Chapter 17 From Gene to Protein PDF

LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Chapter 17 From Gene to Protein...

LECTURE PRESENTATIONS For CAMPBELL BIOLOGY, NINTH EDITION Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson Chapter 17 From Gene to Protein Lectures by Erin Barley Kathleen Fitzpatrick © 2011 Pearson Education, Inc. 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 Proteins are the links between genotype and phenotype same alleles homoz. diff alleles hetero. dna to proteins Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation rna exported outside the nucleus , modified rna will be translated to amino acids amino acids come together to become poly peptide and then many polypeptide become proteins © 2011 Pearson Education, Inc. Figure 17.1 variation/ mutation How does a single faulty gene result in the dramatic (fault protein)/malfunction: gives fault protein appearance of an albino deer? disfunction: completely wrong protein or give none protein Concept 17.1: Genes specify proteins via transcription and translation How was the fundamental relationship between genes and proteins discovered? transcription: read the nucleotide inside the nucleus modify rna in nucleus —> gives us m-rna then it goes to ribosomes translation: after being exported exported rna go to ribosome and it will translate it to amino acids —> polypeptide —> protein 1. eukaryotic: has a nucleus - (the purpose of modification of rna here is: if the rna is not modified, when it goes outside it will break down) - modification protects the rna 2. prokaryotic: doesn’t have nucleus (rna doesn’t require modification here) summary: In prokaryotes, both transcription and translation occur in the cytoplasm. In eukaryotes, transcription takes place in the nucleus, while translation occurs in the ribosomes of the rough endoplasmic reticulum (ER). Explanation: Prokaryotes are single-celled organisms like bacteria, and they lack a true nucleus © 2011 Pearson Education, Inc. no need to know the year but know the scientist Evidence from the Study of Metabolic Defects In 1902, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes are biocatalysts —> 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 © 2011 Pearson Education, Inc. Nutritional Mutants in Neurospora: Scientific Inquiry the enzymes are not involved in the gradient of the reaction its a catalyst pathways: a —> b —> c —> d , (helpful to track them to know which enzyme is deficient if there ins no one outcome) 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 © 2011 Pearson Education, Inc. Figure 17.2a EXPERIMENT Growth: No growth: Wild-type Mutant cells cells growing cannot grow and dividing and divide Minimal medium Figure 17.2b RESULTS no mutation Classes of Neurospora crassa Wild type Class I mutants Class II mutants Class III mutants Minimal Growth medium No (MM) growth (control) MM  ornithine Condition MM  citrulline MM  arginine (control) Can grow with Can grow on Can grow only Require arginine Summary ornithine, or without any on citrulline or to grow of results citrulline, or supplements arginine arginine Figure 17.2c CONCLUSION Gene Class I mutants Class II mutants Class III mutants (codes for (mutation in (mutation in (mutation in enzyme) Wild type gene A) gene B) gene C) Precursor Precursor Precursor Precursor Gene A Enzyme A Enzyme A Enzyme A Enzyme A 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 Arginine Arginine Arginine Arginine feedback inhibition mechanism: - if you stop in the begging the reaction sequence you wont get any products - but if you stop in the last you can get some by products The Products of Gene Expression: A Developing Story Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one all enzymes are protein protein but all proteins are not enzymes one gene gives/controlls 2 diff proteins (control 2 functions) (one protein controlled by 2 diff genes(phenotypes)) 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 gene —> amino acids —> polypeptide(a-lot of modification) —> proteins (some modification) Note that it is common to refer to gene products as proteins rather than polypeptides © 2011 Pearson Education, Inc. Basic Principles of Transcription and Translation RNA is the bridge between genes and the proteins for which they code Transcription is the synthesis of RNA under the direction of DNA Transcription produces messenger RNA (mRNA) Translation is the synthesis of a polypeptide, using information in the mRNA Ribosomes are the sites of translation © 2011 Pearson Education, Inc.  