RNA, which is made up of nucleic acids, has a variety of functions in a cell and is found in many organisms including plants, animals, viruses, and bacteria. Ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) differ functionally. DNA primarily serves as the storage material for genetic information. RNA can function as a carrier of genetic information, a catalyst of biochemical reactions, an adapter molecule in protein synthesis, and a structural molecule in cellular organelles.
Since the discovery of DNA and RNA in the 1950s, scientists have studied the function and structure of the components that makeup these structures. The various types and functions of RNA have been investigated by numerous researchers, including Spanish physiologist Severo Ochoa (1905–1993), who received a Nobel prize in 1959 for his contributions to our understanding of how RNA is synthesized.
There are five major types of RNA that are found in the cells of eukaryotes. These include heterogeneous nuclear RNA (hnRNA), messenger RNA (mRNA), transfer RNA tRNA), ribosomal RNA (rRNA), and small nuclear RNA. Structurally, hnRNA and mRNA are both single stranded, while rRNA and tRNA form three–dimensional molecular configurations. Each type of RNA has a different role in various cellular processes. In addition to these functions, RNA plays an important role in the ability of certain viruses to cause infection.
One of the primary functions of RNA is to facilitate the translation of DNA into protein. This process begins in the nucleus of the cell with a series of enzymatic reactions that transcribe DNA into heterogeneous nuclear RNA by complementary base pairing. Since hnRNA is a direct copy of DNA, it contains exons and introns which are coding and noncoding regions of nucleotides, respectively. hnRNA undergoes post–transcriptional processing that involves removal of the introns and the addition of adenines to the end of single stranded RNA molecules (a process called capping), which are now referred to as mRNA. mRNA is transported out of the nucleus into the cytoplasm of the cell. In this way, it functions as a carrier for information from the cells DNA to the protein synthesizing organelle, called the ribosome.
The mRNA attaches to the ribosome to allow for the initiation of protein synthesis. Part of this process involves another type of RNA that is located in the ribosome called tRNA. tRNA is an adapter molecule, which functions as a bridge between a specific three-base sequence or codon in the mRNA strand and the amino acids that are used to construct the protein. The tRNA carries an amino acid that matches the specific codon and this process begins and stops based on specific sequences in the mRNA. Each amino acid is transferred to the growing polypeptide by chemical interactions to produce a full-length protein. Another type of RNA that is part of the ribosome and is involved in protein synthesis is rRNA. rRNA has two primary functions. First, it provides the structure and shape producing the catalytic regions of the ribosome. Second, it helps speed up, or catalyze, protein synthesis by interactions between the tRNA and the protein synthesis machinery.
While DNA and RNA are very similar in their composition, RNA has a different roles. RNA can serve as a component of the translation machinery and catalyze chemical reactions. For example, in addition to RNA molecules such as rRNA, ribozymes are also a type of RNA that can serve catalytic functions. rRNA functions as a ribozyme during protein synthesis. Another form of RNA that acts as a ribozyme is the small nuclear ribonucleoprotein. During the process of RNA splicing, this ribozyme—like, RNA—containing structure catalyzes reactions in the spliceosome, a group of biomolecules that are involved in removal of the intron, or splicing the hnRNA. These molecules, therefore, play a role in the processing of the hnRNA
Certain viruses contain RNA as their primary genetic material. Viruses bind to a specific protein or receptor on the surface of the cell that it is going to infect. RNA, the virus's genetic material, is injected into the cell. The viral RNA associates with the ribosomes that belong to the cell it is infecting. In a sense, viruses hijack the host's molecular machinery, using the cells transcriptional abilities for its own purpose, to produce viral proteins. The viral proteins then form new viruses. Viral RNA can also form replication complexes where it can copy itself. This copied RNA then gets packaged into the newly created viruses that can cause the cell to lyse, or break open, and these released viruses can infect other cells.
Currently, there is growing interest in small, barely detectable RNA molecules that do not translate into protein, but have been shown to be important in regulating gene expression. Called RNA genes, these small molecules were initially identified in the species of worm Caenorhabditis elegans by American geneticist Victor Ambros and colleagues in the early 1990s. They were shown to turn off gene expression during worm development. This novel function was later demonstrated in other species. American geneticist Stephen R. Holbrook of Lawrence Berkeley National Laboratory in California in a report in the October 1, 2001, journal, Nucleic Acids Research, identified many other potential RNA genes previously undetected using a complex computer program called RNAGENiE. Biotech companies are currently using RNA genes as potential drug targets because of recent interest in RNA genes produced during bacterial infections and their pathogenic effects through the regulation of gene expression of host DNA.
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