# Thread: How many different tRNA molecules do humans have?

1. Hey there, I just found this forum in hope that someone may be able to answer my question. I was wondering how many different tRNA molecules we have an if possible could you include a reference to where you got your answer from? There's a lot of answers flying around without any sources included.

I know a lot of people state that the it's 64, but due to the wobble hypothesis, I am quite reluctant to take this as a definite answer, especially since in other species it's been found to be lower than this.

Any help would be greatly appreciated!

2.

3. tRNA is supposed to transport amino-acids to the ribosomes. The ribosomes construct proteines from those acids by combining them in the sequence which is stated by the mRNA.

The ribosome glides along a string of mRNA and reads the sequence of small nucleotides. From the sequence of nucleotides, it 'knows' which amino acid it needs to add to the proteine: very much like when you construct something from LEGO, you look at your building plan and then rummage through your LEGO box to find the right and fitting piece.

The mRNA actually consists of a single string of Nucleotides (it's an inverse copy of a piece of DNA which is a winded double string, we call that helix form.)
In the mRNA, always three nucleotides in a row stand for one amino acid. That's just as if you had numbered all the different Legos and put them into many boxes with that number on. When you are looking for one certain piece, you look at the number and find the right box easily.

Now the thing is, that mRNA can consist only of 4 different nucleotides, combined in different sequences. That's just as if you had only four lettres to mark the boxes of your lego. Now, the thing is that if you have 21 different lego pieces, how many lettres would you have to write on your boxes in order to distinguish every single sort of lego pieces without having to remember too any combinations of lettres?

For 21 different lego piece sorts, it's 3 lettres: if you put 4 different lettres (nucleotides) into all the different possible combination while the 'words' that you build consist of only three lettres, you will find that there are 64 possible different words that you can build.

If we made our words only 2 lettres long, you could only have 8 lego boxes, because you can only build 8 different words if one word is 2 lettres long and may consist only of 4 different lettres.

So, when nature needed to write down a building plan of proteins, it needed to actually find a different name for each amino acid that could be encoded with a sequence of nucleotides. And to encode (name) 21 different nucleotides, you need 21 different combinations of those 4 available nucleotides. And as we know now, you need a length of 3 to build enough different combinations to name every one of those 21 amino acids. Of course, from those possible 64 names, nature needs only 21 for a human being. So it just gave every one of those 21 different amino acids more than one name for each. You can look that up on wikipedia, the names for the amino acids that are built of 3 nucleotides each (they are called 'codons') are neatly put into a table there.

So to come to the point: if we have 64 different names for our 21 different amino acids, there should be 64 different tRNAs. Because the tRNA actually has to match the coded sequence on the mRNA that is held in the grips of the ribosome. If it matches the codon, the tRNA releases the amino acid that it carries on its back, and the Ribosome takes this amino acid and adds it to the Proteine.

So I'd find it totally logical if there were 64 different tRNA's in a human body.
Maybe there are more or less in a different life form. The reason would be, that the different life form would have a different number of amino acids, so there wouldn't be as many different codons needed, or by contrary more codons would be needed, or maybe that life form doesn't use multiple codons for each amino acid.

I must say that it would need to actually have read a book about the transcription and translation process of genes, or at least to watch many vids on youtube concerning that topic. I can't find a reference that would really explain all that in one single short page. I recommend Campbell's book 'Biology'. It's big, but also very, very interesting.

4. To explain the process some more, we can say that if we want to construct something of LEGO, an airplane for example, we look into our book shelve that's full of file folders. We take one of the file folders, open it, make a quick copy of the building plan to take it to our LEGO building site, and then we rummage throug our LEGO box to find the needed pieces of LEGO and construct our small LEGO airplane.

Nature does just the same: it stores the building plans for proteins in big different folders (Genes contained in the DNA), and if one particular proteine is needed, it makes a copy of one of the building plans (mRNA) and takes it to the building site (ribosome). The ribosome reads the mRNA and accepts the right amino acid from the tRNAs that each carry a different amino acid. That's the process that enables our body to build proteins and at same time, store the building plan for many, many different proteins in a very compact form.

5. Thank you very much for taking the time to answer my question I understand the whole concept of tRNA carrying amino acids to the elongating polypeptide chain, but I was just wondering how many different tRNAs there are. You've state 64 based on the fact that there are 64 different codons which can be made using a combination of 3 different nucleotide bases with the availability of 4 different bases, however there are only 20 amino acids and so many of these codes are degenerate; some being twofold degenerate, threefold degenerate etc. and so there is no need to have 64 different tRNA molecules.

I was wondering if any research has been done to find out exactly how many tRNA species actually exist in our cells.

Thanks

6. Around 500, by current estimates, classified into about 50 isoacceptor families.

ref: BioNumbers

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