In prokaryotes, translation of mRNA can begin before transcription has finished In a eukaryotic cell, the nuclear envelope separates transcription from translation Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA © 2011 Pearson Education, Inc.  A primary transcript is the initial RNA transcript from any gene prior to processing The central dogma is the concept that cells are governed by a cellular chain of command: DNA RNA protein © 2011 Pearson Education, Inc. Figure 17.UN01 Central dogma DNA RNA Protein Figure 17.3a-1 DNA TRANSCRIPTION mRNA (a) Bacterial cell Figure 17.3a-2 DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide (a) Bacterial cell Figure 17.3b-1 Nuclear envelope DNA TRANSCRIPTION Pre-mRNA (b) Eukaryotic cell Figure 17.3b-2 Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA (b) Eukaryotic cell Figure 17.3b-3 Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA TRANSLATION Ribosome Polypeptide (b) Eukaryotic cell 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 How many nucleotides correspond to an amino acid? 20 —> amino acids every amino acids have 3 codes dna code : atcg —> transcribed rna code : uagc reading triple by triple (atc)g.. for amino acids © 2011 Pearson Education, Inc. 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 nonoverlaping three-nucleotide words of mRNA These words are then translated into a chain of amino acids, forming a polypeptide © 2011 Pearson Education, Inc. Figure 17.4 DNA template 3 5 DNA strand A C C A A A C C G A G T molecule T G G T T T G G C T C A 3 Gene 1 5 TRANSCRIPTION Gene 2 U G G U U U G G C U C A mRNA 5 3 Codon TRANSLATION Protein Trp Phe Gly Ser Gene 3 Amino acid  During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript The template strand is always the same strand for a given gene During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction © 2011 Pearson Education, Inc.  Codons along an mRNA molecule are read by translation machinery 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 © 2011 Pearson Education, Inc. 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 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 © 2011 Pearson Education, Inc. Figure 17.5 Second mRNA base imp U C A G UUU UCU UAU UGU U Phe Tyr Cys UUC UCC UAC UGC C U Ser UUA UCA UAA Stop UGA Stop A Leu Third mRNA base (3 end of codon) First mRNA base (5 end of codon) UUG UCG UAG Stop UGG Trp G CUU CCU CAU CGU U His CUC CCC CAC CGC C C Leu Pro Arg CUA CCA CAA CGA A Gln CUG CCG CAG CGG G AUU ACU AAU AGU U Asn Ser AUC Ile ACC AAC AGC C A Thr AUA ACA AAA AGA A rna will have the Lys Arg Met or same nucleotide like AUG start ACG AAG AGG G the dna when it goes to the ribosome GUU GCU GAU GGU U Asp GUC GCC GAC GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G Figure 17.4 DNA template 3 5 DNA strand A C C A A A C C G A G T molecule T G G T T T G G C T C A 3 Gene 1 5 TRANSCRIPTION Gene 2 U G G U U U G G C U C A mRNA 5 3 Codon TRANSLATION Protein Trp Phe Gly Ser Gene 3 Amino acid 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 genes can be inactive or active for many years and new genes can be made and expressed © 2011 Pearson Education, Inc. Figure 17.6 (a) Tobacco plant expressing (b) Pig expressing a jellyfish a firefly gene gene Concept 17.2: Transcription is the DNA- directed synthesis of RNA: a closer look Transcription is the first stage of gene expression © 2011 Pearson Education, Inc. Molecular Components of Transcription RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides The RNA is complementary to the DNA template strand RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine © 2011 Pearson Education, Inc.  The DNA sequence where RNA polymerase attaches is called the promoter; in bacteria, the sequence signaling the end of transcription is called the terminator The stretch of DNA that is transcribed is called a transcription unit how the dna will recognize that this is gene which can be expressed —> by promoter(will recognize the dna and start gene expression) prokaryotic —> at the end of the gene —> have terminator sequence (expresses the end of gene expression) © 2011 Pearson Education, Inc.  note: any replication start from 5’ end to 3’ end in the dna because its double stranded its a 2 way replication so (5’ end —> 3’ end) while in the rna because its a single strand its 1 way replication so only (5’ end —> 3’ end) Animation: Transcription © 2011 Pearson