BIOLOGY. Gene Expression: From Gene to Protein CAMPBELL. Reece Urry Cain Wasserman Minorsky Jackson

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1 CMPBELL BIOLOGY TENTH EDITION Reece Urry Cain Wasserman Minorsky Jackson 17 Gene Expression: From Gene to Protein Lecture Presentation by Nicole Tunbridge and Kathleen Fitzpatrick

2 The Flow of Genetic Information The information content of genes is in the specific sequences of nucleotides The DN 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 DN directs protein synthesis, includes two stages: transcription and translation

3 Figure 17.1

4 Figure 17.1a n albino racoon

5 Concept 17.1: Genes specify proteins via transcription and translation How was the fundamental relationship between genes and proteins discovered?

6 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

7 Nutritional Mutants in Neurospora: Scientific Inquiry George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media Using crosses, they and their coworkers 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

8 Figure 17.2 Precursor Results Table Classes of Neurospora crassa Enzyme Wild type Class I mutants Class II mutants Class III mutants Ornithine Enzyme B Citrulline Enzyme C rginine Minimal medium (MM) (control) MM + ornithine Condition MM + citrulline Growth: Wild-type cells growing and dividing Control: Minimal medium No growth: Mutant cells cannot grow and divide MM + arginine (control) Summary of results Can grow with or without any supplements Can grow on ornithine, citrulline, or arginine Can grow only on citrulline or arginine Require arginine to grow Gene (codes for enzyme) Wild type Class I mutants (mutation in gene ) Class II mutants (mutation in gene B) Class III mutants (mutation in gene C) Precursor Precursor Precursor Precursor Gene Enzyme Enzyme Enzyme Enzyme Ornithine Ornithine Ornithine Ornithine Gene B Enzyme B Enzyme B Enzyme B Enzyme B Citrulline Citrulline Citrulline Citrulline Gene C Enzyme C Enzyme C Enzyme C Enzyme C rginine rginine rginine rginine

9 Figure 17.2a Precursor Enzyme Ornithine Enzyme B Citrulline Enzyme C rginine

10 Figure 17.2b Growth: Wild-type cells growing and dividing No growth: Mutant cells cannot grow and divide Control: Minimal medium

11 Figure 17.2c Results Table Minimal medium (MM) (control) Wild type Classes of Neurospora crassa Class I mutants Class II mutants Class III mutants MM + ornithine Condition MM + citrulline MM + arginine (control) Summary of results Can grow with or without any supplements Can grow on ornithine, citrulline, or arginine Can grow only on citrulline or arginine Require arginine to grow

12 Figure 17.2d Gene (codes for enzyme) Wild type Class I mutants (mutation in gene ) Class II mutants (mutation in gene B) Class III mutants (mutation in gene C) Precursor Precursor Precursor Precursor Gene Enzyme Enzyme Enzyme Enzyme Ornithine Ornithine Ornithine Ornithine Gene B Enzyme B Enzyme B Enzyme B Enzyme B Citrulline Citrulline Citrulline Citrulline Gene C Enzyme C Enzyme C Enzyme C Enzyme C rginine rginine rginine rginine

13 The Products of Gene Expression: 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 the one gene one polypeptide hypothesis It is common to refer to gene products as proteins rather than polypeptides

14 Basic Principles of Transcription and Translation RN is the bridge between genes and the proteins for which they code Transcription is the synthesis of RN using information in DN Transcription produces messenger RN (mrn) Translation is the synthesis of a polypeptide, using information in the mrn Ribosomes are the sites of translation

15 In prokaryotes, translation of mrn can begin before transcription has finished In a eukaryotic cell, the nuclear envelope separates transcription from translation Eukaryotic RN transcripts are modified through RN processing to yield the finished mrn

16 Figure 17.3 Nuclear envelope TRNSCRIPTION DN RN PROCESSING Pre-mRN NUCLEUS mrn TRNSCRIPTION DN CYTOPLSM CYTOPLSM TRNSLTION mrn Ribosome TRNSLTION Ribosome Polypeptide Polypeptide (a) Bacterial cell (b) Eukaryotic cell

