Gene Expression: From Gene to Protein

Transcription

1 14 CMPBELL BIOLOGY IN FOCUS URRY CIN WSSERMN MINORSKY REECE Gene Expression: From Gene to Protein Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University SECOND EDITION Overview: The Flow of Genetic Information The information content of genes is in the form of specific sequences of nucleotides in The inherited by an organism leads to specific traits by dictating the synthesis of proteins Proteins are the links between genotype and phenotype Gene expression, the process by which directs protein synthesis, includes two stages: transcription and translation Figure

2 Concept 14.1: Genes specify proteins via transcription and translation How was the fundamental relationship between genes and proteins discovered? Evidence from the Study of Metabolic Defects In 1902, British physician rchibald 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 Cells synthesize and degrade molecules in a series of steps, a metabolic pathway Nutritional Mutants: Scientific Inquiry Beadle and Tatum disabled genes in bread mold one by one and looked for phenotypic changes They studied the haploid bread mold because it would be easier to detect recessive mutations They studied mutations that altered the ability of the fungus to grow on minimal medium 2

3 Figure 14.2 Individual Neurospora cells placed on complete medium. Cells subjected to X-rays. Each surviving cell formed a colony of genetically identical cells. No growth Cells from each colony placed in a vial with minimal medium. Cells that did not grow identified as nutritional mutants. Cells from one nutritional mutant colony placed in a series of vials. Growth Minimal Minimal Minimal medium medium medium + valine + lysine + arginine Vials observed for growth. Control: Wild-type cells growing on minimal medium Figure Individual Neurospora cells placed on complete medium. Cells subjected to X-rays. Each surviving cell formed a colony of genetically identical cells. Figure No growth Cells from each colony placed in a vial with minimal medium. Cells that did not grow identified as nutritional mutants. Cells from one nutritional mutant colony placed in a series of vials. Growth Minimal medium + valine Minimal medium + lysine Minimal medium + arginine Vials observed for growth. Control: Wild-type cells growing on minimal medium 6 3

4 The researchers amassed a valuable collection of Neurospora mutant strains, catalogued by their defects For example, one set of mutants all required arginine for growth It was determined that different classes of these mutants were blocked at a different step in the biochemical pathway for arginine biosynthesis Figure 14.3 Gene Gene B Gene C Enzyme Enzyme B Enzyme C Precursor Ornithine Citrulline rginine The Products of Gene Expression: Developing Story Some proteins are not enzymes, so researchers later revised the one gene one enzyme hypothesis to 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 the one gene one polypeptide hypothesis It is common to refer to gene products as proteins rather than polypeptides 4

5 Basic Principles of Transcription and Translation RN is the bridge between and protein synthesis RN is chemically similar to, but RN has a ribose sugar instead of deoxyribose and the base uracil (U) rather than thymine (T) RN is usually single-stranded Getting from to protein requires two stages: transcription and translation Transcription is the synthesis of RN using information in Transcription produces messenger RN () Translation is the synthesis of a polypeptide, using information in the Ribosomes are the sites of translation In bacteria, translation of can begin before transcription has finished In eukaryotes, the nuclear envelope separates transcription from translation Eukaryotic RN transcripts are modified through RN processing to yield the finished Eukaryotic must be transported out of the nucleus to be translated 5

6 Figure 14.4 Nuclear envelope TRNSCRIPTION RN PROCESSING Pre- TRNSCRIPTION CYTOPLSM Ribosome TRNSLTION Polypeptide CYTOPLSM NUCLEUS TRNSLTION Ribosome Polypeptide (a) Bacterial cell (b) Eukaryotic cell Figure s1 TRNSCRIPTION CYTOPLSM (a) Bacterial cell Figure s2 TRNSCRIPTION CYTOPLSM TRNSLTION Ribosome Polypeptide (a) Bacterial cell 6