Education, Inc. Figure 17.7-4 in eukaryotic cell Promoter Transcription unit 5 3 3 5 DNA one of the strands will be the gene will recognize the Start point promotor attach to it and RNA polymerase start the transcription 1 Initiation build the rna with dna as a template Nontemplate strand of DNA 5 3 3 5 RNA Template strand of DNA Unwound transcript DNA 2 Elongation building Rewound DNA close and reopen 5 3 3 5 3 5 RNA transcript 3 Termination Text 5 3 3 5 5 3 Completed RNA transcript Direction of transcription (“downstream”) Synthesis of an RNA Transcript The three stages of transcription – Initiation – Elongationoptain rna by building it one by one – Termination © 2011 Pearson Education, Inc. RNA Polymerase Binding and Initiation of Transcription Promoters signal the transcriptional start point in promotor and usually extend several dozen nucleotide pairs based on the movement right or left upstream of the start point Transcription factors mediate the binding of RNA polymerase and the initiation of transcription the expression of specific gene in a specific place —> transcription factor (protein factor) it should be found at the site for the gene to be expressed The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex sequence of repetition , A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes t/f the gene expression start from the tata box which is inside the promotor © 2011 Pearson Education, Inc. Figure 17.8 1 A eukaryotic promoter Promoter Nontemplate strand DNA 5 T A T A A AA 3 3 A T AT T T T 5 TATA box Template strand inhancer and proximer Start point 1. something pushed the rna to attach to the promotor (tata box) 2 Several transcription Transcription factors bind to DNA factors 5 3 3 5 3 Transcription initiation complex forms RNA polymerase II Transcription factors 5 3 3 3 5 5 RNA transcript Transcription initiation complex Elongation of the RNA Strand As RNA polymerase moves along the DNA, it bubble opens untwists the double helix, 10 to 20 bases at a time Transcription progresses at a rate of 40 build in rna nucleotides per second in eukaryotes A gene can be transcribed simultaneously by several RNA polymerases start from 5’ end Nucleotides are added to the 3 end of the growing RNA molecule © 2011 Pearson Education, Inc. Figure 17.9 Nontemplate strand of DNA RNA nucleotides RNA polymerase why not start from here the replication of rna? because of the promotor A T C C A A 3 T 5 T U C 3 end T G U A G A C C A U C C A C A 5 A 3 T A G G T T 5 Direction of transcription Template strand of DNA Newly made RNA 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 mRNA can be translated without further modification In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA stop transcript is released 10–35 nucleotides past this will count the nucleotides polyadenylation sequence © 2011 Pearson Education, Inc. Concept 17.3: Eukaryotic cells modify RNA after transcription Enzymes in the eukaryotic nucleus modify pre- mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm During RNA processing, both ends of the primary transcript are usually altered Also, usually some interior parts of the molecule are cut out, and the other parts spliced together © 2011 Pearson Education, Inc. Alteration of mRNA Ends Each end of a pre-mRNA molecule is modified in a particular way – The 5 end receives a modified nucleotide 5 cap – The 3 end gets a poly-A tail These modifications share several functions – They seem to facilitate the export of mRNA – They protect mRNA from hydrolytic enzymes – They help ribosomes attach to the 5 end © 2011 Pearson Education, Inc. Figure 17.10 if it ends with 3’ end so it started with 5’ end Protein-coding Polyadenylation segment signal 5 3 G P P P AAUAAA AAA… AAA Start Stop 5 Cap 5 UTR 3 UTR Poly-A tail codon codon The choice of which strand (upper or lower) is used as the template for transcription depends on the specific gene's orientation and the direction of transcription, not the position (upper or lower) in the DNA double helix. RNA polymerase always moves along the DNA in the 3' to 5' direction on the template strand, synthesizing RNA in the 5' to 3' direction. Whether the "upper" or "lower" strand is used as the template depends on: Gene orientation: Genes are arranged in specific directions on DNA. Promoter positioning: The promoter region signals where transcription starts and which strand will serve as the template. So, it’s not about choosing the "lower" or "upper" strand but about which one runs in the right direction and has the necessary sequences (like the promoter) for the specific gene being transcribed. 