ch369_f09_transl_notes

ch369_f09_transl_notes - Chapter 20 Protein synthesis...

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Chapter 20 - Protein synthesis Central dogma of molecular biology: 3 base pairs DNA ==> 3 bases of mRNA ==> 1 amino acid
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Translation ” refers to the process of protein synthesis, where a ribosome reads a messenger RNA (mRNA) and synthesizes a protein.
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Stages of translation: Initiation - mRNA binds to the small subunit of ribosome, and then the large subunit joins. Elongation - The protein is synthesized by the ribosome, using amino acids delivered by tRNA. Termination - The newly synthesized protein is released from the ribosome.
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Genetic code: DNA structure was known by 1953, but how it coded for proteins was unknown. Artificial mRNA templates were made and used to break the code. For example, it was observed in 1961 that poly U made poly- phenylalanine. poly U => poly Phenylalanine poly G => poly Glycine Poly A => poly Lysine poly C => poly Proline
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Interestingly, a change in the 3rd nucleotide often does not change the amino acid, or changes to a similar a.a. Some amino acids have one codon, some have 3, 4, or 6 codons.
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Reading frames. Notice there are 3 different reading frames for the triplet code. The a.a. translated depends on where the ribosome starts. Can you see how a mutation that deletes a single nucleotide could change the identities of all the amino acids in a protein?
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All tRNAs have a similar cloverleaf structure. All end in -CCA at their 3’ end (where the amino acid is attached). Amino acids are delivered to the ribosome attached to the 3’ end of transfer RNA (tRNA) at the acceptor stem . The anticodon loop of tRNA reads the mRNA.
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tRNAs contain some modified nucleotides (dihydrouridine, etc.)
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Steps in translation Amino acid activation - attaching the a.a. to the 3’ end of tRNA. Initiation - find the start codon, and start translation. Elongation - add a.a. to the growing peptide chain. Termination - At the stop codon, release the ribosome from mRNA.
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Enzymes called “ aminoacyl tRNA synthetases ” link the a.a.
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