17 Figure 17.3a-1 TRNSCRIPTION CYTOPLSM DN mrn (a) Bacterial cell

18 Figure 17.3a-2 TRNSCRIPTION CYTOPLSM TRNSLTION DN mrn Ribosome Polypeptide (a) Bacterial cell

19 Figure 17.3b-1 Nuclear envelope TRNSCRIPTION DN Pre-mRN NUCLEUS CYTOPLSM (b) Eukaryotic cell

20 Figure 17.3b-2 Nuclear envelope TRNSCRIPTION DN RN PROCESSING Pre-mRN NUCLEUS mrn CYTOPLSM (b) Eukaryotic cell

21 Figure 17.3b-3 Nuclear envelope TRNSCRIPTION DN RN PROCESSING Pre-mRN NUCLEUS mrn CYTOPLSM TRNSLTION Ribosome Polypeptide (b) Eukaryotic cell

22 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: DN RN protein

23 Figure 17.UN01 DN RN Protein

24 The Genetic Code How are the instructions for assembling amino acids into proteins encoded into DN? There are 20 amino acids, but there are only four nucleotide bases in DN How many nucleotides correspond to an amino acid?

25 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 mrn These words are then translated into a chain of amino acids, forming a polypeptide

26 Figure 17.4 DN template strand C C C C G G T T G G T T T G G C T C TRNSCRIPTION mrn U G G U U U G G C U C TRNSLTION Codon Protein Trp Phe Gly Ser mino acid

27 During transcription, one of the two DN 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 a given gene

28 During translation, the mrn base triplets, called codons, are read in the direction Each codon specifies the amino acid (one of 20) to be placed at the corresponding position along a polypeptide

29 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 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

30 Figure 17.5 Second mrn base U C G First mrn base ( end of codon) U C G UUU UUC UU UUG CUU CUC CU CUG UU UC U UG GUU GUC GU GUG Phe Leu Leu Ile Met or start Val UCU UCC UC UCG CCU CCC CC CCG CU CC C CG GCU GCC GC GCG Ser Pro Thr la UU UC U CU CC C CG U C G GU GC G GG Tyr Stop UG Stop His Gln sn Lys sp Glu UGU UGC UG UGG CGU CGC CG CGG GU GC G GG GGU GGC GG GGG Cys Stop Trp rg Ser rg Gly U C G U C G U C G U C G Third mrn base ( end of codon)

31 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

32 Figure 17.6 (a) Tobacco plant expressing a firefly gene (b) Pig expressing a jellyfish gene

33 Figure 17.6a (a) Tobacco plant expressing a firefly gene

34 Figure 17.6b (b) Pig expressing a jellyfish gene

35 Concept 17.2: Transcription is the DN-directed synthesis of RN: closer look Transcription is the first stage of gene expression

36 Molecular Components of Transcription RN synthesis is catalyzed by RN polymerase, which pries the DN strands apart and joins together the RN nucleotides The RN is complementary to the DN template strand RN polymerase does not need any primer RN synthesis follows the same base-pairing rules as DN, except that uracil substitutes for thymine

37 Figure Promoter Transcription unit 1 Initiation Start point RN polymerase Unwound DN RN transcript Template strand of DN

38 Figure Promoter Transcription unit 1 Initiation Start point RN polymerase 2 Elongation Unwound DN Rewound DN RN transcript RN transcript Template strand of DN Direction of transcription ( downstream )

39 Figure Promoter Transcription unit 1 Initiation Start point RN polymerase 2 3 Elongation Termination Unwound DN Rewound DN RN transcript RN transcript Template strand of DN Completed RN transcript Direction of transcription ( downstream )

40 nimation: Transcription

41 The DN 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 DN that is transcribed is called a transcription unit

42 Synthesis of an RN Transcript The three stages of transcription Initiation Elongation Termination

43 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 The completed assembly of transcription factors and RN polymerase II bound to a promoter is called a transcription initiation complex promoter called a TT box is crucial in forming the initiation complex in eukaryotes