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 cuts the exons(joining them after they are cut will give amino acids) and introns(will make different possibilities of protein )(to have alternative proteins) and every protein will give different function © 2011 Pearson Education, Inc. Figure 17.11 5 Exon Intron Exon Intron Exon 3 Pre-mRNA 5 Cap Poly-A tail Codon 130 31104 105 numbers 146 Introns cut out and exons spliced together mRNA 5Cap Poly-A tail 1146 5 UTR 3 UTR Coding segment  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 © 2011 Pearson Education, Inc. Figure 17.12-3 RNA transcript (pre-mRNA) 5 Exon 1 Intron Exon 2 Protein Other snRNA proteins snRNPs Spliceosome 5 Spliceosome components Cut-out mRNA intron 5 Exon 1 Exon 2 Ribozymes Ribozymes are catalytic RNA molecules that function as enzymes and can splice RNA The discovery of ribozymes rendered obsolete the belief that all biological catalysts were proteins © 2011 Pearson Education, Inc.  Three properties of RNA enable it to function as an enzyme only proteins are – 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 can link with other molecules © 2011 Pearson Education, Inc. The Functional and Evolutionary Importance of Introns can be used as a marker to differentiate between different species 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 produced variety of protein This is called alternative RNA splicing Consequently, the number of different proteins an organism can produce is much greater than its number of genes because one gene —> many proteins © 2011 Pearson Education, Inc.  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 © 2011 Pearson Education, Inc. Figure 17.13 Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Polypeptide Concept 17.4: Translation is the RNA- directed synthesis of a polypeptide: a closer look Genetic information flows from mRNA to protein through the process of translation © 2011 Pearson Education, Inc. Molecular Components of Translation A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) tRNA transfer amino acids to the growing polypeptide in a ribosome Translation is a complex process in terms of its biochemistry and mechanics © 2011 Pearson Education, Inc. Figure 17.14 Amino Polypeptide acids tRNA with amino acid attached Ribosome Trp Phe Gly tRNA C C C G A C C G Anticodon A A A U G G U U U G G C 5 Codons 3 mRNA The Structure and Function of Transfer RNA 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 © 2011 Pearson Education, Inc.  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 tRNA model © 2011 Pearson Education, Inc. Figure 17.15a 3 Amino acid attachment site 5 Hydrogen bonds Anticodon (a) Two-dimensional structure  Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule tRNA is roughly L-shaped © 2011 Pearson Education, Inc. Figure 17.15b Amino acid attachment 5 site 3 Hydrogen bonds A A G 3 5 Anticodon Anticodon (c) Symbol used (b) Three-dimensional structure in this book  Accurate translation requires two steps – First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase – Second: a correct match between the tRNA anticodon and an mRNA codon Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon © 2011 Pearson Education, Inc. Figure 17.16-4 Aminoacyl-tRNA synthetase (enzyme) Amino acid P Adenosine P P P Adenosine P Pi ATP Aminoacyl-tRNA Pi Pi tRNA synthetase tRNA Amino acid P Adenosine AMP Computer model Aminoacyl tRNA (“charged tRNA”) Ribosomes Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (rRNA) Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences: some antibiotic drugs (tetracycline and streptomycin) specifically target bacterial ribosomes without harming eukaryotic ribosomes © 2011 Pearson Education, Inc. Figure 17.17a Growing polypeptide Exit tunnel tRNA molecules Large subunit E P A Small subunit 5 mRNA 3 (a) Computer model of functioning ribosome Figure 17.17b P site (Peptidyl-tRNA Exit tunnel binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) E P A Large subunit mRNA binding site Small subunit (b) Schematic model showing binding sites Figure 17.17c Growing polypeptide Amino end Next amino acid to be added to polypeptide chain E tRNA mRNA 3 5 Codons (c) Schematic model with mRNA and tRNA  A ribosome has three binding sites for tRNA – The P site holds the tRNA that carries the growing polypeptide chain – The A site holds the tRNA