7 Figure s1 Nuclear envelope TRNSCRIPTION Pre- NUCLEUS CYTOPLSM (b) Eukaryotic cell Figure s2 Nuclear envelope TRNSCRIPTION RN PROCESSING NUCLEUS Pre- CYTOPLSM (b) Eukaryotic cell Figure s3 Nuclear envelope TRNSCRIPTION RN PROCESSING NUCLEUS Pre- CYTOPLSM TRNSLTION Ribosome Polypeptide (b) Eukaryotic cell 7

8 primary transcript is the initial RN transcript from any gene prior to processing The central dogma is the concept that cells are governed by a cellular chain of command Figure 14.UN01 RN Protein The Genetic Code There are 20 amino acids, but there are only four nucleotide bases in How many nucleotides correspond to an amino acid? 8

9 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 These words are then translated into a chain of amino acids, forming a polypeptide Figure 14.5 molecule Gene 1 Gene 2 Gene 3 template strand TRNSCRIPTION C C C C G G T T G G T T T G G C T C TRNSLTION U G G Codon U U U G G C U C Protein Trp Phe Gly Ser mino acid Figure template strand C C C C G G T T G G T T T G G C T C TRNSCRIPTION TRNSLTION U G G U U U G G C U C Codon Protein Trp Phe Gly Ser mino acid 9

10 During transcription, one of the two strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RN transcript The template strand is always the same strand for any given gene During translation, the base triplets, called codons, are read in the to direction Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide Cracking the Code ll 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 10

11 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 Figure 14.6 U Second base C G First base ( end of codon) UUU U UUC Phe UU Leu UUG CUU C CUC Leu CU CUG UU UC Ile U Met or UG start GUU G GUC GU GUG Val UCU UCC UC UCG CCU CCC CC CCG CU CC C CG GCU GCC GC GCG Ser Pro Thr la UU UC Tyr UGU UGC Cys U C U Stop UG Stop UG Stop UGG Trp G CU CC C CG U C His CGU U CGC C rg CG Gln CGG G GU U sn Ser GC C G Lys rg G GU GG GGU G U sp GC GGC C Gly G GG Glu GG GGG G Third base ( end of codon) Evolution of the Genetic Code The genetic code is nearly universal, shared by the simplest bacteria and the most complex animals Genes can be transcribed and translated after being transplanted from one species to another 11

12 Figure 14.7 (a) Tobacco plant expressing a firefly gene (b) Pig expressing a jellyfish gene Figure (a) Tobacco plant expressing a firefly gene Figure (b) Pig expressing a jellyfish gene 12

13 Concept 14.2: Transcription is the -directed Synthesis of RN: Closer Look Transcription is the first stage of gene expression Molecular Components of Transcription RN synthesis is catalyzed by RN polymerase, which pries the strands apart and joins together the RN nucleotides RN polymerases assemble polynucleotides in the to direction Unlike polymerases, RN polymerases can start a chain without a primer nimation: Transcription Introduction 13

14 Figure 14.8-s1 Promoter Transcription unit Initiation Start point RN polymerase Unwound RN Template strand of transcript Figure 14.8-s2 Promoter Transcription unit Initiation Start point RN polymerase Elongation Unwound Rewound RN transcript RN Template strand of transcript Direction of transcription ( downstream ) Figure 14.8-s3 Promoter Transcription unit Initiation Start point RN polymerase Elongation Termination Unwound Rewound RN transcript RN Template strand of transcript Completed RN transcript Direction of transcription ( downstream ) 14

15 The sequence where RN polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator The stretch of that is transcribed is called a transcription unit Synthesis of an RN Transcript The three stages of transcription Initiation Elongation Termination RN Polymerase Binding and Initiation of Transcription Promoters signal the transcriptional start point and usually extend several dozen nucleotide pairs upstream of the start point Transcription factors mediate the binding of RN polymerase and the initiation of transcription 15

