Chapter 17 From Gene to Protein Power Point

Chapter 17 From Gene to Protein Power. Point® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -3 a-1 TRANSCRIPTION DNA m. RNA (a) Bacterial cell

Fig. 17 -3 a-2 • In prokaryotes, m. RNA produced by transcription is immediately translated without more processing TRANSCRIPTION DNA m. RNA Ribosome TRANSLATION Polypeptide (a) Bacterial cell

Fig. 17 -3 b-1 Nuclear envelope In a eukaryotic cell, the nuclear envelope separates transcription from translation TRANSCRIPTION (b) Eukaryotic cell DNA Pre-m. RNA

Fig. 17 -3 b-2 Nuclear envelope • Eukaryotic RNA transcripts are modified through RNA processing to yield finished m. RNA TRANSCRIPTION RNA PROCESSING m. RNA (b) Eukaryotic cell DNA Pre-m. RNA

Fig. 17 -3 b-3 Nuclear envelope DNA TRANSCRIPTION Pre-m. RNA PROCESSING m. RNA 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 bases correspond to an amino acid? Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -4 DNA molecule Gene 2 Gene 1 Gene 3 DNA template strand TRANSCRIPTION m. RNA Codon TRANSLATION Protein Amino acid

Cracking the Code • All 64 codons were deciphered by the mid 1960 s • Of the 64 triplets, 61 code for amino acids; 3 triplets are “stop” signals to end translation • 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Third m. RNA base (3 end of codon) First m. RNA base (5 end of codon) Fig. 17 -5 Second m. RNA base

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 (a) Tobacco plant expressing a firefly gene (b) Pig expressing a jellyfish gene

Fig. 17 -7 a-1 Promoter Transcription unit 5 3 Start point RNA polymerase DNA 3 5 • RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides • The DNA sequence where RNA polymerase attaches is called the promoter • Promoters signal the initiation of RNA synthesis • The stretch of DNA that is transcribed is called a transcription unit

Fig. 17 -7 a-2 Promoter Transcription unit 5 3 Start point RNA polymerase 3 5 DNA 1 Initiation 5 3 Unwound DNA 3 5 RNA transcript Template strand of DNA

Fig. 17 -7 a-3 Promoter Transcription unit 5 3 Start point RNA polymerase 3 5 DNA 1 Initiation 5 3 3 5 Unwound DNA RNA transcript Template strand of DNA 2 Elongation Rewound DNA 5 3 3 5 RNA transcript 3 5

Fig. 17 -7 Promoter Transcription unit 5 3 Start point RNA polymerase 3 5 DNA 1 Initiation 5 3 3 5 Unwound DNA RNA transcript 3 Rewound DNA 3 end 3 5 5 5 3 Termination 3 5 5 3 5 RNA nucleotides 5 3 RNA transcript RNA polymerase Template strand of DNA 2 Elongation 5 3 Nontemplate strand of DNA Elongation Completed RNA transcript 3 Direction of transcription (“downstream”) Newly made RNA Template strand of DNA

Fig. 17 -8 1 Promoter A eukaryotic promoter includes a TATA box Template 5 3 3 5 TATA box Start point Template DNA strand 2 Transcription factors Several transcription factors must bind to the DNA before RNA polymerase II can do so. 5 3 3 5 3 Additional transcription factors bind to the DNA along with RNA polymerase II, forming the transcription initiation complex. RNA polymerase II Transcription factors 5 3 3 5 5 RNA transcript Transcription initiation complex

Fig. 17 -9 RNA processing 5 G P Protein-coding segment Polyadenylation signal 3 5 Cap 5 UTR Start codon Stop codon AAUAAA AAA…AAA 3 UTR Poly-A tail These modifications share several functions: They seem to facilitate the export of m. RNA They protect m. RNA from hydrolytic enzymes They help ribosomes attach to the 5 end