that carries the next amino acid to be added to the chain – The E site is the exit site, where discharged tRNAs leave the ribosome © 2011 Pearson Education, Inc. Building a Polypeptide The three stages of translation – Initiation – Elongation – Termination All three stages require protein “factors” that aid in the translation process © 2011 Pearson Education, Inc. Ribosome Association and Initiation of Translation The initiation stage of translation brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits First, a small ribosomal subunit binds with mRNA and a special initiator tRNA Then the small subunit moves along the mRNA until it reaches the start codon (AUG) Proteins called initiation factors bring in the large subunit that completes the translation initiation complex © 2011 Pearson Education, Inc. Figure 17.18 Large ribosomal subunit 3 U A C 5 P site Met 5 A U G 3 Met Pi Initiator  tRNA GTP GDP E A mRNA 5 5 3 3 Start codon Small mRNA binding site ribosomal Translation initiation complex subunit Elongation of the Polypeptide Chain During the elongation stage, amino acids are added one by one to the preceding 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 mRNA in a 5′ to 3′ direction © 2011 Pearson Education, Inc. Figure 17.19-4 Amino end of polypeptide E mRNA 3 Ribosome ready for P A site site next aminoacyl tRNA 5 GTP GDP  P i E E P A P A GDP  P i GTP E P A Termination of Translation Termination occurs when a stop codon in the mRNA reaches the A site of the ribosome The A site accepts a protein called a release factor The release factor causes the addition of a water molecule instead of an amino acid This reaction releases the polypeptide, and the translation assembly then comes apart © 2011 Pearson Education, Inc. Figure 17.20-3 Release factor Free polypeptide 5 3 3 2 GTP 3 5 5 2 GDP  2 P i Stop codon (UAG, UAA, or UGA) Polyribosomes A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome) Polyribosomes enable a cell to make many copies of a polypeptide very quickly © 2011 Pearson Education, Inc. Figure 17.21 Completed Growing polypeptide polypeptides Incoming ribosomal subunits Polyribosome Start of mRNA End of (5 end) mRNA (a) (3 end) Ribosomes mRNA (b) 0.1 m 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 © 2011 Pearson Education, Inc. Protein Folding and Post-Translational Modifications During and after synthesis, a polypeptide chain spontaneously coils and folds into its three- dimensional shape Proteins may also require post-translational modifications before doing their job Some polypeptides are activated by enzymes that cleave them Other polypeptides come together to form the subunits of a protein © 2011 Pearson Education, Inc. Targeting Polypeptides to Specific Locations Two populations of ribosomes are evident in cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER) Free ribosomes mostly synthesize proteins that function in the cytosol Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell Ribosomes are identical and can switch from free to bound © 2011 Pearson Education, Inc.  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 © 2011 Pearson Education, Inc.  A signal-recognition particle (SRP) binds to the signal peptide The SRP brings the signal peptide and its ribosome to the ER © 2011 Pearson Education, Inc. Fig. 17-21 Polypeptide An SRP binds to The SRP binds to a receptor The SRP leaves, and The signal The rest of the synthesis the signal peptide protein in the ER membrane. polypetide synthesis cleaving completed polypeptide begins on a halting synthesis This receptor is part of a protein resumes, with enzyme cuts leaves the free ribosome momentarily complex (a transmembrane simultaneous off the signal ribosome and in the cytosol complex) that has a membrane translocation across the peptide folds into its final pore and signal-cleaving membrane. (The signal conformation enzyme peptide stays attached to the translocation complex). Ribosome mRNA Signal peptide ER membrane Signal Signal- peptide Protein recognition removed particle (SRP) CYTOSOL Translocation complex ER LUMEN SRP receptor protein 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 DNA template strand can lead to the production of an abnormal protein © 2011 Pearson Education, Inc. Figure 17.23 Wild-type hemoglobin Sickle-cell hemoglobin Wild-type hemoglobin DNA Mutant hemoglobin DNA 3 C T T 5 3 C A T 5 5 G A A 3 5 G T A 3 mRNA mRNA 5 G A A 3 5 G U A 3 Normal hemoglobin Sickle-cell hemoglobin Glu Val 