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Chapter 17: From Gene to Protein


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17.1 Genes specify proteins via transcription and translation
17.2 Transcription is the DNA- directed synthesis of RNA:
A closer look
17.3 Eukaryotic cells modify RNA after transcription
17.4 Translation is the RNA-directed synthesis of a polypeptide:
A closer look
17.5 Mutations of one or a few nucleotides can affect protein structure and function

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Chapter 17: From Gene to Protein

  1. 1. Chapter 17 From Gene to Protein Transcription and Translation
  2. 2. Overview: The Flow of Genetic Information • The information content of DNA is in the form of specific sequences of nucleotides • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation • The ribosome is part of the cellular machinery for translation, polypeptide synthesis
  3. 3. Evidence from the Study of Metabolic Defects • In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions • He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme • Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway
  4. 4. Nutritional Mutants in Neurospora: Scientific Inquiry • Beadle and Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal medium as a result of inability to synthesize certain molecules • Using crosses, they identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine • They developed a “one gene–one enzyme” hypothesis, which states that each gene dictates production of a specific enzyme
  5. 5. Figure 17.2 Precursor Enzyme A Enzyme B Enzyme C Ornithine Citrulline Arginine No growth: Mutant cells cannot grow and divide Growth: Wild-type cells growing and dividing Control: Minimal medium Results Table Wild type Minimal medium (MM) (control) MM + ornithine MM + citrulline MM + arginine (control) Summary of results Can grow with or without any supplements Gene (codes for enzyme) Wild type Precursor Ornithine Gene A Gene B Gene C Enzyme A Enzyme B Enzyme C Enzyme A Enzyme B Enzyme C Citrulline Arginine Precursor Ornithine Citrulline Arginine Precursor Ornithine Citrulline Arginine Precursor Ornithine Citrulline Arginine Enzyme A Enzyme B Enzyme C Enzyme A Enzyme B Enzyme C Class I mutants (mutation in gene A) Class II mutants (mutation in gene B) Class III mutants (mutation in gene C) Can grow on ornithine, citrulline, or arginine Can grow only on citrulline or arginine Require arginine to grow Class I mutants Class II mutants Class III mutants Classes of Neurospora crassa Condition
  6. 6. The Products of Gene Expression: A Developing Story • Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein • Many proteins are composed of several polypeptides, each of which has its own gene • Therefore, Beadle and Tatum’s hypothesis is now restated as one gene–one polypeptide
  7. 7. Basic Principles of Transcription and Translation • Transcription is the synthesis of RNA under the direction of DNA • Transcription produces messenger RNA (mRNA) • Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA • Ribosomes are the sites of translation
  8. 8. • In prokaryotes, mRNA produced by transcription is immediately translated without more processing--coupled • In a eukaryotic cell, the nuclear envelope separates transcription from translation • Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA • Cells are governed by a cellular chain of command: DNA → RNA → protein
  9. 9. Figure 17.3 Nuclear envelope CYTOPLASM DNA Pre-mRNA mRNA RibosomeTRANSLATION (b) Eukaryotic cell NUCLEUS RNA PROCESSING TRANSCRIPTION (a) Bacterial cell Polypeptide DNA mRNA Ribosome CYTOPLASM TRANSCRIPTION TRANSLATION Polypeptide
  10. 10. The Genetic Code • 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 • So how many bases correspond to an amino acid?
