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Central Dogma – part 2 DNA RNA PROTEIN Translation Central Dogma
of molecular biology
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The Genetic Code—Essential Questions
How are the instructions for assembling amino acids into proteins encoded into DNA? There are 20 amino acids, but there are only four nucleotide bases in DNA How many nucleotides correspond to an amino acid? 2 2
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Codons: Triplets of Nucleotides
The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA These words are then translated into a chain of amino acids, forming a polypeptide 3 3
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During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction
Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide 4 4
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Cracking the Code All 64 codons were deciphered by the mid-1960s
Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation The genetic code is redundant: more than one codon may specify a particular amino acid But it is not ambiguous: no codon specifies more than one amino acid 5 5
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Codons are read one at a time in a nonoverlapping fashion
Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced Codons are read one at a time in a nonoverlapping fashion 6 6
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Concept 14.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look
Genetic information flows from mRNA to protein through the process of translation 7 7
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Molecular Components of Translation
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) tRNAs transfer amino acids to the growing polypeptide in a ribosome Translation is a complex process in terms of its biochemistry and mechanics 8 8
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Amino acids Polypeptide tRNA with amino acid attached Ribosome tRNA
Figure 14.14 Amino acids Polypeptide tRNA with amino acid attached Ribosome Trp Phe Gly Figure Translation: the basic concept tRNA C C C C Anticodon A G A A A U G G U U U G G C 5 Codons 3 mRNA 9
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The Structure and Function of Transfer RNA
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long tRNA molecules can base-pair with themselves Flattened into one plane, a tRNA molecule looks like a cloverleaf In three dimensions, tRNA is roughly L-shaped, where one end of the L contains the anticodon that base-pairs with an mRNA codon Video: tRNA Model 10 10
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The Structure and Function of Transfer RNA
Each tRNA can translate a particular mRNA codon into a given amino acid The tRNA contains an amino acid at one end and at the other end has a nucleotide triplet that can base-pair with the complementary codon on mRNA 11 11
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(b) Three-dimensional structure (c) Symbol used in this book
Figure 14.15 3 Amino acid attachment site A C C A 5 Amino acid attachment site C G G C 5 C G U G 3 U A A U U A U * C C A G G A C U A * A C G * C U * G U G U Hydrogen bonds C * C G A G G * * C A G U * G * G A G C Hydrogen bonds G C U A Figure The structure of transfer RNA (tRNA) * G * A A C A A G * U 3 5 A G A Anticodon Anticodon Anticodon (b) Three-dimensional structure (c) Symbol used in this book (a) Two-dimensional structure 12
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Accurate translation requires two steps
First: a correct match between a tRNA and an amino acid 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 This allows mRNA to be translated with fewer than the 64 tRNAs that would be required without wobble 13 13
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Ribosomes Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons during protein synthesis The large and small ribosomal are made of proteins and ribosomal RNAs (rRNAs) In bacterial and eukaryotic ribosomes the large and small subunits join to form a ribosome only when attached to an mRNA molecule 14 14
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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 15 15
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(b) Schematic model showing binding sites
Figure 14.17b P site (Peptidyl-tRNA binding site) Exit tunnel A site (Aminoacyl- tRNA binding site) E site (Exit site) E P A Large subunit Figure 14.17b The anatomy of a functioning ribosome (part 2: binding sites) mRNA binding site Small subunit (b) Schematic model showing binding sites 16
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(c) Schematic model with mRNA and tRNA
Figure 14.17c Growing polypeptide Amino end Next amino acid to be added to polypeptide chain E tRNA mRNA 3 Figure 14.17c The anatomy of a functioning ribosome (part 3: mRNA and tRNA) Codons 5 (c) Schematic model with mRNA and tRNA 17
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Building a Polypeptide
The three stages of translation Initiation Elongation Termination All three stages require protein “factors” that aid in the translation process 18 18
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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 Then the small subunit moves along the mRNA until it reaches the start codon (AUG) The addition of the large ribosomal subunit is last and completes the formation of the translation initiation complex Proteins called initiation factors bring all these components together 19 19
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Translation initiation complex
Figure 14.18 Large ribosomal subunit 3 U C 5 A P site Met 5 A 3 Met U G P i 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 1 Small ribosomal subunit binds to mRNA. 2 Large ribosomal subunit completes the initiation complex. 20
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Elongation of the Polypeptide Chain
During elongation, amino acids are added one by one to the previous amino acid chain Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation Translation proceeds along the mRNA in a 5 to 3 direction 21 21
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Amino end of polypeptide Codon recognition 1 E E P A 3 mRNA 5 GTP
Figure Amino end of polypeptide 1 Codon recognition E 3 mRNA P site A site 5 GTP GDP P i E P A Figure The elongation cycle of translation (step 1) 22
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Amino end of polypeptide Codon recognition Peptide bond formation 1 E
Figure Amino end of polypeptide 1 Codon recognition E 3 mRNA P site A site 5 GTP GDP P i E P A Figure The elongation cycle of translation (step 2) 2 Peptide bond formation E P A 23
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Amino end of polypeptide Codon recognition Ribosome ready for
Figure Amino end of polypeptide 1 Codon recognition E 3 mRNA Ribosome ready for next aminoacyl tRNA P site A site 5 GTP GDP P i E E P A P A Figure The elongation cycle of translation (step 3) GDP P i 2 Peptide bond formation 3 Translocation GTP E P A 24
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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 25 25
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Release factor Stop codon (UAG, UAA, or UGA) Ribosome reaches a
Figure Release factor 3 5 Stop codon (UAG, UAA, or UGA) 1 Ribosome reaches a stop codon on mRNA. Figure The termination of translation (step 1) 26
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Release factor Free polypeptide Stop codon (UAG, UAA, or UGA)
Figure Release factor Free polypeptide 3 3 5 5 Stop codon (UAG, UAA, or UGA) 1 Ribosome reaches a stop codon on mRNA. 2 Release factor promotes hydrolysis. Figure The termination of translation (step 2) 27
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Release factor Free polypeptide Stop codon (UAG, UAA, or UGA)
Figure Release factor Free polypeptide 5 3 3 3 5 5 2 GTP Stop codon (UAG, UAA, or UGA) 2 GDP P i 1 Ribosome reaches a stop codon on mRNA. 2 Release factor promotes hydrolysis. 3 Ribosomal subunits and other components dissociate. Figure The termination of translation (step 3) 28
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Completing and Targeting the Functional Protein
Often translation is not sufficient to make a functional protein Polypeptide chains are modified after translation or targeted to specific sites in the cell 29 29
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Protein Folding and Post-Translational Modifications
During synthesis, a polypeptide chain spontaneously coils and folds into its three-dimensional shape Proteins may also require post-translational modifications before doing their jobs 30 30
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Polypeptides destined for secretion are marked by a signal peptide
A signal-recognition particle (SRP) binds to the signal peptide The SRP brings the signal peptide and its ribosome to the Endoplasmic Reticulum 31 31
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So Many Things Are Happening In your cells! Figure 14.24 DNA
TRANSCRIPTION 3 Poly-A RNA polymerase 5 RNA transcript Exon RNA PROCESSING RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase So Many Things Are Happening In your cells! Poly-A NUCLEUS Amino acid AMINO ACID ACTIVATION CYTOPLASM tRNA mRNA 5 Cap 3 A Aminoacyl (charged) tRNA P Poly-A E Ribosomal subunits Figure A summary of transcription and translation in a eukaryotic cell 5 Cap TRANSLATION C A C U A E A C Anticodon A A A U G G U U U A U G Codon Ribosome 32
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