PROTEIN SYNTHESIS Protein Synthesis Process where DNA encodes for the production of amino acids and proteins. Protein synthesis starts in the nucleus, where DNA is held and ends in the cytosol, where proteins are made. It is divided into two main parts: transcription and translation Transcription Process where DNA is used as a template for the production of mRNA (mRNA is transcribed from DNA) Transcription has three phases: Initiation, elongation and termination 1. Initiation RNA polymerase binds to DNA at the promoter region (high in A and T, takes less energy to break these bonds) As in DNA replication, the DNA is "unzipped" by the enzyme helicase, leaving the single nucleotide chain open to be copied. When DNA is unwound it exposes the template strand The part of DNA not being used to make mRNA is called the coding strand 2. Elongation RNA polymerase reads the DNA strand and synthesizes a single strand of mRNA in the 5’ to 3’ direction Uracil complements adenine (instead of thymine) As DNA is transcribed, it goes back into double-helical form 3. Termination When RNA polymerase reaches the termination sequence at the end of the gene, RNA synthesis stops mRNA and RNA polymerase are released mRNA is not yet ready to leave the cell. First it has a 5’ cap added to protect it and initiate translation. Also, a 3’ end is added containing a polyA tail (200 – 300 adenine base pairs). This single strand of mRNA then leaves the nucleus through nuclear pores, and goes into the cytoplasm where it joins with ribosomes, where protein synthesis occurs by the formation of peptide bonds and polypeptide chains. Translation The process of converting the mRNA codon sequences into an amino acid polypeptide chain. It also has 3 stages: initiation, elongation and termination The making of an amino acid takes place on the ribosomes. Ribosomes consist of two subunits (large and small). The two subunits bind to mRNA and clamp the mRNA between them. The ribosome moves along the mRNA in the 5’ to 3’ direction, adding a new amino acid each time it reads a codon. 1. Initiation A ribosome attaches to the mRNA and starts to code at the start codon (AUG). 2. Elongation tRNA brings the corresponding amino acid to each codon as the ribosome moves down the mRNA strand (See handouts, you should know these handouts for the test) 3. Termination Reading of the final mRNA codon (aka the STOP codon, UAG, UAA, UGA), which ends the synthesis of the peptide chain and releases it. http://www.accessexcellence.o rg/RC/VL/GG/protein_synthesi s.php Protein Synthesis Video http://www.wisconline.com/objects/index_tj.asp?objID =AP1302 Questions on Handout P. 241 Questions 1, 2, 3, 4, 9, 10, 11, 13 Readings P. 237 – 241 P. 242-244 P. 250-253