RNA Splicing s mall n uclear r ibo n ucleo p roteins = snRNPs snRNP = snRNA + protein Pronounced “ snurps ” Recognize splice sites snRNPs join with other proteins to form a spliceosome Spliceosomes catalyze the process of removing introns and joining exons Ribozyme = RNA acts as enzyme (catalytic role)
Why have introns? Some regulate gene activity Alternative RNA Splicing : produce different combinations of exons One gene can make more than one polypeptide! 20,000 genes 100,000 polypeptides
Concept 14.4: Translation is the RNA-directed synthesis of a polypeptide
Components of Translation 1. mRNA = message 2. tRNA = interpreter 3. Ribosome = site of translation
Summary of Translation
tRNA Transcribed in nucleus Specific to each amino acid Transfer AA to ribosomes Anticodon : pairs with complementary mRNA codon Base-pairing rules between 3 rd base of codon & anticodon are not as strict. This is called wobble .
tRNA Aminoacyl-tRNA-synthetase : enzyme that binds tRNA to specific amino acid
Ribosomes Ribosome = rRNA + proteins made in nucleolus 2 subunits
Ribosomes Active sites : A site : holds AA to be added P site : holds growing p olypeptide chain E site : e xit site for tRNA
Translation: 1. Initiation • Small subunit binds to start codon (AUG) on mRNA • tRNA carrying Met attaches to P site • Large subunit attaches
2. Elongation Codon recognition : tRNA anticodon matches codon in A site
2. Elongation Peptide bond formation : AA in A site forms bond with peptide in P site
2. Elongation Translocation : tRNA in A site moves to P site ; tRNA in P site moves to E site (then exits)
3.Termination Stop codon reached and translation stops Release factor binds to stop codon; polypeptide is released Ribosomal subunits dissociate
Protein Folding During synthesis, polypeptide chain coils and folds spontaneously Chaperonin : protein that helps polypeptide fold correctly
Post-Translational Modifications Attach sugars, lipids, phosphate groups, etc. Remove amino acids from ends Cut into several pieces Subunits come together Insulin Production
Types of Ribosomes Free ribosomes : synthesize proteins that stay in cytosol and function there Bound ribosomes (to ER): make proteins of endomembrane system (nuclear envelope, ER, Golgi, lysosomes, vacuoles, plasma membrane) & proteins for secretion Uses signal peptide to target location
Cellular “ Zip Codes ” Signal peptide : 20 AA at leading end of polypeptide determines destination Signal-recognition particle (SRP) : brings ribosome to ER
Polyribosomes A single mRNA can be translated by several ribosomes at the same time
Prokaryotes can transcribe + translate
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