  11. 11. Codons: Triplets of Bases • The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words • These triplets are the smallest units of uniform length that can code for all the amino acids • Example: AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced
  12. 12. • During transcription, a DNA strand called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript • During translation, the mRNA base triplets, called codons, are read in the 5′ to 3′ direction • Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide
  13. 13. Figure 17.4 A C C A A A C C G A G T ACTTTT CGGGGT U G G U U U G G C CU A SerGlyPheTrp Codon TRANSLATION TRANSCRIPTION Protein mRNA 5′ 5′ 3′ Amino acid DNA template strand 5′ 3′ 3′
  14. 14. Cracking the Code • All 64 codons were deciphered by the mid-1960s • The genetic code is redundant but not ambiguous; no codon specifies more than one amino acid • Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced
  15. 15. Figure 17.5 Second mRNA base ThirdmRNAbase(3′endofcodon) FirstmRNAbase(5′endofcodon) UUU UUC UUA UUG Phe Leu Leu Ile Val CUU CUC CUA CUG AUU AUC AUA AUG GUU GUC GUA GUG UCU UCC UCA UCG CCU CCC CCA CCG ACU ACC ACA ACG GCU GCC GCA GCG GAG GAA GAC GAU AAG AAA AAC AAU CAG CAA CAC CAU Ser Pro Thr Ala Glu Asp Lys Asn Gln His Tyr Cys Trp Arg Ser Arg Gly GGG GGA GGC GGU AGG AGA AGC AGU CGG CGA CGC CGU UGG UGA UGC UGUUAU UAC UAA UAG Stop Stop Stop Met or start U C A G U C A G U C A G U C A G U C A G G A C U
  16. 16. Evolution of the Genetic Code • The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals • Genes can be transcribed and translated after being transplanted from one species to another Pig expressing a jellyfish gene (b)Tobacco plant expressing a firefly gene (a)
  17. 17. Molecular Components of Transcription • RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides • RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine • The DNA sequence where RNA polymerase attaches is called the promoter; in prokaryotes, the sequence signaling the end of transcription is called the terminator • The stretch of DNA that is transcribed is called a transcription unit
  18. 18. Synthesis of an RNA Transcript • The three stages of transcription: – Initiation – Elongation – Termination
  19. 19. Figure 17.7-3 Promoter Transcription unit RNA polymerase Start point 1 Template strand of DNARNA transcript Unwound DNA Rewound DNA RNA transcript Direction of transcription (“downstream”) Completed RNA transcript Initiation Elongation Termination 2 3 5′ 3′ 5′ 5′ 3′ 5′ 3′ 5′ 3′ 5′ 5′ 3′ 3′ 5′ 3′ 3′ 5′ 3′ 5′ 3′
  20. 20. Figure 17.9 Nontemplate strand of DNA 5′ 3′ 3′ end 5′ 3′ 5′ Direction of transcription RNA polymerase Template strand of DNA Newly made RNA RNA nucleotides A A A A A AA A A C C G G T T T C C CU U C CT T C A T T G G U
  21. 21. RNA Polymerase Binding and Initiation of Transcription • Promoters signal the initiation of RNA synthesis • Transcription factors mediate the binding of RNA polymerase and initiation of transcription in eukaryotes • The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex • A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes
  22. 22. Promoter Nontemplate strand 15′ 3′ 5′ 3′ Start point RNA polymerase II Template strand TATA box Transcription factors DNA 3′ 5′ 3′ 5′ 3′ 5′ 2 3 Transcription factors RNA transcript Transcription initiation complex 3′5′ 5′ 3′ A eukaryotic promoter Several transcription factors bind to DNA. Transcription initiation complex forms. T A T AAAA TA AT T T T
  23. 23. Elongation of the RNA Strand • As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time • Transcription progresses at a rate of 40 nucleotides per second in eukaryotes • A gene can be transcribed simultaneously by several RNA polymerases
  24. 24. Termination of Transcription • The mechanisms of termination are different in prokaryotes and eukaryotes • In prokaryotes, the polymerase stops transcription at the end of the terminator • In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence
  25. 25. Concept 17.3: Eukaryotic cells modify RNA after transcription • Enzymes in the eukaryotic nucleus modify pre- mRNA (RNA processing) before the messages are dispatched to the cytoplasm • During RNA processing, both ends of the primary transcript are usually altered (5’ cap and 3’ polyA tail addition) • Also, usually some interior parts of the molecule are cut out, and the other parts spliced together (RNA splicing)