44 Figure 17.8 DN T T T T T T T TT box Transcription factors Promoter Start point Nontemplate strand Template strand 1 eukaryotic promoter RN polymerase II Transcription factors 2 Several transcription factors bind to DN. RN transcript 3 Transcription initiation complex forms. Transcription initiation complex

45 Elongation of the RN Strand s RN polymerase moves along the DN, 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 Nucleotides are added to the end of the growing RN molecule

46 Figure 17.9 RN polymerase Nontemplate strand of DN RN nucleotides C T U C C end C C T U U T G G T T Newly made RN Direction of transcription Template strand of DN

47 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 mrn 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

48 Concept 17.3: Eukaryotic cells modify RN after transcription Enzymes in the eukaryotic nucleus modify premrn (RN processing) before the genetic messages are dispatched to the cytoplasm During RN processing, both ends of the primary transcript are usually altered lso, usually certain interior sections of the molecule are cut out, and the remaining parts spliced together

49 lteration of mrn Ends Each end of a pre-mrn molecule is modified in a particular way The end receives a modified nucleotide cap The end gets a poly- tail These modifications share several functions They seem to facilitate the export of mrn to the cytoplasm They protect mrn from hydrolytic enzymes They help ribosomes attach to the end

50 Figure modified guanine nucleotide added to the end G P P P Cap UTR Region that includes protein-coding segments Start codon Stop codon Polyadenylation signal U UTR adenine nucleotides added to the end Poly- tail

51 Split Genes and RN Splicing Most eukaryotic genes and their RN 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 RN splicing removes introns and joins exons, creating an mrn molecule with a continuous coding sequence

52 Figure Pre-mRN Intron Cap Intron Poly- tail Introns cut out and exons spliced together mrn Cap Poly- tail UTR Coding UTR segment

53 In some cases, RN splicing is carried out by spliceosomes Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (snrnps) that recognize the splice sites The RNs of the spliceosome also catalyze the splicing reaction

54 Figure Spliceosome Small RNs Pre-mRN Exon 1 Exon 2 Intron Spliceosome components mrn Exon 1 Exon 2 Cut-out intron

55 Ribozymes Ribozymes are catalytic RN molecules that function as enzymes and can splice RN The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins

56 Three properties of RN 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 RN contain functional groups that may participate in catalysis RN may hydrogen-bond with other nucleic acid molecules

57 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 RN splicing Consequently, the number of different proteins an organism can produce is much greater than its number of genes

58 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 Exon shuffling may result in the evolution of new proteins

59 Figure DN Gene Exon 1 Intron Exon 2 Intron Exon 3 Transcription RN processing Translation Domain 3 Domain 2 Domain 1 Polypeptide

60 Concept 17.4: Translation is the RN-directed synthesis of a polypeptide: closer look Genetic information flows from mrn to protein through the process of translation

61 Molecular Components of Translation cell translates an mrn 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

62 Figure Polypeptide mino acids Trp Ribosome trn with amino acid attached Phe Gly trn nticodon U G G U U U G G C mrn Codons

63 The Structure and Function of Transfer RN Molecules of trn 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 mrn

64 trn molecule consists of a single RN strand that is only about 80 nucleotides long Flattened into one plane to reveal its base pairing, a trn molecule looks like a cloverleaf

65 Because of hydrogen bonds, trn actually twists and folds into a three-dimensional molecule trn is roughly L-shaped

66 Figure mino acid attachment site G * C U * C C * G U C G C C C G C U U G U C * U * G G G U * * * G C G G U U U * * C C C G C U G nticodon G * C U C * G G G G G Hydrogen bonds Hydrogen bonds nticodon (a) Two-dimensional structure (b) Three-dimensional (c) structure mino acid attachment site G nticodon Symbol used in this book

67 Figure 17.15a mino acid attachment site Hydrogen bonds (a) Two-dimensional structure nticodon C C C C G G C U U G G U U U G C C C * C C U G * * G G G G G C * * * * * U G C C C G C U G * * * U U U G G C * G G G U

68 Figure 17.15b mino acid attachment site Hydrogen bonds nticodon (b) Three-dimensional structure (c) G nticodon Symbol used in this book