16 The completed assembly of transcription factors and RN polymerase II bound to a promoter is called a transcription initiation complex promoter sequence called a TT box is crucial in forming the initiation complex in eukaryotes Figure 14.9 T T T T T T T TT box Promoter Transcription factors Start point Nontemplate strand Template strand eukaryotic promoter Several transcription factors bind to. RN polymerase II Transcription factors Transcription initiation complex forms. RN transcript Transcription initiation complex Elongation of the RN Strand s RN polymerase moves along the, it untwists the double helix, 10 to 20 bases at a time Transcription progresses at a rate of 40 nucleotides per second in eukaryotes gene can be transcribed simultaneously by several RN polymerases 16

17 Figure RN polymerase Nontemplate strand of RN nucleotides T C C end U U C C T G G T T Newly made RN Direction of transcription Template strand of Termination of Transcription The mechanisms of termination are different in bacteria and eukaryotes In bacteria, the polymerase stops transcription at the end of the terminator and the can be translated without further modification In eukaryotes, RN polymerase II transcribes the polyadenylation signal sequence; the RN transcript is released nucleotides past this polyadenylation sequence Concept 14.3: Eukaryotic cells modify RN after transcription Enzymes in the eukaryotic nucleus modify pre (RN processing) before the genetic messages are dispatched to the cytoplasm During RN processing, both ends of the primary transcript are altered lso, usually some interior parts of the molecule are cut out and the other parts spliced together 17

18 lteration of Ends Each end of a pre- molecule is modified in a particular way The end receives a modified G nucleotide cap The end gets a poly- tail These modifications share several functions Facilitating the export of to the cytoplasm Protecting from hydrolytic enzymes Helping ribosomes attach to the end Figure modified guanine nucleotide added to the end Region that includes protein-coding segments Polyadenylation signal adenine nucleotides added to the end G P P P U Cap UTR Start Stop codon codon UTR Poly- tail Split Genes and RN Splicing Most eukaryotic s have long noncoding stretches of nucleotides that lie between coding regions The noncoding regions are called intervening sequences, or introns The other regions are called exons and are usually translated into amino acid sequences RN splicing removes introns and joins exons, creating an molecule with a continuous coding sequence 18

19 Figure Pre- Cap Intron Intron Poly- tail Cap UTR Coding segment Introns cut out and exons spliced together Poly- tail UTR Many genes can give rise to two or more different polypeptides, depending on which segments are used as exons This process is called alternative RN splicing RN splicing is carried out by spliceosomes Spliceosomes consist of proteins and small RNs Figure Spliceosome Small RNs Pre- Exon 1 Exon 2 Intron Spliceosome components Exon 1 Exon 2 Cut-out intron 19

20 Ribozymes Ribozymes are RN molecules that function as enzymes In some organisms, RN splicing can occur without proteins, or even additional RN molecules The introns can catalyze their own splicing Concept 14.4: Translation is the RN-directed Synthesis of a Polypeptide: Closer Look Genetic information flows from to protein through the process of translation Molecular Components of Translation cell translates an message into protein with the help of transfer RN (trn) trns transfer amino acids to the growing polypeptide in a ribosome Translation is a complex process in terms of its biochemistry and mechanics 20

21 Bioflix nimation: Protein Synthesis Figure Polypeptide mino acids Ribosome trn with amino acid attached trn nticodon U G G U U U G G C Codons The Structure and Function of Transfer RN Each trn can translate a particular codon into a given amino acid The trn contains an amino acid at one end and at the other end has a nucleotide triplet that can basepair with the complementary codon on 21

22 trn molecule consists of a single RN strand that is about 80 nucleotides long trn molecules can base-pair with themselves Flattened into one plane, a trn molecule looks like a cloverleaf In three dimensions, trn is roughly L-shaped, where one end of the L contains the anticodon that base-pairs with an codon Video: trn Model Figure mino acid attachment site G * C U * * C G C nticodon (a) Two-dimensional structure C C C G G C U U G U G U * C U * G G G U * * * C G G U U U G * * C U C * C G G G G * G C Hydrogen C bonds G C U G Hydrogen bonds nticodon (b) Three-dimensional structure mino acid attachment site G nticodon (c) Symbol used in this book 22

