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Protein Synthesis Chapter 17
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Protein synthesis DNA Responsible for hereditary information DNA divided into genes Gene: Sequence of nucleotides Determines amino acid sequence in proteins Genes provide information to make proteins
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Protein synthesis DNA RNA protein
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Central Dogma Mechanism of reading & expressing genes Information passes from the genes (DNA) to an RNA copy Directs sequence of amino acids to make proteins
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Protein synthesis Transcription: DNA sequence is copied into an RNA Translation: Information from the RNA is turned into an amino acid sequence
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RNA RNA (ribonucleic acid) Single strand Sugar –ribose (-OH on 2’ carbon) Uracil instead of thymine
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RNA mRNA: Messenger RNA Transcribes information from DNA Codons (3 nucleotides) CGU mRNA Codes for amino acids rRNA: Ribosomal RNA Polypeptides are assembled
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RNA tRNA: Transfer RNA Transports aa to build proteins Positions aa on rRNA Anticodons (3 complementary nucleotides) GCA
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Cracking the code Francis Crick Codons (Triplet code)-mRNA Each codon corresponds to an aa 20 amino acids Reading frame Reading symbols in correct groupings
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Cracking the code 1 or 2 deletions or additions Gene was transcribed incorrectly 3 deletions Reading frame would shift Gene was transcribed correctly
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WHYDIDTHEREDCATEATTHEFATRAT WHYIDTHEREDCATEATTHEFATRAT WHYDTHEREDCATEATTHEFATRAT WHYTHEREDCATEATTHEFATRAT
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The code Universal code AGA codes for amino acid Arginine Humans & bacteria Genes from humans can be transcribed by mRNA from bacteria Produce human proteins Insulin
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Protein synthesis DNA RNA Protein Transcription Translation
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Prokaryotes Transcription Getting the code from DNA Template strand Strand of DNA that is transcribed or read Transcribed RNA is complementary to the DNA
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Prokaryotes Coding strand DNA strand not coded Same sequence of nucleotides as the RNA transcript Only T instead of U.
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Prokaryotes RNA polymerase Enzyme Adds nucleotides to the 3’end 5’to3’ direction Does not need a primer to start
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Prokaryotes Stages of transcription Initiation Elongation Termination
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Prokaryotes Initiation Promoters: Sequence on DNA where transcription starts -35 sequence TTGACA -10 sequence TATAAT Sequences are not transcribed
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Prokaryotes RNA polymerase binds promoter Unwinds DNA Uses an ATP or GTP to start Uses phosphate group Transcription bubble: RNA polymerase, DNA & growing RNA strand
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Prokaryotes Termination Stop signal Sequence on DNA RNA transcript signals polymerase to detach from DNA RNA strand separates from the DNA
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Prokaryotes Translation Passing the code to make a polypeptide mRNA binds to rRNA on the ribosome mRNA attaches so only one codon is exposed at a time
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Ribosome Located in the cytoplasm Site of translation 2 subunits composed of protein & RNA Small (20 proteins and 1 RNA) Large (30 proteins and 2 RNA) 3 sites on ribosome surface involved in protein synthesis E, P, and A sites
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Ribosome
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Prokaryotes tRNA (anti-codon) Complementary sequence Binds to mRNA tRNA carries a specific amino acid Adds to growing polypeptide 45 tRNA’s
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Prokaryotes Aminoacyl-t-RNA synthetases Activating enzymes Link correct tRNA code to correct aa One for each 20 amino acids Some read one code, some read several codes
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Prokaryotes Nonsense codes UAA, UAG, UGA code to stop AUG codes for start as well as methionine Ribosome starts at the first AUG it comes across in the code
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Prokaryotes Translation 1. Initiation 2. Elongation 3. Termination
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Prokaryotes Initiation Initiation complex 1. tRNA with formylmethionine attached binds to a small ribosome 2. Initiation factors position the tRNA on the P site 3. A site (aminoacyl) where other tRNA’s form
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Prokaryotes 4. tRNA is positioned on to the mRNA at AUG 5. Attachment of large ribosomal unit
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Prokaryotes Elongation factors Help second tRNA bind to the A-site Two amino acids bind (peptide bond) Translocation: Ribosome moves 3 more nucleotides along mRNA in the 5’to 3’ direction
