Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PowerPoint Lectures for Biology, Seventh Edition Neil Campbell and Jane Reece Lectures by Chris Romero Chapter 17 From Gene to Protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: The Flow of Genetic Information Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation DNA mRNA Protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Products of Gene Expression: A Developing Story One gene–One protein – Many proteins are composed of several polypeptides, each of which has its own gene Therefore, the definition is now restated as one gene–one polypeptide
LE TRANSCRIPTION DNA Prokaryotic cell
LE TRANSCRIPTION DNA Prokaryotic cell Ribosome Polypeptide mRNA Prokaryotic cell
LE TRANSCRIPTION TRANSLATION DNA mRNA Ribosome Polypeptide DNA Prokaryotic cell Nuclear envelope TRANSCRIPTION Eukaryotic cell
LE TRANSCRIPTION TRANSLATION DNA mRNA Ribosome Polypeptide DNA Pre-mRNA Prokaryotic cell Nuclear envelope mRNA TRANSCRIPTION RNA PROCESSING Eukaryotic cell
LE TRANSCRIPTION TRANSLATION DNA mRNA Ribosome Polypeptide DNA Pre-mRNA Prokaryotic cell Nuclear envelope mRNA TRANSLATION TRANSCRIPTION RNA PROCESSING Ribosome Polypeptide Eukaryotic cell
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Genetic Code There are 20 amino acids, but there are only four nucleotide bases in DNA So how many bases correspond to an amino acid? Codon
LE 17-4 DNA molecule Gene 1 Gene 2 Gene 3 DNA strand (template) 3 TRANSCRIPTION Codon mRNA TRANSLATION Protein Amino acid 3 5 5
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Cracking the Code The genetic code is redundant but not ambiguous; no codon specifies more than one amino acid
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Genetic Code
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Dictionary of the Genetic Code
LE 17-5 Second mRNA base First mRNA base (5 end) Third mRNA base (3 end)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Synthesis of an RNA Transcript The three stages of transcription: – Initiation – Elongation – Termination
LE 17-8 Promoter TATA box Start point Transcription factors Several transcription factors Additional transcription factors RNA polymerase II Transcription factors RNA transcript Transcription initiation complex Eukaryotic promoters Template DNA strand
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA synthesis is catalyzed by RNA polymerase – promoter – terminator Promoter Transcription unit RNA polymerase Start point DNA Animation: Transcription Animation: Transcription
LE 17-7 Promoter Transcription unit DNA Initiation Start point RNA polymerase Unwound DNA RNA tran- script Template strand of DNA
LE 17-7 Promoter 5 3 Transcription unit 3 5 DNA Start point RNA polymerase Initiation Unwound DNA RNA tran- script Template strand of DNA Elongation Rewound DNA RNA transcript
LE 17-7 Promoter 3 5 Transcription unit DNA Initiation RNA polymerase Start point Template strand of DNA RNA tran- script Unwound DNA Elongation Rewound DNA RNA transcript Termination Completed RNA transcript
LE 17-7 Elongation Non-template strand of DNA RNA polymerase RNA nucleotides 3 end Newly made RNA Template strand of DNA Direction of transcription (“downstream”)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 5 Protein-coding segment 5 Start codon Stop codon Poly-A tail Polyadenylation signal 5 3 Cap UTR Concept 17.3: Eukaryotic cells modify RNA after transcription During RNA processing, both ends of the primary transcript are usually altered
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Split Genes and RNA Splicing Introns Exons RNA splicing removes introns and joins exons 5 ExonIntronExonIntronExon 3 Pre-mRNA Coding segment Introns cut out and exons spliced together Cap Poly-A tail 5 3 UTR
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites Exon 1 5 IntronExon 2 Other proteins Protein snRNA snRNPs RNA transcript (pre-mRNA) Spliceosome 5 components Cut-out intron mRNA Exon 1Exon 2 5
LE Gene Transcription RNA processing Translation Domain 2 Domain 3 Domain 1 Polypeptide Exon 1IntronExon 2IntronExon 3 DNA
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look Transfer RNA (tRNA) Polypeptide tRNA with amino acid attached Ribosome tRNA Anticodon 3 5 mRNA Amino acids Codons
LE 17-14a Amino acid attachment site Hydrogen bonds 3 5 Two-dimensional structure Anticodon Amino acid attachment site 3 5 Hydrogen bonds Anticodon Symbol used in this book Three-dimensional structure 35