Fig. 17 -10 • 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 5 Exon Intron 3 Pre-m. RNA 5 Cap Poly-A tail 1 30 31 Coding segment m. RNA 5 Cap 1 5 UTR 104 105 146 Introns cut out and exons spliced together Poly-A tail 146 3 UTR • Exons expressed, translated into amino acid sequences • RNA splicing removes introns and joins exons, creating an m. RNA molecule with a continuous coding sequence

Fig. 17 -11 -3 5 RNA transcript (pre-m. RNA) Exon 1 Intron Protein sn. RNA • In some cases, RNA splicing is carried out by spliceosomes • Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (sn. RNPs) that recognize the splice sites Exon 2 Other proteins sn. RNPs Spliceosome 5 Spliceosome components 5 m. RNA Exon 1 Exon 2 Cut-out intron

Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during RNA splicing Exons Because of alternative splicing, the number of different proteins an organism can produce is much greater than its number of genes DNA Troponin T gene Primary RNA transcript RNA splicing m. RNA or

Fig. 17 -12 Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Proteins often consist Transcription of discrete regions called domains RNA processing In many cases, different exons code for the Translation different domains in a protein Exon shuffling may result in the evolution of new proteins Domain 3 Domain 2 Domain 1 Polypeptide

Translation • A cell translates an m. RNA message into protein with the help of transfer RNA (t. RNA) • Molecules of t. RNA 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 m. RNA Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -14 3 Amino acid attachment site 5 • A t. RNA molecule consists of a single RNA strand that is only about 80 nucleotides long • Because of hydrogen bonds, t. RNA actually twists and folds into a threedimensional molecule Hydrogen bonds Anticodon (a) Two-dimensional structure 5 3 Amino acid attachment site Hydrogen bonds Anticodon (b) Three-dimensional structure 3 5 Anticodon (c) Symbol used in this book

Fig. 17 -15 -4 Aminoacyl-t. RNA synthetase (enzyme) Amino acid Accurate translation requires two steps: – – First: a correct match between a t. RNA and an amino acid, done by the enzyme aminoacylt. RNA synthetase Second: a correct match between the t. RNA anticodon and an m. RNA codon Flexible pairing at the third base of a codon is called wobble and allows some t. RNAs to bind to more than one codon P P P Adenosine ATP P P Pi Pi Adenosine t. RNA Aminoacyl-t. RNA synthetase Pi t. RNA P Adenosine AMP Computer model Aminoacyl-t. RNA (“charged t. RNA”)

Ribosomes • Ribosomes facilitate specific coupling of t. RNA anticodons with m. RNA codons in protein synthesis • The two ribosomal subunits (large and small) are made of proteins and ribosomal RNA (r. RNA) Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -16 • A ribosome has three binding sites for t. RNA: • The P site holds the t. RNA that carries the growing polypeptide chain • The A site holds the t. RNA that carries the next amino acid to be added to the chain • The E site is the exit site, where discharged t. RNAs leave the ribosome Growing polypeptide Exit tunnel t. RNA molecules EP Large subunit A Small subunit 5 m. RNA 3 (a) Computer model of functioning ribosome P site (Peptidyl-t. RNA binding site) E site (Exit site) A site (Aminoacylt. RNA binding site) E P A m. RNA binding site Large subunit Small subunit (b) Schematic model showing binding sites Growing polypeptide Amino end Next amino acid to be added to polypeptide chain m. RNA 5 E t. RNA 3 Codons (c) Schematic model with m. RNA and t. RNA

Building a Polypeptide • The three stages of translation: – Initiation – Elongation – Termination • All three stages require protein “factors” that aid in the translation process Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -17 The initiation stage of translation brings together m. RNA, a t. RNA with the first amino acid, and the two ribosomal subunits 1. First, a small ribosomal subunit binds with m. RNA and a special initiator t. RNA 3 U A C 5 A U G 3 Met 5 Initiator t. RNA P site Met Large ribosomal subunit GTP GDP E m. RNA 5 Start codon m. RNA binding site 3 Small ribosomal subunit A 5 3 Translation initiation complex 2. small subunit moves along the m. RNA until it reaches the start codon (AUG) 3. large subunit that completes the translation initiation complex