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Prokaryotes Initial tRNA moves to E site Released New tRNA moves into A site Continues to add more aa to form the polypeptide
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Prokaryotes Release factors: Proteins that release newly made polypeptides Codon (UAG, UAA, UGA) Release factor binds to the codon Polypeptide chain is released from A site
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Eukaryotes Transcription (nucleus) Initiation Elongation Termination
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Eukaryotes Initiation Transcription Initiation Complex is formed Transcription factors bind first to the promoter RNA pol II binds DNA Starts to transcribe
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Fig. 17-7b Elongation RNA polymerase Nontemplate strand of DNA RNA nucleotides 3 end Direction of transcription (“downstream”) Template strand of DNA Newly made RNA 3 5 5
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Fig. 17-UN1 Transcription unit Promoter RNA transcript RNA polymerase Template strand of DNA 5 5 53 3 3
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Eukaryotes Termination Polyadenylation signal sequence Recognized by RNA polymerase II mRNA is released
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Eukaryotes mRNA is modified Nucleus RNA processing
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Eukaryotes 5’ cap Addition of a GTP 5’ phosphate of the first base of mRNA Methyl group is added to the GTP 3’poly-A-tail Several A’s on the end of the mRNA
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Eukaryotes Introns: non-coding sequences of nucleic acids Exons: coding sequences of nucleic acids
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Euraryotes RNA splicing Cut out introns Reconnect exons snRNP’s (small nuclear RNA’s) Spliceosome: Many snRNP’s come together & remove introns
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Eukaryotes Translation 1. Initiating aa is methionine 2. Initiation complex is more detailed
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Fig. 17-16b P site (Peptidyl-tRNA binding site) A site (Aminoacyl- tRNA binding site) E site (Exit site) mRNA binding site Large subunit Small subunit (b) Schematic model showing binding sites Next amino acid to be added to polypeptide chain Amino end Growing polypeptide mRNA tRNA EP A E Codons (c) Schematic model with mRNA and tRNA 5 3
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Second nucleotide
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Fig. 17-UN3 mRNA Ribosome Polypeptide
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Similarities DNA RNA Protein Transcription Translation
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Differences in gene expression Transcription 1. Prokaryotes one RNA polymerase Eukaryotes 3 RNA polymerases (poli-II mRNA synthesis) 2. Prokaryotes mRNA contain transcripts of several genes Eukaryotes only one gene 3. Prokaryotes no nucleus so start translation before transcription is done
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Differences in gene expression 3. Eukaryotes complete transcription before leaving the nucleus 4. Eukaryotes modify RNA Introns/exons 5. Prokaryotes Polymerase binds promoters Eukaryotes transcription factors bind first then enzyme 6. Termination
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Differences in gene expression Translation 1. Prokaryotes start translation with AUG Eukaryotes 5’cap initiates translation 2. Prokaryotes smaller ribosomes
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Mutations Changes in genetic information Point mutations: Change in a single base pair Sickle cell mutation
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Mutations Two types 1. Base-pair substitution Exchange one nucleotide and base pair with another Silent mutations No effect on proteins
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Mutations Missense mutations: Substitutions that change one aa for another Little effect
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Mutations Nonsense mutations Point mutation codes for stop codon Stops translation too soon Shortens protein Non-functional proteins
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Mutations 2. Insertions or deletions Additions or losses of nucleotides Frameshift mutations Improperly grouped codons Nonfuctional proteins
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Fig. 17-23 Wild-type 3 DNA template strand 5 5 5 3 3 Stop Carboxyl end Amino end Protein mRNA 3 3 3 5 5 5 A instead of G U instead of C Silent (no effect on amino acid sequence) Stop T instead of C 3 3 3 5 5 5 A instead of G Stop Missense A instead of T U instead of A 3 3 3 5 5 5 Stop Nonsense No frameshift, but one amino acid missing (3 base-pair deletion) Frameshift causing extensive missense (1 base-pair deletion) Frameshift causing immediate nonsense (1 base-pair insertion) 5 5 5 3 3 3 Stop missing 3 3 3 5 5 5 Stop 5 5 5 3 3 3 Extra U Extra A (a) Base-pair substitution(b) Base-pair insertion or deletion
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Mutagens Chemical or physical agents Mutations in DNA
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