tRNA types

[gonelli]

When it comes to tRNA molcules involved with translation, they each will have a certain anticodon so they can put in their amino acid when the corresponding mRNA codon comes up.

 

But how many different types of tRNA molecules would there be? Because you can say there are 64 (4 x 4 x 4), however are there ones for the 3 stop codons?

 

So I'm guessing either 64 or 61 (i'm leaning more towards 61), but can anyone help me out here?

[carol]

I think there would be 61 types of tRNA. A release factor binds instead of a tRNA when the stop codon is reached.

[muchado]

Hypothetically there should be 61 types of tRNA. However, to this date reports from various eukaryotic organisms suggest there are far less then 61 types of tRNA. The reason behind that is the ability to wobble base pair and the presence of modified bases. Wobble base pairing means that one of the three bases in a given codon does not have to follow watson-crick base pairing rule. We also have non-conventional base like inosine that can base pair with A, U, or C. So how many you ask? Most people will just say >20 to stay on the safe side:)

[lucaspa]

According to Lehninger's Biochemistry (a standard college textbook on the subject), "there is more than one specific tRNA for each amino acid." This varies from species to species. "altogether, E. coli cells contain some 80 different tRNAs for the 20 amino acids."

 

This means that sometimes there is more than 1 tRNA for a specific codon sequence. For instance, yeast have 5 different tRNAs for serine, but there are only 4 codon sequences that code for serine. There are 4 tRNA's for lysine, but only 2 codons for lysine. On the other side, there are 5 tRNAs for leucine but 6 codons for leucine. So there isn't a one-to-one correspondence between number of codons and number of tRNAs.