Fig. 17 -18 -1 Amino end of polypeptide E 3 m. RNA 5 P A site • During the elongation stage, amino acids are added one by one to the preceding amino acid • occurs in three steps: • codon recognition • peptide bond formation • translocation

Fig. 17 -18 -2 Amino end of polypeptide E 3 m. RNA 5 P A site GTP GDP E Peptide bond formation P A

Fig. 17 -18 -3 Amino end of polypeptide E 3 m. RNA 5 P A site GTP GDP E P A

Fig. 17 -18 -4 Amino end of polypeptide E 3 m. RNA Ribosome ready for next aminoacyl t. RNA 5 P A site GTP GDP E E P A GDP GTP translocation E P A

Fig. 17 -19 -3 • Termination occurs when a stop codon in the m. RNA 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 Release factor Free polypeptide 5 3 5 5 Stop codon (UAG, UAA, or UGA) 3 2 GTP 2 GDP 3

Fig. 17 -20 Completed polypeptide Growing polypeptides • A number of ribosomes can translate a single m. RNA simultaneously, forming a polyribosome (or polysome) • Polyribosomes enable a cell to make many copies of a polypeptide very quickly Incoming ribosomal subunits Start of m. RNA (5 end) (a) Polyribosome End of m. RNA (3 end) Ribosomes m. RNA (b) 0. 1 µm

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Protein Folding and Post-Translational Modifications • During and after synthesis, a polypeptide chain spontaneously coils and folds into its threedimensional 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Concept 17. 5: 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 can lead to the production of an abnormal protein Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -22 Wild-type hemoglobin DNA Mutant hemoglobin DNA C T T C A T 3 5 G T A G A A 3 5 m. RNA 5 G A A Normal hemoglobin Glu m. RNA 3 5 G U A Sickle-cell hemoglobin Val 5 3 3

Types of Point Mutations • Point mutations within a gene can be divided into two general categories – Base-pair substitutions – Base-pair insertions or deletions Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -23 Wild-type DNA template strand 3 5 5 3 m. RNA 5 3 Protein Stop Amino end Carboxyl end A instead of G 3 5 Extra A 5 3 3 5 3 5 U instead of C 5 5 3 Extra U 3 Stop Silent (no effect on amino acid sequence) Frameshift causing immediate nonsense (1 base-pair insertion) T instead of C missing 3 5 5 3 3 5 3 5 5 3 A instead of G missing 5 3 Stop Missense Frameshift causing extensive missense (1 base-pair deletion) missing A instead of T 5 3 3 5 U instead of A 5 5 3 3 5 missing 3 5 Stop Nonsense (a) Base-pair substitution 3 No frameshift, but one amino acid missing (3 base-pair deletion) (b) Base-pair insertion or deletion

Mutagens • Spontaneous mutations can occur during DNA replication, recombination, or repair • Mutagens are physical or chemical agents that can cause mutations Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 17 -25 DNA TRANSCRIPTION 3 http: //www. bing. c om/videos/searc h? q=youtube+pr otein+synthesis& mid=0892 DC 519 F 771664223708 92 DC 519 F 77166 42237&view=det ail&FORM=VIRE 1 l Po A y- RNA polymerase 5 RNA transcript RNA PROCESSING Exon RNA transcript (pre-m. RNA) Intron Aminoacyl-t. RNA synthetase y-A Pol NUCLEUS Amino acid CYTOPLASM AMINO ACID ACTIVATION t. RNA m. RNA Growing polypeptide 3 p Ca A P E A y- Activated amino acid Ribosomal subunits l Po Cap 5 TRANSLATION E A Codon Ribosome Anticodon