mid 28 - Eukaryotic Translation Initiation Elongation...

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Eukaryotic Translation • Initiation • Elongation • Termination Ch.4: pp 123-131s Ch12: pp 515, 523 Ch16: pp 657-661
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In eukaryotes, transcription and translation are separated in space and time
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Eukaryotic Translation • Genetic code: identical • tRNAs: same; charged by 20 different aminoacyl-tRNA synthetase (ARS) • Ribosomes: larger subunits but identical functions • More factors are involved in eukaryotic translation • With the exception of translation initiation, the other steps are similar to prokaryotic translation.
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• Ribosomes are large (10 3 kDa) RNA-protein complexes composed of two subunits. More than 80% of cellular RNA is ribosomal RNA • The large subunit (60S) is composed of 3 rRNAs and 50 proteins • The small subunit (40S) is composed of 1 rRNA and 33 proteins Composition of Ribosomes
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Eukaryotic Ribosomes Cytoskeleton Ribosomes Centriole Lysosome Flagellum Not in most plant cells Nucleus Smooth endoplasmic reticulum (ER) Golgi apparatus Rough endoplasmic reticulum (ER) Mitochondrion Plasma membrane • Ribosomes are found in two places: –Membrane attached •Are bound to the rough endoplasmic reticulum (ER) •Proteins that are in the secretory pathway are synthesized by these ribosomes –Membrane unattached •Are free in the cytosol •Proteins in the non- secretory pathway are synthesized by these ribosomes A mammalian cell contains up to 10,000 different proteins.
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Eukaryotic Ribosomes
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“Free” ribosomes: • Electron micrograph of “free” ribosomes in the cytosol • Not attached to ER membrane • Synthesized proteins likely released into cytosol where they will remain, or will be transported to the nucleus, or incorporated into proper organelle Eukaryotic Ribosomes
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Eukaryotic Ribosomes Where does translation take place?
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Fig. 4-26 Ribosome Growing polypeptide chain tRNA mRNA tRNA 4 leaving tRNA 7 arriving Translation: Protein Synthesized by Ribosomes
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General Overview of Translation: Occurs in Three steps 1. Initiation - finding the start codon and assembling the ribosomal subunits 2. Elongation - reading the mRNA sequence and polymerizing the addition of corresponding amino acids to growing polypeptide chain 3. Termination - recognition of the stop codon and release of the new polypeptide
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The ribosome has three important sites 3 Sites of Ribosome: – A site – P site – E site
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