69 Video: Stick and Ribbon Rendering of a trn

70 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 mrn codon Flexible pairing at the third base of a codon is called wobble and allows some trns to bind to more than one codon

71 Figure mino acid and trn enter active site. Tyrosine (Tyr) (amino acid) Tyrosyl-tRN synthetase Tyr-tRN U Complementary trn anticodon

72 Figure mino acid and trn enter active site. Tyrosine (Tyr) (amino acid) Tyrosyl-tRN synthetase Tyr-tRN U TP minoacyl-trn synthetase Complementary trn anticodon MP + 2 P i trn 2 Using TP, synthetase catalyzes covalent bonding. mino acid Computer model

73 Figure mino acid and trn enter active site. Tyrosine (Tyr) (amino acid) Tyrosyl-tRN synthetase Tyr-tRN U TP minoacyl-trn synthetase Complementary trn anticodon MP + 2 P i trn 3 minoacyl trn released. 2 Using TP, synthetase catalyzes covalent bonding. mino acid Computer model

74 Ribosomes Ribosomes facilitate specific coupling of trn anticodons with mrn codons in protein synthesis The two ribosomal subunits (large and small) are made of proteins and ribosomal RN (rrn) Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences: some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosomes

75 Figure trn molecules Growing polypeptide Exit tunnel E P Large subunit Small subunit mrn (a) Computer model of functioning ribosome P site (Peptidyl-tRN binding site) E site (Exit site) mrn binding site E P Exit tunnel site (minoacyltrn binding site) Large subunit Small subunit mino end mrn E Growing polypeptide Next amino acid to be added to polypeptide chain Codons trn (b) Schematic model showing binding sites (c) Schematic model with mrn and trn

76 Figure 17.17a trn molecules Growing polypeptide Exit tunnel E P Large subunit Small subunit mrn (a) Computer model of functioning ribosome

77 Figure 17.17b P site (Peptidyl-tRN binding site) Exit tunnel E site (Exit site) mrn binding site E P site (minoacyltrn binding site) Large subunit Small subunit (b) Schematic model showing binding sites

78 Figure 17.17c mino end Growing polypeptide Next amino acid to be added to polypeptide chain mrn E trn Codons (c) Schematic model with mrn and trn

79 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

80 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 required for some steps also

81 Ribosome ssociation and Initiation of Translation Initiation brings together mrn, a trn with the first amino acid, and the two ribosomal subunits First, a small ribosomal subunit binds with mrn and a special initiator trn Then the small subunit moves along the mrn until it reaches the start codon (UG) Proteins called initiation factors bring in the large subunit that completes the translation initiation complex

82 Figure Met U C U G P site Met Large ribosomal subunit Initiator trn mrn GTP P i + GDP E Start codon mrn binding site Small ribosomal subunit Translation initiation complex 1 Small ribosomal subunit binds 2 to mrn. Large ribosomal subunit completes the initiation complex.

83 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 and third steps Translation proceeds along the mrn in a direction

84 Figure mino end of polypeptide E P site site 1 mrn GTP Codon recognition GDP + P i E P

85 Figure mino end of polypeptide E P site site 1 mrn GTP Codon recognition GDP + P i E P 2 Peptide bond formation E P

86 Figure Ribosome ready for next aminoacyl trn mino end of polypeptide E P site site 1 mrn GTP Codon recognition GDP + P i E E P GDP + P i 3 Translocation GTP P 2 Peptide bond formation E P

87 Termination of Translation Termination occurs when a stop codon in the mrn 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 comes apart

88 Figure Release factor Stop codon (UG, U, or UG) 1 Ribosome reaches a stop codon on mrn.

89 Figure Release factor Free polypeptide Stop codon (UG, U, or UG) 1 Ribosome reaches a 2 stop codon on mrn. Release factor promotes hydrolysis.

90 Figure Release factor Free polypeptide 2 GTP Stop codon (UG, U, or UG) 2 GDP + 2 P i 1 Ribosome reaches a 2 stop codon on mrn. Release factor promotes hydrolysis. 3 Ribosomal subunits and other components dissociate.

