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Chapter 17~ From Gene to Protein. Protein Synthesis: overview. One gene-one enzyme hypothesis (Beadle and Tatum) One gene-one polypeptide (protein) hypothesis Transcription : synthesis of RNA under the direction of DNA (mRNA)
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Protein Synthesis: overview • One gene-one enzyme hypothesis (Beadle and Tatum) • One gene-one polypeptide (protein) hypothesis • Transcription: synthesis of RNA under the direction of DNA (mRNA) • Translation: actual synthesis of a polypeptide under the direction of mRNA
The “Central Dogma” • Flow of genetic information in a cell • How do we move information from DNA to proteins? transcription translation RNA DNA protein trait DNA gets all the glory, but proteins do all the work! replication
aa aa aa aa aa ribosome aa aa aa aa aa aa From gene to protein nucleus cytoplasm transcription translation DNA mRNA protein trait
Transcription fromDNA nucleic acid languagetoRNA nucleic acid language
RNA • ribose sugar • N-bases • uracil instead of thymine • U : A • C : G • single stranded • lots of RNAs • mRNA, tRNA, rRNA, siRNA… transcription DNA RNA
Transcription • Making mRNA • transcribed DNA strand = template strand • untranscribed DNA strand = coding strand • same sequence as RNA • synthesis of complementary RNA strand • transcription bubble • enzyme • RNA polymerase coding strand 3 A G C A T C G T 5 A G A A A C G T T T T C A T C G A C T DNA 3 C T G A A 5 T G G C A U C G U T C unwinding 3 G T A G C A rewinding mRNA template strand RNA polymerase 5 build RNA 53
RNA polymerases • 3 RNA polymerase enzymes • RNA polymerase 1 • only transcribes rRNA genes • makes ribosomes • RNA polymerase 2 • transcribes genes into mRNA • RNA polymerase 3 • only transcribes tRNA genes • each has a specific promoter sequence it recognizes
Which gene is read? • Promoter region • binding site before beginning of gene • TATA box binding site • binding site for RNA polymerase & transcription factors • Enhancer region • binding site far upstream of gene • turns transcription on HIGH
Transcription Factors • Initiation complex • transcription factors bind to promoter region • suite of proteins which bind to DNA • hormones? • turn on or off transcription • trigger the binding of RNA polymerase to DNA
RNA polymerase Matching bases of DNA & RNA A • Match RNA bases to DNA bases on one of the DNA strands C U G A G G U C U U G C A C A U A G A C U A 5' 3' G C C A T G G T A C A G C T A G T C A T C G T A C C G T
Transcription: the process • 1.Initiation~ transcription factors mediate the binding of RNA polymerase to an initiation sequence (TATA box) • 2.Elongation~ RNA polymerase continues unwinding DNA and adding nucleotides to the 3’ end • 3.Termination~ RNA polymerase reaches terminator sequence
intron = noncoding (inbetween) sequence exon = coding (expressed) sequence Eukaryotic genes have junk! • Eukaryotic genes are not continuous • exons = the real gene • expressed / coding DNA • introns = the junk • inbetween sequence intronscome out! eukaryotic DNA
intron = noncoding (inbetween) sequence exon = coding (expressed) sequence mRNA splicing • Post-transcriptional processing • eukaryotic mRNA needs work after transcription • primary transcript = pre-mRNA • mRNA splicing • edit out introns • make mature mRNA transcript ~10,000 bases eukaryotic DNA pre-mRNA primary mRNA transcript ~1,000 bases mature mRNA transcript spliced mRNA
Discovery of exons/introns 1977 | 1993 Richard Roberts Philip Sharp adenovirus CSHL MIT common cold beta-thalassemia
Splicing must be accurate • No room for mistakes! • a single base added or lost throws off the reading frame AUGCGGCTATGGGUCCGAUAAGGGCCAU AUGCGGUCCGAUAAGGGCCAU AUG|CGG|UCC|GAU|AAG|GGC|CAU Met|Arg|Ser|Asp|Lys|Gly|His AUGCGGCTATGGGUCCGAUAAGGGCCAU AUGCGGGUCCGAUAAGGGCCAU AUG|CGG|GUC|CGA|UAA|GGG|CCA|U Met|Arg|Val|Arg|STOP|
snRNPs snRNA intron exon exon 5' 3' spliceosome 5' 3' lariat 5' 3' exon exon mature mRNA excised intron 5' 3' Whoa! I think we just brokea biological “rule”! RNA splicing enzymes • snRNPs • small nuclear RNA • proteins • Spliceosome • several snRNPs • recognize splice site sequence • cut & paste gene No, not smurfs! “snurps”
Alternative splicing • Alternative mRNAs produced from same gene • when is an intron not an intron… • different segments treated as exons Starting to gethard to define a gene!