  26. 26. Figure 17.10 A modified guanine nucleotide added to the 5′ end Region that includes protein-coding segments 5′ 5′ Cap 5′ UTR Start codon Stop codon G P P P 3′ UTR 3′ AAUAAA AAA AAA Poly-A tail Polyadenylation signal 50–250 adenine nucleotides added to the 3′ end … These modifications share several functions: • They seem to facilitate the export of mRNA • They protect mRNA from hydrolytic enzymes
  27. 27. Split Genes and RNA Splicing • Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions • These noncoding regions are called intervening sequences, or introns • The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences • RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
  28. 28. Figure 17.11 Pre-mRNA Intron Intron Introns cut out and exons spliced together Poly-A tail5′ Cap 5′ Cap Poly-A tail 1–30 31–104 105–146 1–146 3′ UTR5′ UTR Coding segment mRNA • In some cases, RNA splicing is carried out by spliceosomes • Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites
  29. 29. Figure 17.12 Spliceosome Small RNAs Exon 2 Cut-out intron Spliceosome components mRNA Exon 1 Exon 2 Pre-mRNA Exon 1 Intron 5′ 5′
  30. 30. Ribozymes • Ribozymes are catalytic RNA molecules • Here they function as enzymes that can splice RNA • The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins
  31. 31. • Three properties of RNA enable it to function as an enzyme – It can form a three-dimensional structure because of its ability to base-pair with itself – Some bases in RNA contain functional groups that may participate in catalysis – RNA may hydrogen-bond with other nucleic acid molecules
  32. 32. The Functional and Evolutionary Importance of Introns • Some introns contain sequences that may regulate gene expression • Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during splicing • This is called alternative RNA splicing • Consequently, the number of different proteins an organism can produce is much greater than its number of genes
  33. 33. • Proteins often have a modular architecture consisting of discrete regions called domains • In many cases, different exons code for the different domains in a protein
  34. 34. Figure 17.13 Gene DNA Transcription RNA processing Translation Domain 3 Exon 1 Exon 3Exon 2 IntronIntron Domain 2 Domain 1 Polypeptide
  35. 35. Molecular Components of Translation • A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) & ribosomes • tRNAs transfer amino acids to the growing polypeptide in a ribosome • 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
  36. 36. Figure 17.14 Polypeptide Amino acids tRNA with amino acid attached Ribosome tRNA Anticodon Codons mRNA 5′ U U U UG G G G C A C C A A A C C G Phe Trp 3′ Gly
  37. 37. 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 • Amino acid at one end of sock and anticodon at the other • Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule • tRNA is roughly L-shaped A C C
  38. 38. Figure 17.15 Amino acid attachment site Amino acid attachment site 5′ 3′ A C C A C G C U U A G G C G A U U U A A GAA CC CU * ** C G G U UG C * * * *C CU A G G G G A GAGC C C *U * G A G G U * * * A A A G C U G A A Hydrogen bonds Hydrogen bonds Anticodon Anticodon Symbol used in this book Three-dimensional structure (a) Two-dimensional structure (b) (c) Anticodon 5′3′ A A G 3′ 5′
  39. 39. • Accurate translation requires two steps: – First step: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase – Second step: 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
  40. 40. Figure 17.16-3 Aminoacyl-tRNA synthetase 1 Amino acid and tRNA enter active site. Tyr-tRNA Tyrosine (Tyr) (amino acid) Tyrosyl-tRNA synthetase Complementary tRNA anticodon A AU tRNA Amino acid Using ATP, synthetase catalyzes covalent bonding. 23 ATP AMP + 2 P i Aminoacyl tRNA released. Computer model
  41. 41. 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)
  42. 42. Figure 17.17 Exit tunnel Large subunit Small subunit mRNA 3′ 5′ E P A tRNA molecules Growing polypeptide (a) Computer model of functioning ribosome Growing polypeptide Next amino acid to be added to polypeptide chain tRNA 3′ 5′ mRNA Amino end Codons E (c) Schematic model with mRNA and tRNA(b) Schematic model showing binding sites Small subunit Large subunit Exit tunnel A site (Aminoacyl- tRNA binding site) P site (Peptidyl-tRNA binding site) E site (Exit site) mRNA binding site E P A
  43. 43. • 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 uncharged tRNAs leave the ribosome
  44. 44. Building a Polypeptide • The three stages of translation: –Initiation –Elongation –Termination –All 3 stages require protein factors and some require energy in the form of GTP.