91 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

92 Protein Folding and Post-Translational Modifications During its synthesis, a polypeptide chain begins to coil and fold spontaneously to form a protein with a specific shape a three-dimensional molecule with secondary and tertiary structure gene determines primary structure, and primary structure in turn determines shape Post-translational modifications may be required before the protein can begin doing its particular job in the cell

93 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 Ribosomes are identical and can switch from free to bound

94 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

95 signal-recognition particle (SRP) binds to the signal peptide The SRP brings the signal peptide and its ribosome to the ER

96 Figure Polypeptide synthesis begins. Ribosome SRP binds to signal peptide. mrn SRP binds to receptor protein. SRP detaches and polypeptide synthesis resumes. Signalcleaving enzyme cuts off signal peptide. Completed polypeptide folds into final conformation. SRP Signal peptide Signal peptide removed ER membrane Protein CYTOSOL ER LUMEN SRP receptor protein Translocation complex

97 Making Multiple Polypeptides in Bacteria and Eukaryotes Multiple ribosomes can translate a single mrn simultaneously, forming a polyribosome (or polysome) Polyribosomes enable a cell to make many copies of a polypeptide very quickly

98 Figure Incoming ribosomal subunits Growing polypeptides Completed polypeptide Start of mrn ( end) Polyribosome End of mrn ( end) (a) Several ribosomes simultaneously translating one mrn molecule Ribosomes mrn (b) large polyribosome in a bacterial cell (TEM) 0.1 µm

99 Figure 17.22a Growing polypeptides Completed polypeptide Incoming ribosomal subunits Start of mrn ( end) Polyribosome End of mrn ( end) (a) Several ribosomes simultaneously translating one mrn molecule

100 Figure 17.22b Ribosomes mrn (b) large polyribosome in a bacterial cell (TEM) 0.1 µm

101 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

102 Figure RN polymerase DN mrn Polyribosome RN polymerase Direction of transcription 0.25 µm DN Polyribosome Polypeptide (amino end) Ribosome mrn ( end)

103 Figure 17.23a RN polymerase DN Polyribosome mrn 0.25 µm

104 In eukaryotes, the nuclear envelop separates the processes of transcription and translation RN undergoes processes before leaving the nucleus

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

106 Figure 17.24a TRNSCRIPTION DN RN transcript RN PROCESSING CYTOPLSM Exon NUCLEUS RN polymerase RN transcript (pre-mrn) Intron mino acid trn minoacyl-trn synthetase MINO CID CTIVTION mrn minoacyl (charged) trn

107 Figure 17.24b mrn Growing polypeptide minoacyl E P Ribosomal (charged) subunits trn E U G G U U U U G Codon Ribosome nticodon TRNSLTION

108 BioFlix: Protein Synthesis

109 nimation: Translation

110 Concept 17.5: 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 base pair of a gene The change of a single nucleotide in a DN template strand can lead to the production of an abnormal protein

111 If a mutation has an adverse effect on the phenotype of the organism the condition is referred to as a genetic disorder or hereditary disease

112 Figure Wild-type β-globin Sickle-cell β-globin Wild-type β-globin DN C T C G G Mutant β-globin DN C C G T G mrn mrn G G G U G Normal hemoglobin Glu Sickle-cell hemoglobin Val

113 Types of Small-Scale Mutations Point mutations within a gene can be divided into two general categories Nucleotide-pair substitutions One or more nucleotide-pair insertions or deletions

114 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 Missense mutations still code for an amino acid, but not the correct amino acid Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein

115 Figure 17.26a Wild type DN template strand T C T T C C C G T T T G G T T T G G C T mrn Protein mino end U G G U U U G G C U Met Lys Phe Gly Stop Carboxyl end Nucleotide-pair substitution: silent T U G G Met Lys Phe Gly U U U instead of G C T T C C C T T T G G T T T G G T T U instead of C G G U U Stop

116 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, producing a frameshift mutation