More post-transcriptional processing • Need to protect mRNA on its trip from nucleus to cytoplasm • enzymes in cytoplasm attack mRNA • protect the ends of the molecule • add 5 GTP cap • add poly-A tail • longer tail, mRNA lasts longer: produces more protein
aa aa aa aa aa ribosome aa aa aa aa aa aa From gene to protein nucleus cytoplasm transcription translation DNA mRNA protein trait
Translation fromnucleic acid languagetoamino acid language
TACGCACATTTACGTACGCGG DNA AUGCGUGUAAAUGCAUGCGCC mRNA MetArgValAsnAlaCysAla protein ? How does mRNA code for proteins? How can you code for 20 amino acids with only 4 nucleotide bases (A,U,G,C)? 4 ATCG 4 AUCG 20
TACGCACATTTACGTACGCGG DNA AUGCGUGUAAAUGCAUGCGCC mRNA AUGCGUGUAAAUGCAUGCGCC mRNA codon MetArgValAsnAlaCysAla protein ? mRNA codes for proteins in triplets
Cracking the code 1960 | 1968 Nirenberg & Khorana • Crick • determined 3-letter (triplet) codon system WHYDIDTHEREDBATEATTHEFATRAT WHYDIDTHEREDBATEATTHEFATRAT • Nirenberg (47) & Khorana (17) • determined mRNA–amino acid match • added fabricated mRNA to test tube of ribosomes, tRNA & amino acids • created artificial UUUUU… mRNA • found that UUU coded for phenylalanine
1960 | 1968 Marshall Nirenberg Har Khorana
The code • Code for ALL life! • strongest support for a common origin for all life • Code is redundant • several codons for each amino acid • 3rd base “wobble” Why is thewobble good? • Start codon • AUG • methionine • Stop codons • UGA, UAA, UAG
GCA UAC CAU Met Arg Val How are the codons matched to amino acids? 3 5 TACGCACATTTACGTACGCGG DNA 5 3 AUGCGUGUAAAUGCAUGCGCC mRNA codon 3 5 tRNA anti-codon aminoacid
aa aa aa aa aa ribosome aa aa aa aa aa aa From gene to protein nucleus cytoplasm transcription translation DNA mRNA protein trait
Transfer RNA structure • “Clover leaf” structure • anticodon on “clover leaf” end • amino acid attached on 3 end
Loading tRNA • Aminoacyl tRNA synthetase • enzyme which bonds amino acid to tRNA • bond requires energy • ATP AMP • bond is unstable • so it can release amino acid at ribosome easily Trp C=O Trp Trp C=O H2O OH O OH C=O O activating enzyme tRNATrp A C C mRNA U G G anticodon tryptophan attached to tRNATrp tRNATrp binds to UGG condon of mRNA
Ribosomes • Facilitate coupling of tRNA anticodon to mRNA codon • organelle or enzyme? • Structure • ribosomal RNA (rRNA) & proteins • 2 subunits • large • small E P A
Ribosomes • A site (aminoacyl-tRNA site) • holds tRNA carrying next amino acid to be added to chain • P site (peptidyl-tRNA site) • holds tRNA carrying growing polypeptide chain • E site (exit site) • empty tRNA leaves ribosome from exit site Met C A U 5' G U A 3' E P A
3 2 1 Building a polypeptide • Initiation • brings together mRNA, ribosome subunits, initiator tRNA • Elongation • adding amino acids based on codon sequence • Termination • end codon release factor Leu Val Ser Met Met Ala Leu Met Met Leu Leu Trp tRNA C A G C G A C C C A A G A G C U A C C A U A U U A U G A A 5' 5' A A 5' C U U 5' A A G G A G U U G U C U U U G C A C U 3' G G U A A U A A C C mRNA 3' 3' 3' U G G U A A 3' E P A
Protein targeting • Destinations: • secretion • nucleus • mitochondria • chloroplasts • cell membrane • cytoplasm • etc… • Signal peptide • address label start of a secretory pathway
RNA polymerase DNA Can you tell the story? aminoacids exon intron tRNA pre-mRNA 5' GTP cap mature mRNA aminoacyl tRNAsynthetase poly-A tail 3' large ribosomal subunit polypeptide 5' tRNA small ribosomal subunit E P A ribosome
Prokaryotes DNA in cytoplasm circular chromosome naked DNA no introns Eukaryotes DNA in nucleus linear chromosomes DNA wound on histone proteins introns vs. exons intron = noncoding (inbetween) sequence exon = coding (expressed) sequence Prokaryote vs. Eukaryote genes intronscome out! eukaryotic DNA
Translation in Prokaryotes • Transcription & translation are simultaneous in bacteria • DNA is in cytoplasm • no mRNA editing • ribosomesread mRNA as it is being transcribed
Translation: prokaryotes vs. eukaryotes • Differences between prokaryotes & eukaryotes • time & physical separation between processes • takes eukaryote ~1 hour from DNA to protein • no RNA processing
Mutations • Point mutations • single base change • base-pair substitution • silent mutation • no amino acid change • redundancy in code • missense • change amino acid • nonsense • change to stop codon When do mutationsaffect the nextgeneration?
Point mutation leads to Sickle cell anemia What kind of mutation? Missense!
Sickle cell anemia • Primarily Africans • recessive inheritance pattern • strikes 1 out of 400 African Americans hydrophilicamino acid hydrophobic amino acid
Mutations • Frameshift • shift in the reading frame • changes everything “downstream” • insertions • adding base(s) • deletions • losing base(s) Where would this mutation cause the most change:beginning or end of gene?
Cystic fibrosis • Primarily whites of European descent • strikes 1 in 2500 births • 1 in 25 whites is a carrier (Aa) • normal allele codes for a membrane protein that transports Cl- across cell membrane • defective or absent channels limit transport of Cl- (& H2O) across cell membrane • thicker & stickier mucus coats around cells • mucus build-up in the pancreas, lungs, digestive tract & causes bacterial infections • without treatment children die before 5; with treatment can live past their late 20s
Deletion leads to Cystic fibrosis delta F508 loss of oneamino acid