  45. 45. Ribosome Association and Initiation of Translation • Initiation 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
  46. 46. Figure 17.18 1 2 P site 3′ Large ribosomal subunit Translation initiation complex Large ribosomal subunit completes the initiation complex. Small ribosomal subunit binds to mRNA. mRNA binding site Small ribosomal subunit Initiator tRNA Start codon 3′5′ E A 3′ 5′ GTP GDP P i + 3′ 5′ 5′ U A C GA UMet Met mRNA
  47. 47. Elongation of the Polypeptide Chain • During elongation, amino acids are added one by one to the C-terminus of the growing chain • Each addition involves proteins called elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation • Energy expenditure occurs in the first (docking in A site) and third steps(translocation) • Translation proceeds along the mRNA in a 5 → 3 direction′ ′
  48. 48. Amino end of polypeptide Codon recognition 1 3′ 5′ E P A sitesite E P A mRNA GTP P iGDP +
  49. 49. Amino end of polypeptide Codon recognition 1 3′ 5′ E P A sitesite E P A mRNA GTP P i 2 GDP + E P A Peptide bond formation
  50. 50. Amino end of polypeptide Codon recognition 1 3′ 5′ E P A sitesite E P A mRNA GTP P i 23 GTP P i GDP + GDP + Translocation E P A Peptide bond formation E P A Ribosome ready for next aminoacyl tRNA
  51. 51. 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 comes apart
  52. 52. Figure 17.20-3 31 2 Release factor 3′ 5′5′ 3′ Stop codon (UAG, UAA, or UGA) Ribosome reaches a stop codon on mRNA. Release factor promotes hydrolysis. Ribosomal subunits and other components dissociate. Free polypeptide 3′ 5′ 2 GTP 2 GDP + 2 P i
  53. 53. Polyribosomes • A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome • Polyribosomes enable a cell to make many copies of a polypeptide very quickly
  54. 54. LE 17-20 Ribosomes mRNA 0.1 µm This micrograph shows a large polyribosome in a prokaryotic cell (TEM). An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes. Incoming ribosomal subunits Growing polypeptides End of mRNA (3′ end) Start of mRNA (5′ end) Polyribosome Completed polypeptides
  55. 55. 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
  56. 56. 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 (rough ER) • Ribosomes are identical and can switch from free to bound
  57. 57. • 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 • A signal-recognition particle (SRP) binds to the signal peptide • The SRP brings the signal peptide and its ribosome to the ER
  58. 58. Figure 17.21 Protein ER membrane Signal peptide removed SRP receptor protein Translocation complex ER LUMEN CYTOSOL SRP Signal peptide mRNA Ribosome Polypeptide synthesis begins. SRP binds to signal peptide. SRP binds to receptor protein. SRP detaches and polypeptide synthesis resumes. Signal- cleaving enzyme cuts off signal peptide. Completed polypeptide folds into final conformation. 1 2 3 4 5 6
  59. 59. • A bacterial cell ensures a streamlined process by coupling transcription and translation • In this case the newly made protein can quickly diffuse to its site of function
  60. 60. RNA polymerase Polyribosome DNA mRNA 0.25 µm Ribosome DNA mRNA (5′ end) Polyribosome Polypeptide (amino end) Direction of transcriptionRNA polymerase
  61. 61. • In eukaryotes, the nuclear envelop separates the processes of transcription and translation • RNA undergoes processes before leaving the nucleus
  62. 62. DNA RNA polymerase RNA transcript (pre-mRNA) Intron Exon Aminoacyl-tRNA synthetase Amino acid tRNA Aminoacyl (charged) tRNA mRNA CYTOPLASM NUCLEUS RNA transcript 3′ 5′ Poly-A 5′ Cap 5′ C ap TRANSCRIPTION RNA PROCESSING Poly-A Poly-A AMINO ACID ACTIVATION TRANSLATION Ribosomal subunits E P A E A A C C U U U U U GGG A A A Anticodon Codon Ribosome C A U 3′ A
