RNA (ribonucleic acid)
Figure 1. Comparison of RNA and DNA molecules.
Figure 2. The magnetotatic bacterium Magnetospirillum magnetotacticum.
Figure 3. Introns and exons.
Figure 4. Typical cloverleaf diagram of a tRNA molecule.
RNA (ribonucleic acid) is a class of nucleic acids characterized by the presence of the sugar ribose and the organic base uracil. By contrast, DNA contains deoxyribose and thymine, respectively (see Figure 1). Most RNA molecules, including messenger RNA and transfer RNA (see below), act as cellular intermediaries; that is, they convert the genetic information stored in DNA into the proteins that provide cells with structure and enable them to carry out metabolism. In some viruses, RNA also serves as the hereditary material.
Biologists believe that RNA evolved on Earth before DNA. However, a major debate is in progress about the sequence of events leading to the first cellular life-forms, and, in particular, about which came first, RNA or proteins.
Messenger RNA (mRNA) is a molecule of RNA that serves as a template for protein synthesis. mRNA is transcribed from a gene and then translated by ribosomes in order to manufacture a protein. Each set of three bases, called codons, specifies a certain protein in the sequence of amino acids that comprise the protein. The sequence of a strand of mRNA is based on the sequence of a complementary strand of DNA (see Figure 2).
An intron is a portion of messenger RNA (mRNA), as transcribed from the DNA of a eukaryote, which is removed by enzymes before the mRNA matures and is transplanted into protein. The portions of mRNA that remain after the excision of the introns are known as exons.
An exon is the region of a gene that contains the code for producing the gene's protein. In other words, an exon is a section of DNA that is both translated and transcribed into a protein. Each exon codes for a specific portion of the complete protein. In some species (including humans), a gene's exons are separated by long regions of DNA called introns, or "junk DNA," that have no apparent function.
Transfer RNA (tRNA) is a class of small RNA's, each containing about 80 nucleotides. A tRNA molecule has two functional sites. At one of these sites, an activating enzyme adds a specific amino acid. The other functional site carries the anticodon codon) specific for that amino acid. Each type of tRNA transfers one particular amino acid to a growing polypeptide chain as specified by the nucleotide sequence of the messenger RNA being translated (see Figure 4).