
• A special group of enzymes pairs up the proper tRNA molecules
with their corresponding amino acids.
• tRNA brings the amino acids to the ribosomes
The “anticodon” is the 3 RNA bases that
matches the 3 bases of the codon on the
mRNA molecule
Two-dimensional structure. The four base-paired regions and
(a) 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.)
3
A
C
C
A
C
G
C
U
U
A
A
U C
C A C AG
*
G
GU GU *
C
* *
U C
* G AG
G
U
Amino acid
attachment site
*
A*
*
A
5
G
C
G
G
A
U
U
UA
A G *
* CUC
*
G
C G A G
A G G
*
C
Hydrogen
C
A
bonds
G
A
C
U
G
A
Anticodon
Transfer RNA
• 3 dimensional tRNA molecule is roughly “L” shaped
5
3
Amino acid
attachment site
Hydrogen
bonds
A AG
3
Anticodon
(b) Three-dimensional structure
Anticodon
5
Ribosomes
• Ribosomes facilitate the specific coupling of tRNA anticodons
with mRNA codons during protein synthesis
• The 2 ribosomal subunits are constructed of proteins and RNA
molecules named ribosomal RNA or rRNA
DNA
TRANSCRIPTION
mRNA
Ribosome
TRANSLATION
Polypeptide
Growing
polypeptide
Exit tunnel
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.
Ribosome
• The ribosome has three binding sites for tRNA
– The P site
– The A site
– The E site
P site (Peptidyl-tRNA
binding site)
A site (AminoacyltRNA binding site)
E site
(Exit site)
Large
subunit
E
mRNA
binding site
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.
Building a Polypeptide
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 basepairs 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.
Building a Polypeptide
• We can divide translation into three stages
– Initiation
– Elongation
– Termination
• The AUG start codon is recognized by methionyl-tRNA
or Met
• Once the start codon has been identified, the ribosome
incorporates amino acids into a polypeptide chain
• RNA is decoded by tRNA (transfer RNA) molecules,
which each transport specific amino acids to the
growing chain
• Translation ends when a stop codon (UAA, UAG, UGA)
is reached
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
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).
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, amino acids are added one
by one to the preceding amino acid
TRANSCRIPTION
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.
Amino end
of
polypeptide
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
E
mRNA
Ribosome ready for
next aminoacyl tRNA
5
3
P A
site site
2 GTP
2 GDP
E
E
P
P
A
GDP
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
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 step in translation is termination. When the ribosome reaches
a STOP codon, there is no corresponding transfer RNA.
Instead, a small protein called a “release factor” attaches to the stop
codon.
The release factor causes the whole complex to fall apart: messenger
RNA, the two ribosome subunits, the new polypeptide.
The messenger RNA can be translated many times, to produce many
protein copies.
•
•
•
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.
Comparing Gene Expression In Prokaryotes And Eukaryotes
•
In a eukaryotic cell:
–
–
•
The nuclear envelope separates transcription from translation
Extensive RNA processing occurs in the nucleus
Prokaryotic cells lack a nuclear envelope, allowing translation to
begin while transcription progresses
RNA polymerase
DNA
mRNA
Polyribosome
RNA
polymerase
Direction of
transcription
0.25 mm
DNA
Polyribosome
Polypeptide
(amino end)
Ribosome
mRNA (5 end)
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
Exon
RNA PROCESSING
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
FORMATION OF
INITIATION COMPLEX
CYTOPLASM
AMINO ACID ACTIVATION
tRNA
3 After leaving the
4
Each amino acid
attaches to its proper tRNA
with the help of a specific
enzyme and ATP.
nucleus, mRNA attaches
to the ribosome.
mRNA
Growing
polypeptide
Activated
amino acid
Ribosomal
subunits
5
TRANSLATION
5
E
A
A A A
UG GUU U A U G
Codon
Figure 17.26
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.)