A ribosome has four binding sites: the A-site, P-site and E-site work with tRNAs and the fourth works with mRNAs. The A-site and P-site are close to each other and thus force their two tRNA molecules to form base-pairs with now-adjacent codons on the mRNA molecule (the strict utilization of two sites allows for wobble at the third position). In two translocation steps, three nucleotides are processed and the cycle is repeated.
When the two subunits are joined, together they have a mass of several million daltons.
Slides R-1, R-2, R-3, R-4, R-5, R-6, R-7, R-8, R-9, R-10, R-11
1. Initiation and termination of peptide chain in translation- start and stop codon role in this process.
The codon AUG starts the RNA translation process. AUG translates as methionine (in bacteria, it is formylmethionine). Hence, the front/first end of the protein – the N-terminus – always has methionine. Proteases subsequently remove this methionine. In Eukaryotes, this methionine enters the small ribosomal subunit along with eukaryotic initiation factors (knows as “eIFs”). (0 percent of the time, translation begins at the first AUG codon encountered, and, when this is encountered, the two subunits assemble. The initiator tRNA joins with the P-site and at this point, protein synthesis is ready to begin.
Any one of three different codons signals the end of translation: UAA, UAG or UGA. When one of these stop codons enters the A-site, instead of synthesizing an amino acid, the ribosome synthesizes a water molecule. The synthesis of the water molecule releases the carboxyl end of the assembled polypeptide chain from the tRNA molecule and thus releases the polypeptide chain/protein into the cytoplasm. After the polypeptide chain is released, the ribosomal subunits separate and are now able to rejoin with a new mRNA and produce another protein.
Slides: T-12, T-21, T-22, T-23
Saturday, June 26, 2010
Things by Louis J. Sheehan, Esquire
Why was I locked out? Google seems to read too much into things.
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