bio1a handout2 - Bio1A Fall 09 RIBOSOMES Sections 115, 120,...

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Bio1A Fall ‘09 Sections 115, 120, 121 GSI: Amiee Ho email: amieeho@berkeley.edu Handout #2 RIBOSOMES Ribosome Composition Large subunit Small subunit # RNAs # proteins # RNAs # proteins Ribosome MW Eukaryote 3 49 1 33 4.2 x 10^6 Prokaryote 2 34 1 21 2.5 x 10^6 Ribosome characteristics: Subunits (large and small) come together when making proteins and then separate when it’s done making proteins ***What is the structural difference between free ribosomes and bound ribosomes?? Two types of translation via ribosomes: 1. Co-translational (Fig 45) a. Occurs on rough ER b. Ribosome sits on MB of ER – as protein is being made, it is inserted into the lumen of ER there is NEVER a free protein in cytosol that goes into lumen of rough ER c. After protein is done – ribosomes dissociate and go away 2. Post-translational (Fig 45) a. Made by free ribosomes in the cytosol b. Synthesis of proteins that are in the cytosol, nucleus, mitochondrion, chloroplast and peroxisome i. Free ribosomes in cytosol and then transported to these organelles c. mRNA runs through the ribosome – as RNA runs through ribosome, the protein is being made – once made, then it is free and then it will be imported into an organelle d. For mitochondria, chloroplast and peroxisomes proteins : i. Proteins destined to organelles have a SIGNAL sequence – at the N terminal end (not always but most of the time) there is 10-20aa’s has info that direct protein to various organelles (Table 47) 1. When protein enters organelle the signal sequence is clipped off and you have the mature protein e. For nucleus proteins: Proteins are made with free ribosomes in cytosol, then reactions with protein IMPORTIN – this complex goes into nucleus through nuclear pore Importin separates once it gets in and then goes back into the cytosol ***What kind of signal sequence do cytosolic proteins have?? Endomembrane system Membranes related by direct physical contact or by transfer of components via vesicles o Note:vesicles are membrane bound sacs Components include: nuclear MB , endoplasmic reticulum (smooth and rough), golgi complex, lysosomes, vacuoles (plants), plasma membrane o Nuclear MB and ER have a continuous endomembrane Rough ER :
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bumps on the ER MB are from the bound ribosomes and give it a “rough” appearance Rough ER is involved in the synthesis of proteins It is involved in CO - TRANSLATIONAL import (Fig 45) Fate of proteins in Rough ER : Proteins in rough ER lumen are glycosylated (sugar added to protein) – this directs them to different places in the cell Proteins are then transported (fig 49) from ER lumen to golgi complex o transported via vesicles (MB bound sacs) Smooth ER : lipid biosynthesis (steroid hormones), polysaccharide break down, breakdown of other compounds like drugs
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bio1a handout2 - Bio1A Fall 09 RIBOSOMES Sections 115, 120,...

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