生物化学资料：3-Protein synthesis.ppt

Translation system Synthesis of proteins in the cytoplasm Involves the followings: 1. mRNA (codons) 2. tRNA (anticodons) 3. rRNA and ribosomes 4. amino acids (building blocks) 5. enzymes and protein factors 6. energy TOPICS OUTLINE 1. The genetic code 2. Components required for translation 3. Steps in prokaryotic protein synthesis 4. Steps in eukaryotic protein synthesis 5. Posttranslational modification of polypeptide chain 6. Protein targeting  TOPIC 1 THE GENETIC CODE Messenger RNA (mRNA) mRNA: The template for protein synthesis mRNA carries information from DNA in a triple codon The main role of mRNA in protein synthesis: offering template Nirenberg’s Experiments 1961 If the synthesized poly U is added to an in vitro system for protein synthesis, the polypeptides synthesized are all phenylalanine. This study demonstrated that the codon encoding Phe is UUU. There are four different bases in DNA (and also in mRNA), so there are 4×4×4 or 64 possible triplets to be allocated, of which 61 specify amino acids and the remaining three determine peptide chain termination. Characteristics of the genetic code 2. Directionality: Open Reading Frame (ORF) From a specific initiation codon AUG to the Stop codon, excluding untranslated regions in mRNA. Each ORF encodes a polypeptide chain. 4. Degeneracy 5. Universality The genetic code is “universal” in most living organisms Most living organisms use the same basic genetic code. Evidence: Comparisons of DNA and protein sequence reveal perfect correspondence between codons and amino acids among most organisms. Translational systems can use mRNA from another organism to generate protein. The genetic code is (almost) universal 6. Wobble An anticodon is complementary to the codon representing its amino acid . The anticodon enables the tRNA to recognize the codon via complementary base pairing. Codon Wobble Summary for topic 1 Codon consists of a triplet. Each of the codons specifies an ami