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Lecture 5

BIOSC 0160 Lecture 5: Biology 2 Lecture 1.5: Translation

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Biological Sciences
BIOSC 0160

Lecture 1.5 12417 How do mRNAs direct protein synthesis? Ribosomes: Organelles where protein synthesis occurs, consists of ribosomal RNA and protein o Small subunit and large subunit, mRNA binds in between o Prokaryotic ribosomes are smaller than eukaryotic ribosomes and contain some unique proteins and rRNA Translation in prokaryotes can occur at the same time as transcription because of the lack of a nucleus everything is in the cytosol Multiple ribosomes can attach to a single mRNA forming a polyribosome, attaching to the 5 end and read 5 to 3 o mRNA features important for nuclear export: PolyA tail and 5 cap Export proteins bind to mRNA ProteinRNA complex (mRNP) docks, passes through nuclear pores and then dissociates o If its longer, its the 3 end How does an RNA codon specify an amino acid? Crick: disregarded hypothesis that mRNA interacts directly with amino acids. Not all amino acids can form hydrogen bonds o Adapter molecules hold amino acids and interact with mRNA codons o Discovered unknown RNAs through in vitro translation experiments o This discovery supported the discovery of tRNA which transferred amino acids to proteins Intramolecular basepairing results in a clover shaped stem and loop secondary structure Anticodon base pairs with mRNA on one loop, amino acid is attached to the 3 end Ester linkage joins carboxyl group of the amino acid to the free 3 hydroxyl of the tRNA Tertiary structure looks like an upsidedown L When amino acid is attached, it is called an aminoacyl tRNA AminoacyltRNA synthetases charge tRNAs by catalyzing the ester linkage between tRNA and the amino acid requires input of ATP We need 61 proteincoding tRNAs, but there are only 40 o Crick: proposed wobble in the third position which allows for nonstandard base pairing; allowing fewer tRNAs to cover more codons Experimental evidence supports this Anticodon G can pair with U or C Anticodon C can pair with G Anticodon A can pair with U Anticodon base U can pair with A or G Anticodon base I can pair with U, C, or A
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