117 Figure 17.26b Wild type DN template strand T C T T C C C G T T T G G T T T G G C T mrn Protein mino end Nucleotide-pair substitution: missense U G G U U U G G C U Met Lys Phe Gly T instead of C T C T T C T C T G G U G G U U U instead of G Met Lys Phe Ser T T T G G G C C Stop Carboxyl end T T T U Stop

118 New Mutations and Mutagens Spontaneous mutations can occur during DN replication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations

119 What Is a Gene? Revisiting the Question The idea of the 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 DN sequence that codes for a specific polypeptide chain

120 gene can be defined as a region of DN that can be expressed to produce a final functional product that is either a polypeptide or an RN molecule

121 Figure DN template strand mrn Protein mino end Wild type T C T T C C C G T G G T T T G G C U G G Met Lys Phe Gly U U U T G G C U T T Stop Carboxyl end (a) Nucleotide-pair substitution instead of G T T C G T T C G T T T C G C G T T T T U instead of C U G G U U U G G U U Met Lys Phe Gly Stop Silent T Missense T instead of C C T T C T C G T G G T T T G C U G G instead of G U U U G Met Lys Phe Ser C T U T T Stop (b) Nucleotide-pair insertion or deletion Extra T C T G T T T G G Extra U U G U G U U U Met Stop Frameshift (1 nucleotide-pair insertion) T T C C C G T T G T C T U G G missing missing G G C U Met Lys Leu la G G C U T C T T C C C G T T T G G T T G G C T Frameshift (1 nucleotide-pair deletion) U U U instead of T missing T C T C C C G T T T C C C T G T G T T T G G C T G G G U instead of G missing U G U G U U U G G U U G G G Met Stop Met Phe Gly Nonsense 3 nucleotide-pair deletion T T C G T T T T C T U U U U C U T T Stop

122 Figure 17.26c Wild type DN template strand T C T T C C C G T T T G G T T T G G C T mrn Protein mino end U G G U U U G G C U Met Lys Phe Gly Stop Carboxyl end Nucleotide-pair substitution: nonsense instead of T T C T G U instead of U G Met T U T C C C G T T G T T T G G C T Stop G U U U G G U U

123 Figure 17.26d Wild type DN template strand T C T T C C C G T T T G G T T T G G C T mrn 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 T C T G Extra T T T C C C G T T G T T T G G C T U G U G U U U G G C U Met Stop

124 Figure 17.26e Wild type DN template strand T C T T C C C G T T T G G T T T G G C T mrn Protein mino end U G G U U U G G C U Met Lys Phe Gly Stop Carboxyl end Nucleotide-pair deletion: frameshift causing extensive missense 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 Met Lys Leu la

125 Figure 17.26f Wild type DN template strand T C T G T T C C C G T T G T T T G G C T mrn Protein mino end U G G U U U G G Met Lys Phe Gly C U Stop Carboxyl end 3 nucleotide-pair deletion: no frameshift, but one amino acid missing T T C missing T C C C G T 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

126 Figure 17.UN02 TRNSCRIPTION DN RN PROCESSING Pre-mRN mrn TRNSLTION Ribosome Polypeptide

127 Figure 17.UN03 TRNSCRIPTION DN RN PROCESSING Pre-mRN mrn TRNSLTION Ribosome Polypeptide

128 Figure 17.UN04 TRNSCRIPTION DN TRNSLTION mrn Ribosome Polypeptide

129 Figure 17.UN05a thr lac lacy lacz lacl rec galr metj lex trpr Sequence alignment

130 Figure 17.UN05b Sequence logo 8

131 Figure 17.UN05c

132 Figure 17.UN06 Transcription unit Promoter RN transcript RN polymerase Template strand of DN

133 Figure 17.UN07 Cap Pre-mRN Exon Intron Exon Intron Poly- tail Exon mrn UTR Coding UTR segment

134 Figure 17.UN08 trn Polypeptide mino acid E nticodon Ribosome Codon mrn

135 Figure 17.UN09 Type of RN Functions Messenger RN (mrn) Transfer RN (trn) Plays catalytic (ribozyme) roles and structural roles in ribosomes Primary transcript Small RNs in the spliceosome

136 Figure 17.UN10