ID #2247

In lecture 20, Prof Mehrtens said that we take " the many and add them to the one", he's referring to the chain of amino acids attaching to the single amino acid. However, one of the slides from lec 20 shows an arrow of the single amino terminus pointing to the C terminus of the polypeptide chain. If we are adding the polypeptide chain to the single amino acid wouldn't that be C->N direction rather than the N->C direction it needs to be synthesized in? I'm confused because poly-> single is contradictory to the amino ->carboxyl direction?

Protein synthesis always occurs in the N-->C direction. 
Prof. Mehrtens did say that you take the many (ie. polypeptide chain attached to a tRNA at the P site) and add the one (ie. incoming amino acid attached to a tRNA at the A site). The slide that you are referring to from lec 20 focuses on the amino acids only, which shows the peptide bond is formed between amino terminus of the amino acid attached to the tRNA at the A site and the carboxyl terminus of the last amino acid in the polypeptide chain attached to the tRNA at the P site. Even though the slide doesn't show the tRNA's it's important to think of this process in that context. 

It's also important to remember that each amino acid individually has an amino group (amino end) and a carboxylic group (carboxyl end) like we discussed for Exam 1 material. And, you can can discuss the polypeptide chain and ultimately the final protein as having its own amino end (or N-terminus) and carboxyl end (or C-terminus). 

For that reason the arrow on one of the slide from lecture 20 and the statement that protein synthesis occurs N-->C is not contradictory. The arrow is showing the process of peptide bond formation between the respective amino and carboxyl ends of the amino acids, but due to that peptide bond being formed, the direction of protein synthesis overall is N-->C because we started with the first amino acid drawn all the way on the left of that slide, which now exists at the polypeptide's N-terminus and we just added the final amino acid by creating that peptide bond, which now exists at the polypeptide's C-terminus (so N-->C). 

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