23 Figure mino acid attachment site G * C U * C C C G G C C G U G U U U C C G U G * * C U C * C G * G U G U * G G C G U * G * G G C G U * * C G C U G nticodon (a) Two-dimensional structure * Hydrogen bonds Figure mino acid attachment site Hydrogen bonds nticodon (b) Three-dimensional structure G nticodon (c) Symbol used in this book ccurate translation requires two steps First, a correct match between a trn and an amino acid, done by the enzyme aminoacyl-trn synthetase Second, a correct match between the trn anticodon and an codon Flexible pairing at the third base of a codon is called wobble and allows some trns to bind to more than one codon 23

24 Figure mino acid and trn enter active site. Tyrosine (Tyr) (amino acid) Tyrosyl-tRN synthetase Tyr-tRN U Complementary trn anticodon Using TP, synthetase catalyzes covalent bonding. minoacyl trn released. TP MP + 2 P i trn minoacyl-trn synthetase mino acid Computer model Ribosomes Ribosomes facilitate specific coupling of trn anticodons with codons during protein synthesis The large and small ribosomal are made of proteins and ribosomal RNs (rrns) In bacterial and eukaryotic ribosomes the large and small subunits join to form a ribosome only when attached to an molecule Figure trn molecules Growing polypeptide Exit tunnel E P Large subunit Small subunit E site (exit site) binding site (a) Computer model of functioning ribosome P site (peptidyl-trn _ binding site) Exit tunnel E P site (aminoacyltrn binding _ site) Large subunit Small subunit mino end E Codons Growing polypeptide Next amino acid to be added to polypeptide chain trn (b) Schematic model showing binding sites Pearson Education, Inc. (c) Schematic model with and trn 24

25 Figure Growing polypeptide Exit tunnel trn molecules E P Large subunit Small subunit (a) Computer model of functioning ribosome Pearson Education, Inc. Figure P site (peptidyl-trn binding site) Exit tunnel E site (exit site) binding site E P site (aminoacyltrn binding site) Large subunit Small subunit (b) Schematic model showing binding sites Pearson Education, Inc. Figure mino end Growing polypeptide E Next amino acid to be added to polypeptide chain trn Codons (c) Schematic model with and trn Pearson Education, Inc. 25

26 ribosome has three binding sites for trn The P site holds the trn that carries the growing polypeptide chain The site holds the trn that carries the next amino acid to be added to the chain The E site is the exit site, where discharged trns leave the ribosome Building a Polypeptide The three stages of translation Initiation Elongation Termination ll three stages require protein factors that aid in the translation process Energy is provided by hydrolysis of GTP Ribosome ssociation and Initiation of Translation The initiation stage of translation brings together, a trn with the first amino acid, and the two ribosomal subunits small ribosomal subunit binds with and a special initiator trn Then the small subunit moves along the until it reaches the start codon (UG) 26

27 nimation: Translation Introduction Figure Initiator trn Start codon binding site U C U G Small ribosomal subunit binds to. P i + GTP GDP Small ribosomal subunit P site E Large ribosomal subunit completes the initiation complex. Large ribosomal subunit Translation initiation complex The start codon is important because it establishes the reading frame for the 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 27

28 Elongation of the Polypeptide Chain During elongation, amino acids are added one by one to the previous amino acid at 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 Translation proceeds along the in a to direction Figure s1 mino end of polypeptide E P site site GTP Codon recognition GDP + P i E P Figure s2 mino end of polypeptide E P site site GTP Codon recognition GDP + P i E P Peptide bond formation E P 28

29 Figure s3 mino end of polypeptide Ribosome ready for next aminoacyl trn E P site site GTP Codon recognition GDP + P i E E P P Translocation GDP + P i GTP Peptide bond formation E P Termination of Translation Termination occurs when a stop codon in the reaches the site of the ribosome The 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 Figure s1 Release factor Stop codon (UG, U, or UG) Ribosome reaches a stop codon on. 29

