Altius FL #6 C/P #21

[Drmcat@]

Hey guys,

I have a hard time understanding why D instead of C is the correct answer. I know that one anticodon (tRNA) can recognize and base pair with multiple codons on mRNA --> this is what Wobble Hypothesis is about. However, I do not get why that would make one amino acid bind to multiple tRNA molecules. Any explanation would be helpful. Thanks in advance!

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[Doogie.Howser]

If there are 20 amino acids, and 63 possible codons, then one amino acid can bind to multiple tRNAs.

For example, the amino acid phenylalanine can bind to the tRNAs with anticodons UUU and UUC. One amino acid for multiple tRNAs.

Think about what would happen if C were correct. If more than one AA can bind to a single tRNA, then arbitrarily the anticodon UUU could wield a phenylalanine, serine, or cysteine.

Make sense?

 

[Drmcat@]

Thanks for your reply. Yes this makes sense now that I thought about what you just said.

But is the actual number of tRNAs 20? Because, to me, it sounds like if there were 63 possible tRNAs matching those 63 codons that you talked about, then each aa must pair with more than one of these 63 tRNAs. But the question stem says there is only 20 tRNAs, which is a bit confusing. How would one aa bind to more than one tRNA if there is only 20 aa and 20 tRNAs??

 

[Doogie.Howser]

The actual number of tRNAs is 63 - one for each codon.
Yes, (almost) every AA does pair with more than one of the 63 tRNAs.

The question tries to throw off the reader. It states that there are 20 aminoacyl-tRNA synthetase enzymes. This is just the enzyme that charges each tRNA with it's respective AA. There is one enzyme for each amino acid, so there are 20 of them.

Does that help?

 

[Drmcat@]

OMG totally missed that. I have been thinking it says 20 tRNA molecules the whole time, which confused the heck out of me. That makes total sense now.
Thanks so much for the clarification!

 

[Doogie.Howser]

Good luck!