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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 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 Fig. 17-13 Amino acids Polypeptide tRNA with amino acid attached Ribosome tRNA Anticodon Codons 5 mRNA 3 The Structure and Function of Transfer RNA • A tRNA molecule consists of a single RNA A C strand that is only about 80 nucleotides long C • Flattened into one plane to reveal its base pairing, a tRNA molecule looks like a cloverleaf Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 17-14 3 Amino acid attachment site 5 Hydrogen bonds Anticodon (a) Two-dimensional structure Amino acid attachment site 5 3 Hydrogen bonds 3 Anticodon (b) Three-dimensional structure 5 Anticodon (c) Symbol used in this book Fig. 17-14a 3 Amino acid attachment site 5 Hydrogen bonds Anticodon (a) Two-dimensional structure Fig. 17-14b Amino acid attachment site 5 3 Hydrogen bonds 3 Anticodon (b) Three-dimensional structure 5 Anticodon (c) Symbol used in this book • 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 • Accurate translation requires two steps: – First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyltRNA 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 Fig. 17-15-1 Amino acid P P P ATP Adenosine Aminoacyl-tRNA synthetase (enzyme) Fig. 17-15-2 Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P P Pi Pi Pi Adenosine Fig. 17-15-3 Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P P Pi Pi Pi Adenosine tRNA Aminoacyl-tRNA synthetase tRNA P Adenosine AMP Computer model Fig. 17-15-4 Aminoacyl-tRNA synthetase (enzyme) Amino acid P P P Adenosine ATP P P Pi Pi Adenosine tRNA Aminoacyl-tRNA synthetase Pi tRNA P Adenosine AMP Computer model Aminoacyl-tRNA (“charged tRNA”) 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 Fig. 17-16 Growing polypeptide Exit tunnel tRNA molecules EP Large subunit A Small subunit 5 mRNA 3 (a) Computer model of functioning ribosome P site (Peptidyl-tRNA binding site) E site (Exit site) A site (AminoacyltRNA binding site) E P A mRNA binding site Large subunit Small subunit (b) Schematic model showing binding sites Growing polypeptide Amino end Next amino acid to be added to polypeptide chain E mRNA 5 tRNA 3 Codons (c) Schematic model with mRNA and tRNA Fig. 17-16a Growing polypeptide Exit tunnel tRNA molecules Large subunit E PA Small subunit 5 mRNA 3 (a) Computer model of functioning ribosome Fig. 17-16b P site (Peptidyl-tRNA binding site) E site (Exit site) A site (AminoacyltRNA binding site) E P A mRNA binding site Large subunit Small subunit (b) Schematic model showing binding sites Growing polypeptide Amino end Next amino acid to be added to polypeptide chain E tRNA 3 mRNA 5 Codons (c) Schematic model with mRNA and tRNA • 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 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 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 Fig. 17-17 3 U A C 5 5 A U G 3 Initiator tRNA Large ribosomal subunit P site GTP GDP E mRNA 5 Start codon mRNA binding site 3 Small ribosomal subunit 5 A 3 Translation initiation complex 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 Fig. 17-18-1 Amino end of polypeptide E 3 mRNA 5 P A site site Fig. 17-18-2 Amino end of polypeptide E 3 mRNA 5 P A site site GTP GDP E P A Fig. 17-18-3 Amino end of polypeptide E 3 mRNA 5 P A site site GTP GDP E P A E P A Fig. 17-18-4 Amino end of polypeptide E 3 mRNA Ribosome ready for next aminoacyl tRNA P A site site 5 GTP GDP E E P A P A GDP GTP E P A 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 Fig. 17-19-1 Release factor 3 5 Stop codon (UAG, UAA, or UGA) Fig. 17-19-2 Release factor Free polypeptide 3 5 5 Stop codon (UAG, UAA, or UGA) 3 2 GTP 2 GDP Fig. 17-19-3 Release factor Free polypeptide 5 3 5 5 Stop codon (UAG, UAA, or UGA) 3 2 GTP 2 GDP 3 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 Fig. 17-20 Growing polypeptides Completed polypeptide Incoming ribosomal subunits Start of mRNA (5 end) (a) End of mRNA (3 end) Ribosomes mRNA (b) 0.1 µm 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 Protein Folding and Post-Translational Modifications • During and after synthesis, a polypeptide chain spontaneously coils and folds into its threedimensional 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 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 • 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 • 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 Fig. 17-21 Ribosome mRNA Signal peptide Signal peptide removed Signalrecognition particle (SRP) CYTOSOL ER LUMEN Translocation complex SRP receptor protein ER membrane Protein