30 Figure s2 Release factor Free polypeptide Stop codon (UG, U, or UG) Ribosome reaches a stop codon on. Release factor promotes hydrolysis. Figure s3 Release factor Free polypeptide 2 GTP Stop codon (UG, U, or UG) 2 GDP + 2 P i Ribosome reaches a stop codon on. Release factor promotes hydrolysis. Ribosomal subunits and other components dissociate. 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 30

31 Protein Folding and Post-Translational Modifications During synthesis, a polypeptide chain spontaneously coils and folds into its threedimensional shape Proteins may also require post-translational modifications before doing their jobs Targeting Polypeptides to Specific Locations Two populations of ribosomes are evident in cells: free ribosomes (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 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 31

32 signal-recognition particle (SRP) binds to the signal peptide The SRP brings the signal peptide and its ribosome to the ER Figure Polypeptide synthesis begins. SRP binds to signal peptide. SRP binds to receptor protein. SRP detaches and polypeptide synthesis resumes. Signalcleaving polypeptide Completed enzyme cuts folds into off signal final peptide. conformation. Ribosome Signal peptide SRP CYTOSOL ER LUMEN SRP receptor protein Translocation complex Signal peptide removed ER membrane Protein Making Multiple Polypeptides in Bacteria and Eukaryotes In bacteria and eukaryotes, multiple ribosomes translate an at the same time Once a ribosome is far enough past the start codon, another ribosome can attach to the Strings of ribosomes called polyribosomes (or polysomes) can be seen with an electron microscope 32

33 Figure Incoming ribosomal subunits Growing polypeptides Completed polypeptide Start of ( end) End of ( end) (a) n molecule translated simultaneously by several ribosomes Ribosomes (b) large polyribosome in a bacterial cell (TEM) 0.1 mm Figure Growing polypeptides Completed polypeptide Incoming ribosomal subunits Start of ( end) End of ( end) (a) n molecule translated simultaneously by several ribosomes Figure Ribosomes (b) large polyribosome in a bacterial cell (TEM) 0.1 mm 33

34 Bacteria and eukaryotes can also transcribe multiple s from the same gene In bacteria, the transcription and translation can take place simultaneously In eukaryotes, the nuclear envelope separates transcription and translation Figure RN polymerase Polyribosome RN polymerase Direction of transcription 0.25 mm Polyribosome Polypeptide (amino end) Ribosome ( end) Figure RN polymerase Polyribosome 0.25 mm 34

35 Figure TRNSCRIPTION RN transcript RN PROCESSING RN polymerase Exon RN transcript (pre-) Intron minoacyl-trn synthetase NUCLEUS CYTOPLSM mino acid trn MINO CID CTIVTION Cap E P Ribosomal subunits minoacyl (charged) trn Cap TRNSLTION E U G G UU U U G Codon Ribosome nticodon Figure TRNSCRIPTION RN transcript RN PROCESSING CYTOPLSM Exon NUCLEUS Cap RN polymerase RN transcript (pre-) Intron mino acid trn minoacyl-trn synthetase minoacyl (charged) trn MINO CID CTIVTION Figure Growing polypeptide E P Cap Ribosomal subunits minoacyl (charged) trn Cap E U G G U U U U G Codon Ribosome nticodon TRNSLTION 35

36 Concept 14.5: Mutations of one or a few nucleotides 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 nucleotide pair of a gene The change of a single nucleotide in a template strand can lead to the production of an abnormal protein If a point mutation occurs in a gamete, it may be transmitted to offspring Figure Wild-type b-globin Sickle-cell b-globin Wild-type b-globin C T C G G Mutant b-globin C C G T G G G G U G Normal hemoglobin Glu Sickle-cell hemoglobin Val Types of Small-Scale Mutations Point mutations within a gene can be divided into two general categories Single nucleotide-pair substitutions Nucleotide-pair insertions or deletions 36