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 © 2011 Pearson Education, Inc. Substitutions A 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 © 2011 Pearson Education, Inc. Figure 17.24a Wild type DNA template strand 3 T A C T T C A A A C C G A T T 5 5 A T G A A G T T T G G C T A A 3 mRNA5 A U G A A G U U U G G C U A A 3 Protein Met Lys Phe Gly Stop Amino end Carboxyl end (a) Nucleotide-pair substitution: silent A instead of G 3 T A C T T C A A A C C A A T T 5 5 A T G A A G T T T G G T T A A 3 U instead of C 5 A U G A A G U U U G G U U A A 3 Met Lys Phe Gly Stop Figure 17.24b Wild type DNA template strand 3 T A C T T C A A A C C G A T T 5 5 A T G A A G T T T G G C T A A 3 mRNA5 A U G A A G U U U G G C U A A 3 Protein Met Lys Phe Gly Stop Amino end Carboxyl end (a) Nucleotide-pair substitution: missense T instead of C 3 T A C T T C A A A T C G A T T 5 5 A T G A A G T T T A G C T A A 3 A instead of G 5 A U G A A G U U U A G C U A A 3 Met Lys Phe Ser Stop Figure 17.24c Wild type DNA template strand 3 T A C T T C A A A C C G A T T 5 5 A T G A A G T T T G G C T A A 3 mRNA5 A U G A A G U U U G G C U A A 3 Protein Met Lys Phe Gly Stop Amino end Carboxyl end (a) Nucleotide-pair substitution: nonsense A instead of T T instead of C 3 T A C A T C A A A C C G A T T 5 5 A T G T A G T T T G G C T A A 3 U instead of A 5 A U G U A G U U U G G C U A A 3 Met Stop 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 © 2011 Pearson Education, Inc. Figure 17.24d Wild type DNA template strand 3 T A C T T C A A A C C G A T T 5 5 A T G A A G T T T G G C T A A 3 mRNA5 A U G A A G U U U G G C U A A 3 Protein Met Lys Phe Gly Stop Amino end Carboxyl end (b) Nucleotide-pair insertion or deletion: frameshift causing immediate nonsense Extra A 3 T A C A T T C A A A C G G A T T 5 5 A T G T A A G T T T G G C T A A 3 Extra U 5 A U G U A A G U U U G G C U A A 3 Met Stop 1 nucleotide-pair insertion Figure 17.24e Wild type DNA template strand 3 T A C T T C A A A C C G A T T 5 5 A T G A A G T T T G G C T A A 3 mRNA5 A U G A A G U U U G G C U A A 3 Protein Met Lys Phe Gly Stop Amino end Carboxyl end (b) Nucleotide-pair insertion or deletion: frameshift causing extensive missense A missing 3 T A C T T C A A C C G A T T 5 5 A T G A A G T T G G C T A A 3 U missing 5 A U G A A G U U G G C U A A 3 Met Lys Leu Ala 1 nucleotide-pair deletion Figure 17.24f Wild type DNA template strand 3 T A C T T C A A A C C G A T T 5 5 A T G A A G T T T G G C T A A 3 mRNA5 A U G A A G U U U G G C U A A 3 Protein Met Lys Phe Gly Stop Amino end Carboxyl end (b) Nucleotide-pair insertion or deletion: no frameshift, but one amino acid missing T T C missing 3 T A C A A A C C G A T T 5 5 A T G T T T G G C T A A 3 A A G missing 5 A U G U U U G G C U A A 3 Met Phe Gly Stop 3 nucleotide-pair deletion Mutagens Spontaneous mutations can occur during DNA replication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations © 2011 Pearson Education, Inc. Concept 17.6: While gene expression differs among the domains of life, the concept of a gene is universal Archaea are prokaryotes, but share many features of gene expression with eukaryotes © 2011 Pearson Education, Inc. Comparing Gene Expression in Bacteria, Archaea, and Eukarya Bacteria and eukarya differ in their RNA polymerases, termination of transcription, and ribosomes; archaea tend to resemble eukarya in these respects Bacteria can simultaneously transcribe and translate the same gene In eukarya, transcription and translation are separated by the nuclear envelope In archaea, transcription and translation are likely coupled © 2011 Pearson Education, Inc. Figure 17.25 RNA polymerase DNA mRNA Polyribosome Direction of 0.25 m RNA transcription polymerase DNA Polyribosome Polypeptide (amino end) Ribosome mRNA (5 end) 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 – A discrete unit of inheritance – A region of specific nucleotide sequence in a chromosome – A DNA sequence that codes for a specific polypeptide chain © 2011 Pearson Education, Inc. Figure 17.26 DNA TRANSCRIPTION 3 -A ly Po RNA 5 RNA polymerase transcript Exon RNA RNA transcript PROCESSING (pre-mRNA) Intron Aminoacyl- y-A Pol tRNA synthetase NUCLEUS Amino acid AMINO ACID CYTOPLASM tRNA ACTIVATION Growing mRNA polypeptide ap 3 C 5 A -A P Aminoacyl Po ly E (charged) Ribosomal tRNA subunits ap 5 C TRANSLATION E A Anticodon Codon Ribosome  In summary, a gene can be defined as a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule © 2011 Pearson Education, Inc. END