  63. 63. Concept 17.5: RNA plays multiple roles in the cell: a review Type of RNA Functions Messenger RNA (mRNA) Carries information specifying amino acid sequences of proteins from DNA to ribosomes Transfer RNA (tRNA) Serves as adapter molecule in protein synthesis; translates mRNA codons into amino acids Ribosomal RNA (rRNA) Plays catalytic (ribozyme) roles and structural roles in ribosomes
  64. 64. Type of RNA Functions Primary transcript Serves as a precursor to mRNA, rRNA, or tRNA, before being processed by splicing or cleavage Small nuclear RNA (snRNA) Plays structural and catalytic roles in spliceosomes SRP RNA Is a component of the the signal- recognition particle (SRP) Video: protein synthesis
  65. 65. Concept 17.7: Point mutations can affect protein structure and function • Mutations are changes in the genetic material of a cell or virus • Point mutations are chemical changes in just one base pair of a gene • The change of a single nucleotide in a DNA template strand leads to production of an abnormal protein
  66. 66. Figure 17.25 Wild-type β-globin Sickle-cell β-globin Mutant β-globin DNAWild-type β-globin DNA mRNA mRNA Normal hemoglobin Sickle-cell hemoglobin Val U GG A CC G GT Glu G GA G GA C CT3′ 5′ 5′ 5′ 5′ 5′ 3′ 3′ 3′ 3′ 3′ 5′
  67. 67. Types of Point Mutations • Point mutations within a gene can be divided into two general categories – Base-pair substitutions – Base-pair insertions or deletions
  68. 68. Substitutions • A base-pair substitution replaces one nucleotide and its partner with another pair of nucleotides • Base-pair substitution can cause missense or nonsense mutations • Missense mutations still code for an amino acid, but not necessarily the right amino acid • Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein • Missense mutations are more common
  69. 69. Wild type Stop 5′ 3′ 3′ Stop 3′ 5′ 5′ Met (b) Nucleotide-pair insertion or deletion Carboxyl endAmino end mRNA Protein 3′ 5′ 5′ DNA template strand (a) Nucleotide-pair substitution A instead of G U instead of C 5′ 3′ 3′ Stop Extra A Extra U Frameshift (1 nucleotide-pair insertion) missing missing Frameshift (1 nucleotide-pair deletion) missing 3′ 5′ 5′ missing 3′ 5′ 3′ Met Lys Leu Ala 3′ 5′ 3′ T A C T T C A A A C C G A T TA TA G T A A A AC TTTTG G G G G C U A AAAA U U UUUGG A U G A A G G G C UUU U A A AATCGGTTT GG A AA T A C T T C A A C C G A T T A Met Lys Phe Gly T A C C C C GA A A A A AAAA A U G G G G G T T T T TTTTT C G G G C UUUUAA A A 3′ 5′ 5′ T A A A A A T T T TA A A A AACCCC T T T T TTGGGG G G G GUUU U A AA A AU Met Lys Phe Gly Stop U Silent 3′ 5′ 5′ 5′ 3′ 3′ Stop Stop Stop 3 nucleotide-pair deletionNonsense A instead of T U instead of A 5′ 5′ 5′ 3′ 3′ 3′ Met Lys Phe Ser Met Met Phe Gly A T A A T A A T A T instead of C A instead of G Missense U G G G G A AU U U U UU A A A A T T TA A A A AGCCCC A T T T T T TCGGGG 3′ 5′ 5′ 5′ 3′ 3′ T T A A AA A A G U U U U UCGGG T T T T T AGCC CGGG C CTT AAAA T T T TT A AA AA C C C GA A A A ATTTT GGGT C A A AAUCGGGU U U U A
  70. 70. Mutagens • Spontaneous mutations can occur during DNA replication, recombination, or repair • Mutagens are physical or chemical agents that can cause mutations
  71. 71. What is a gene? revisiting the question • A gene is a region of DNA whose final product is either a polypeptide or an RNA molecule
  72. 72. LE 17-26 TRANSCRIPTION RNA PROCESSING RNA transcript 5′ Exon NUCLEUS FORMATION OF INITIATION COMPLEX CYTOPLASM 3′ DNA RNA polymerase RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase Amino acid tRNA AMINO ACID ACTIVATION 3′ mRNA A P E Ribosomal subunits 5′ Growing polypeptide E A Activated amino acid Anticodon TRANSLATION Codon Ribosome Review compartments for eukaryotes