37 Substitutions nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code Figure Wild type template strand T C T T C C C G T T T G G T T T G G C T Protein mino end Nucleotide-pair substitution: silent U G G U U U G G C U Met Lys Phe Gly instead of G Stop Carboxyl end T C T T C C C T T T G G T T T G G T T U instead of C U G G U U U G G U U Met Lys Phe Gly Stop Figure Wild type template strand T C T T C C C G T T T G G T T T G G C T Protein mino end U G G U U U G G C U Met Lys Phe Gly Stop Carboxyl end Nucleotide-pair substitution: missense T instead of C T C T T C T C G T T T G G T T T G C T instead of G U G G U U U G C U Met Lys Phe Ser Stop 37

38 Figure Wild type template strand T C T T C C C G T T T G G T T T G G C T Protein mino end Met Lys Phe Gly Nucleotide-pair substitution: nonsense U G G U U U G G C U Stop Carboxyl end instead of T T C T C C C G T T T G T G T T T G G C T U instead of U G U G U U U G G U U Met Stop Figure Wild type template strand T C T T C C C G T T T G G T T T G G C T Protein mino end U G G U U U G G C U Met Lys Phe Gly Stop Carboxyl end Nucleotide-pair insertion: frameshift causing immediate nonsense Extra T C T T C C T G T G T T T G Extra U U G U G U U U G G C U Met Stop C G G T T C T Figure Wild type template strand T C T T C C C G T T T G G T T T G G C T Protein mino end U G G U U U G G C U Met Lys Phe Gly missing T C T T C C C G T T T G G T T G G C T U missing U G G U U G G C U Stop Carboxyl end Nucleotide-pair deletion: frameshift causing extensive missense Met Lys Leu la 38

39 Figure Wild type template strand T C T T C C C G T T T G G T T T G G C T Protein mino end U G G U U U G G C U Met Lys Phe Gly T T C missing T C C C G T T G T T T G G C T G missing U G U U U G G C U Met Phe Gly Stop T Stop Carboxyl end 3 nucleotide-pair deletion: no frameshift, but one amino acid missing Missense mutations still code for an amino acid, but not the correct amino acid Substitution mutations are usually missense mutations Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein Insertions and Deletions Insertions and deletions are additions or losses of nucleotide pairs in a gene These mutations have a disastrous effect on the resulting protein more often than substitutions do Insertion or deletion of nucleotides may alter the reading frame of the genetic message, producing a frameshift mutation 39

40 New Mutations and Mutagens Spontaneous mutations can occur during replication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations Researchers have developed methods to test the mutagenic activity of chemicals Most cancer-causing chemicals (carcinogens) are mutagenic, and the converse is also true What Is a Gene? Revisiting the Question The definition of a gene has evolved through the history of genetics We have considered a gene as discrete unit of inheritance region of specific nucleotide sequence in a chromosome sequence that codes for a specific polypeptide chain gene can be defined as a region of that can be expressed to produce a final functional product, either a polypeptide or an RN molecule 40

41 Figure 14.UN02 TRNSCRIPTION RN PROCESSING Pre- TRNSLTION Ribosome Polypeptide Figure 14.UN03 TRNSCRIPTION RN PROCESSING Pre- TRNSLTION Ribosome Polypeptide Pearson Education, Inc. Figure 14.UN04 TRNSCRIPTION TRNSLTION Ribosome Polypeptide Pearson Education, Inc. 41

42 Figure 14.UN05-1 thr lac lacy lacz laci rec galr Met J lex trpr Sequence alignment Figure 14.UN Sequence logo Figure 14.UN

43 Figure 14.UN05-4 Ribosome binding site on Figure 14.UN06 Transcription unit Promoter RN transcript RN polymerase Template strand of Figure 14.UN07 Cap Exon Intron Exon Pre- Intron RN splicing Poly- tail Exon UTR Coding segment UTR 43

44 Figure 14.UN08 trn Polypeptide mino acid E nticodon Ribosome Codon Figure 14.UN09 Figure 14.UN10 44