Episode #9 of the course The molecular building blocks of life by Dr. Bill Thomas
Over the last several lessons, we have been looking at different forms of the amazingly versatile molecule RNA and its involvement in the process of building proteins. We’ve already seen how messenger RNA (mRNA) carries a copy of a gene—the blueprint for a protein—to the protein-making machine called a ribosome. The ribosome, which is made of ribosomal RNA, or rRNA, uses this blueprint to assemble a protein out of amino acids. In this lesson, we’ll look at another role for RNA in producing proteins, performed by transfer RNA.
What Is a Transfer RNA?
Because hundreds or even thousands of amino acids may be needed to construct a single protein, ribosomes need a steady supply of these building blocks in order to do their job. Transfer RNA, or tRNA, is the molecule responsible for carrying amino acids to the ribosome so they can be assembled into a protein.
Like messenger RNA and ribosomal RNA, transfer RNA has a key role to play in the process of producing a protein from the blueprints encoded in gene. Like rRNA, tRNA does not contain any genetic information; however, unlike rRNA, a tRNA consists of a single strand of RNA attached at one end to a specific amino acid. Internal attractions within a tRNA molecule cause it to fold and draw in upon itself, forming a complicated three-dimensional structure. Part of this structure is a loop that contains an anticodon, a sequence of three nucleotides that serves a very important purpose.
The Codon-Anticodon Connection
If you recall from Lesson 4, the genetic code is made up of three-nucleotide codons. In messenger RNA, the genetic code is spelled out in codons that tell the ribosome which amino acids will be used in a protein. For each of these codons, there is a tRNA with a matching anticodon: three nucleotides that complement the nucleotides of the codon and are drawn together by mutual attraction. When the anticodon of a transfer RNA finds the matching codon of a messenger RNA, its amino acid is transferred from the tRNA to the growing chain of amino acids that will become a protein. The tRNA is recycled and the process of translation starts again.
High Demand for tRNA
At any moment in time, there are many thousands of different proteins being manufactured in a cell. Because of the high volume of proteins being produced, many transfer RNAs must be present in a cell at all times. Unsurprisingly, organisms compensate for this by encoding many, many tRNAs in their genomes. In fact, the human genome codes for nearly 500 tRNAs!
The Versatility of RNA
Given the extreme differences in the form and function of transfer RNA, messenger RNA, and ribosomal RNA, it is clear that RNA is a very versatile molecule. In fact, although we have discussed three different types of RNA, we have hardly scratched the surface of what this molecule can do. For instance, the comparatively recent discovery of micro RNA (miRNA) and small interfering RNA (siRNA) revolutionized the field of molecular biology. These remarkable molecules give cells another level of control over the production of proteins by targeting messenger RNAs for destruction, thus shutting down the construction of a particular protein. In addition, they can target foreign RNA and DNA from viruses, defending cells from these invaders.
It’s an RNA World
The versatility of RNA is not limited to protein production. Many viruses have genomes that consist of single- or double-stranded RNA; examples include well-known human pathogens such as HIV, hepatitis C, West Nile, and the Zika virus. Furthermore, because RNA can form complex three-dimensional structures, some RNA molecules can act like enzymes, which are usually proteins. Since RNA can perform many of the functions carried out by proteins and DNA, it has been hypothesized that RNA was the earliest biological molecule and that all life evolved from this “RNA world.”
In our next lesson, we will take a look at the final piece of the puzzle for building a cell: lipids. With this building block, we can construct the cellular environment within which all this biological activity takes place.
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