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The translation of mRNA to protein can be examined in more detail

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1 The translation of mRNA to protein can be examined in more detail
Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look The translation of mRNA to protein can be examined in more detail Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

2 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 carries a specific amino acid on one end Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA BioFlix: Protein Synthesis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

3 Amino acids tRNA with amino acid attached Ribosome tRNA Anticodon 5
Fig Amino acids Polypeptide tRNA with amino acid attached Ribosome Trp Phe Gly Figure Translation: the basic concept tRNA Anticodon 5 Codons 3 mRNA

4 The Structure and Function of Transfer RNA
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf A C C For the Cell Biology Video A Stick and Ribbon Rendering of a tRNA, go to Animation and Video Files. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

5 Figure 17.14 The structure of transfer RNA (tRNA)
3 Amino acid attachment site 5 Hydrogen bonds Anticodon (a) Two-dimensional structure 5 Amino acid attachment site 3 Figure The structure of transfer RNA (tRNA) Hydrogen bonds 3 5 Anticodon Anticodon (c) Symbol used in this book (b) Three-dimensional structure

6 (a) Two-dimensional structure
Fig a 3 Amino acid attachment site 5 Hydrogen bonds Figure The structure of transfer RNA (tRNA) Anticodon (a) Two-dimensional structure

7 (b) Three-dimensional structure
Fig b Amino acid attachment site 5 3 Hydrogen bonds Figure The structure of transfer RNA (tRNA) 3 5 Anticodon Anticodon (c) Symbol used in this book (b) Three-dimensional structure

8 tRNA is roughly L-shaped
Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule tRNA is roughly L-shaped Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

9 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

10 Aminoacyl-tRNA Amino acid synthetase (enzyme) Fig. 17-15-1
P P P Adenosine ATP Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA

11 Aminoacyl-tRNA Amino acid synthetase (enzyme) Fig. 17-15-2
P P P Adenosine ATP P Adenosine P P i P i P i Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA

12 Aminoacyl-tRNA Amino acid synthetase (enzyme) tRNA Aminoacyl-tRNA
Fig Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P Adenosine tRNA P P i Aminoacyl-tRNA synthetase P i P i tRNA Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA P Adenosine AMP Computer model

13 Aminoacyl-tRNA Amino acid synthetase (enzyme) tRNA Aminoacyl-tRNA
Fig Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P Adenosine tRNA P P i Aminoacyl-tRNA synthetase P i P i tRNA Figure An aminoacyl-tRNA synthetase joining a specific amino acid to a tRNA P Adenosine AMP Computer model Aminoacyl-tRNA (“charged tRNA”)

14 Ribosomes Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

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

16 (a) Computer model of functioning ribosome
Fig a Growing polypeptide Exit tunnel tRNA molecules Large subunit E P A Small subunit Figure The anatomy of a functioning ribosome 5 3 mRNA (a) Computer model of functioning ribosome

17 (b) Schematic model showing binding sites
Fig b P site (Peptidyl-tRNA binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) E P A Large subunit mRNA binding site Small subunit (b) Schematic model showing binding sites Growing polypeptide Amino end Next amino acid to be added to polypeptide chain Figure The anatomy of a functioning ribosome E tRNA mRNA 3 Codons 5 (c) Schematic model with mRNA and tRNA

18 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

19 Building a Polypeptide
The three stages of translation: Initiation Elongation Termination All three stages require protein “factors” that aid in the translation process Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

20 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 First, 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) Proteins called initiation factors bring in the large subunit that completes the translation initiation complex Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

21 Translation initiation complex
Fig Large ribosomal subunit 3 U C 5 A P site Met 5 A Met U G 3 Initiator tRNA GTP GDP E A mRNA 5 5 3 3 Start codon Figure The initiation of translation Small ribosomal subunit mRNA binding site Translation initiation complex

22 Elongation of the Polypeptide Chain
During the elongation stage, amino acids are added one by one to the preceding amino acid Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

23 Amino end of polypeptide E 3 mRNA 5 Fig. 17-18-1 P A site site
Figure The elongation cycle of translation

24 GDP Amino end of polypeptide E 3 mRNA 5 E P A Fig. 17-18-2 P A site
GTP GDP E P A Figure The elongation cycle of translation

25 GDP Amino end of polypeptide E 3 mRNA 5 E P A E P A Fig. 17-18-3 P A
site A site 5 GTP GDP E P A Figure The elongation cycle of translation E P A

26 GDP GDP Amino end of polypeptide E 3 mRNA Ribosome ready for
Fig Amino end of polypeptide E 3 mRNA Ribosome ready for next aminoacyl tRNA P site A site 5 GTP GDP E E P A P A Figure The elongation cycle of translation GDP GTP E P A

27 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 Animation: Translation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

28 Release factor 3 5 Stop codon (UAG, UAA, or UGA) Fig. 17-19-1
Figure The termination of translation

29 Release factor Free polypeptide 3 3 2 5 5 Stop codon
Fig Release factor Free polypeptide 3 3 2 5 5 GTP Stop codon (UAG, UAA, or UGA) 2 GDP Figure The termination of translation

30 Release factor Free polypeptide 5 3 3 3 2 5 5 Stop codon
Fig Release factor Free polypeptide 5 3 3 3 2 5 5 GTP Stop codon (UAG, UAA, or UGA) 2 GDP Figure The termination of translation

31 Polyribosomes A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) Polyribosomes enable a cell to make many copies of a polypeptide very quickly Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

32 Completed polypeptide Growing polypeptides Incoming ribosomal subunits
Fig Completed polypeptide Growing polypeptides Incoming ribosomal subunits Polyribosome Start of mRNA (5 end) End of mRNA (3 end) (a) Ribosomes Figure Polyribosomes mRNA (b) 0.1 µm

33 Completing and Targeting the Functional Protein
Often translation is not sufficient to make a functional protein Polypeptide chains are modified after translation Completed proteins are targeted to specific sites in the cell Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

34 Protein Folding and Post-Translational Modifications
During and after synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape Proteins may also require post-translational modifications before doing their job Some polypeptides are activated by enzymes that cleave them Other polypeptides come together to form the subunits of a protein Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

35 Targeting Polypeptides to Specific Locations
Two populations of ribosomes are evident in cells: free ribsomes (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 Ribosomes are identical and can switch from free to bound Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

36 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 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

37 A signal-recognition particle (SRP) binds to the signal peptide
The SRP brings the signal peptide and its ribosome to the ER Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

38 Ribosome mRNA Signal peptide ER membrane Signal peptide removed
Fig Ribosome mRNA Signal peptide ER membrane Signal peptide removed Signal- recognition particle (SRP) Protein CYTOSOL Translocation complex Figure The signal mechanism for targeting proteins to the ER ER LUMEN SRP receptor protein

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