tRNA: Role, Function & Synthesis
This lesson focuses on transfer RNA (tRNA). It covers what tRNA is, what it does in our cells, and how it is made. It also gives a brief description of the history and discovery of tRNA.
What Is tRNA?
Transfer RNA, or tRNA, is a member of a nucleic acid family called ribonucleic acids. RNA molecules are comprised of nucleotides, which are small building blocks for both RNA and DNA. tRNA has a very specific purpose: to bring protein subunits, known as amino acids, to the ribosome where proteins are constructed.
One of the discoverers of DNA, Francis Crick, first suggested the existence of tRNA. At the time, scientists knew that genetic information was kept in the nucleus as DNA and that DNA carried the instructions on how to make proteins. DNA doesn’t leave the nucleus, though, so our cells make a copy of the DNA called messenger RNA, or mRNA.
mRNA leaves the nucleus and is bound by ribosomes, the molecular machines that act as the factory that makes proteins. Scientists understood that while DNA and RNA have almost the same alphabet, proteins are very different. Francis Crick proposed that there must be a small molecule capable of translating mRNA into proteins. Other scientists proved his theory with the discovery of tRNA.
Figure 1: The structure of tRNA
Figure 1: image of tRNA structure
Function of tRNA
The job of tRNA is to read the message of nucleic acids, or nucleotides, and translate it into proteins, or amino acids. The process of making a protein from an mRNA template is called translation.
How does tRNA read the mRNA? It reads the mRNA in three-letter nucleotide sequences called codons. Each individual codon corresponds to an amino acid. There are four nucleotides in mRNA. If you do the math to figure out how many different codons exist, you arrive at 64, or four cubed (4^3). There is one tRNA molecule for each and every codon.
Interestingly, there are only 21 amino acids. This brings up the idea that our genetic code is redundant. That is, we have 64 codons but only 21 amino acids. How do we resolve this? More than one codon can specify for an amino acid.
This table (Figure 2) shows all the combinations of nucleic acids, or codons, as well as which amino acid is specified by which codon. As you can see, not every amino acid has four codons. In fact, methionine only has one.
Notice, however, that each codon has only one corresponding amino acid. Thus, we say that the genetic code is redundant, but not ambiguous. For example, the codons GUU, GUC, GUA, and GUG all code for Valine (redundancy), and none of them specify any other amino acid (no ambiguity).
Figure 2: The table of Amino Acids and Codons
Table of Codons and Amino Acids
So we now know that the job of tRNA is to bring an amino acid to the ribosome. We also know that each codon has its own tRNA and that each tRNA has its own amino acid attached to it. Further, we know that the job of tRNA is to transport amino acids to the ribosome for protein production.
The tRNA doesn’t become part of the protein which suggests that tRNA can either be attached to an amino acid or free. We call this charged or uncharged. How does this work?
Briefly, an uncharged amino acid goes to a pool of amino acids. Here, it finds the one specific amino acid to which it can attach and binds it or becomes charged. Then it carries the amino acid back to the ribosome where the amino acid is transferred from the tRNA to the growing protein.
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