LE 17-14b Hydrogen bonds Amino acid attachment site Anticodon Symbol used in this book Anticodon Three-dimensional structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Accurate translation requires two steps: Amino acid Aminoacyl-tRNA synthetase (enzyme) Pyrophosphate Phosphates tRNA AMP Aminoacyl tRNA (an “activated amino acid”)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Ribosomes Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis tRNA molecules Exit tunnel Growing polypeptide Large subunit mRNA 3 Computer model of functioning ribosome Small subunit 5 E P A
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings A ribosome has three binding sites for tRNA: – A site – P site – E site P site (Peptidyl-tRNA binding site) E site (Exit site) mRNA binding site A site (Aminoacyl- tRNA binding site) Large subunit Small subunit Schematic model showing binding sites EPA
LE 17-16c Amino end mRNA 5 3 Growing polypeptide Next amino acid to be added to polypeptide chain tRNA Codons Schematic model with mRNA and tRNA E
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Building a Polypeptide The three stages of translation: – Initiation – Elongation – Termination
LE Met GTP Initiator tRNA mRNA 5 3 mRNA binding site Small ribosomal subunit Start codon P site 5 3 Translation initiation complex E A Large ribosomal subunit GDP Met Initiation
LE Ribosome ready for next aminoacyl tRNA mRNA 5 Amino end of polypeptide E P site A site GDP E PA GTP GDP E PA E PA Animation: Translation Animation: Translation Elongation
LE The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome. The two ribosomal subunits and the other components of the assembly dissociate. Release factor Stop codon (UAG, UAA, or UGA) Free polypeptide When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA. Termination
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Polyribosomes A number of ribosomes can translate a single mRNA simultaneously, forming a polyribosome Ribosomes mRNA 0.1 m An mRNA molecule is generally translated simultaneously by several ribosomes in clusters called polyribosomes. Incoming ribosomal subunits Growing polypeptides End of mRNA (3 end) Start of mRNA (5 end) Polyribosome Completed polypeptides
LE Ribosomes mRNA Signal peptide Signal- recognition particle (SRP) SRP receptor protein CYTOSOL ER LUMEN Translocation complex Signal peptide removed ER membrane Protein
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 17.5: RNA plays multiple roles in the cell: a review Type of RNA Functions Messenger RNA (mRNA) Carries information specifying amino acid sequences of proteins from DNA to ribosomes Transfer RNA (tRNA) Serves as adapter molecule in protein synthesis; translates mRNA codons into amino acids Ribosomal RNA (rRNA) Plays catalytic (ribozyme) roles and structural roles in ribosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Type of RNA Functions Primary transcript Serves as a precursor to mRNA, rRNA, or tRNA, before being processed by splicing or cleavage Small nuclear RNA (snRNA) Plays structural and catalytic roles in spliceosomes
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 17.6: Comparing gene expression in prokaryotes and eukaryotes reveals key differences RNA polymerase DNA Polyribosome RNA polymerase Direction of transcription mRNA 0.25 m DNA Polyribosome Polypeptide (amino end) Ribosome mRNA (5 end) Prokaryotic cells
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 17.7: Point mutations can affect protein structure and function Point mutations are chemical changes in just one base pair of a gene Wild-type hemoglobin DNA mRNA Mutant hemoglobin DNA mRNA Normal hemoglobinSickle-cell hemoglobin
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Types of Point Mutations Point mutations within a gene can be divided into two general categories – Base-pair substitutions – Base-pair insertions or deletions
LE Base-pair substitution No effect on amino acid sequence U instead of C Missense A instead of G Nonsense U instead of A Stop Amino end Protein 53 Carboxyl end Stop mRNA Wild type
LE Base-pair insertion or deletion Frameshift causing immediate nonsense Extra U Missing Frameshift causing extensive missense Insertion or deletion of 3 nucleotides: no frameshift but extra or missing amino acid Missing Stop Amino end Carboxyl end Stop Wild type mRNA Protein 53
LE TRANSCRIPTION RNA PROCESSING RNA transcript 5 Exon NUCLEUS FORMATION OF INITIATION COMPLEX CYTOPLASM 3 DNA RNA polymerase RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase Amino acid tRNA AMINO ACID ACTIVATION 3 mRNA A P E Ribosomal subunits 5 Growing polypeptide E A Activated amino acid Anticodon TRANSLATION Codon Ribosome