Your lab director requests that you add new growth medium to the mammalian cell cultures before heading home from the lab on a Friday night. Unfortunately, you need to make fresh medium because all the premixed bottles of medium have been used
One of the ingredients you know you need to add is a mix of the essential amino acids (those that cannot be made by the cells, but are needed in proteins). On the shelf of dry chemicals you find the amino acids you need, and you mix them into your medium, along with all the other necessary nutrients, and replace the old medium with your new medium. On Sunday, you come in to the lab just to check on your cells and find that the cells have not grown. You are sure you made the medium correctly, but on checking you see that somebody wrote a note on the dry mixture of amino acids you used: "Note: this mixture contains only ?-amino acids."
A. What is the meaning of the note and how does it explain the lack of cell growth in your culture?
B. Are there any organisms that could grow using this mixture? Justify your answer.
A. The note indicates that the mixture contains only one of the two possible stereoisomers (? or ?). Because mammalian cells use only the ? stereoisomer, the ?-amino acid mixture could not be used and therefore it is as though no amino acids were added at all.
B. Not unless ?-amino acids were also mixed in. Certain types of bacteria use ?-amino acids to produce their cell walls, but they would still require ?-amino acids for the rest of the proteins they make.
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