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Lecture 14

BIO206 Lecture 14 Chapter 7.pdf

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George S Espie

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Lecture 14/ Chapter 7 November-04-13 8:05 PM tRNA Non-standard bases & base pairing „Charging tRNA‟ aminoacyl tRNA synthetases Ribosomes Composition and structure, rRNA transcription Functional sites: A, P, E, EF-Tu, EF-G, mRNA tRNA & rRNA transcript processing Ribosomes: Protein-Manufacturing Machines • Synthesis and subunit assembly in nucleus • Subunits transported to cytoplasm • Typical cell contains millions of ribosomes • Free in cytoplasm or attached to ER - can see the small grains that are the ribosomes • need lots of ribosomes as the cell continuously is making thousands of proteins Sites of protein synthesis where ribosomes are found Lot of biosynthesis is required to make necessary ribosomes Ribosome Location • Subunit pool in cytoplasm • Assembly with mRNA • Remain free in cytosol if not in “use” • After translation dissociate and return to pool • - ribosome subunits are dissociated from each other in the cytoplasm pool. THE ONLY RIBOSOMES ATTACHED TO ER ARE ACTUALLY SYNTHESIZING PROTEIN - note that the reaction is actually taking place in the cytoplasm in all cases (we will see later how transfer into the ER takes place) - the complex for protein synthesis is assembles and translation occurs either in cytosol or into ER depending on PROPERTIES OF THE mRNA - ribosomes subunits dissociate and are returned to the pool where they can start again in the cycle. Ribosome Composition • Synthesis in 3 steps • Synthesis of proteins and rRNA • rRNA and proteins assemble to subunits • Subunits assemble to form ribosome complex • - again stress that a ribosome is a complex assembly of many molecules, multi-step process to assemble. this is a schematic of eukaryotic ribosomes, the inset picture is prokaryotic. • representation is stylized but useful to talk about how it functions Ribosomes are relatively large structures - Includes rRNA and different proteins Prokaryotic and Eukaryotic Ribosomes REVIEW can see that the eukaryotic ribosome is larger and more complex (more rRNAs as well as more proteins) more proteins) A problem S = svedberg unit= a measure of how well a molecule move in centrifugal force field (represent where they sediment in the force field) - It is not additive - THIS IS AN E. COLI RIBOSOME - similar in eukaryotes small subunit head, base, cleft, platform - large subunit ridge, valley, stalk, central protuberance - when they join you can see a “hole” up the centre (look at later) - electron micrograph you can see the structure of the small subunit on the right. - mRNA interacts with the ribosome in specific binding, -there are 3 sites where tRNA can bind. Stylized version is good for discussing function but we will look at the actual shape first Can see the A site, the P site and the E site on left are represented by specific colours on the right - A is pink, P is green, E is yellow Red chain of beads is the mRNA strand threaded through the sites EF-Tu: where protein that binds before the A site in the smaller subunit - from the different angle you can see the “hole” or cavity in the ribosome that shelters the reactions in translation from the cytoplasm. Structures reflect the function of each subunit - small matches the mRNA, so it holds the mRNA large subunit catalyzes bond formation, its channel holds the peptide chain. up to 35 amino acids can fit in the large subunit channel Ribosome in process of translating mRNA. Ribosome binds to the mRNA at the 5‟ end and 5‟ 3‟ New polypeptide is from N terminal T terminal 4 major classes of RNA the first 3 (Red Bracked) are involved in
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