Messenger RNA, or mRNA, consists of a single unpaired strand of nucleotides. RNA nucleotides have three parts: a five-carbon sugar in ring form, a phosphate and a base. The bases are the functional part of the RNA molecule. They contain coded information for the synthesis of proteins. The four RNA bases are cytosine, guanine, uracil and adenine. In each nucleotide, one of these four bases forms a bond with the sugar, called ribose, while the sugar and the phosphate bind with one another to complete the nucleotide. These nucleotides unite with one another by sugar-phosphate bonds until the molecule is complete. The mRNA molecule forms a long but simple strand, with only a few coils here and there.
Ribosomes serve as sites for protein synthesis. DNA in the nucleus contains genes, the blueprints for individual proteins. With the catalytic help of enzymes, information for the synthesis of a specific gene is copied unto an mRNA molecule. The mRNA carries this information to a ribosome, and construction begins. Ribosomes consist of proteins and another form of RNA called ribosomal RNA, or rRNA. The rRNA molecule consists of a long strand with the same linear arrangements of nucleotides as mRNA, but its form is more complex. It contains sections in which the strand loops and twists around, so two sections of the strand come close to one another. In these sections, the bases on the two neighboring parts of the strand pair up by forming hydrogen bonds. The sections that intervene between the coiled sections have unpaired bases.
Transfer RNA, or tRNA, is a small molecule with an elegant structure. Its single strand makes a complete circuit, so the beginning and end of the molecule lie in close proximity. Before reaching its terminus, the strand makes several loops. After each loop, it returns where it came from, running close to the section of the strand that approached the loop. These two neighboring sections unite with each other by hydrogen bonds. The resultant figure, when flattened out, looks like a clover leaf. The tRNA molecule brings a specific amino acid to the ribosome-mRNA complex. The terminus of the molecule at the end of the cloverleaf stem has a site for attaching the amino acid. The loop opposite the stem has three unpaired nucleotides called codons, which become attached to a complementary site on mRNA, so that its amino acid can be added to the growing protein molecule.
Both mRNA and rRNA have precursor RNA forms. Precursor mRNA, or premRNA, has long sections not needed for protein synthesis. Enzymes remove these sections and splice the molecule back together, so that only the functional nucleotides remain. Besides, mRNA has protective structures at the beginning and at the end of the molecule, which do not occur on premRNA. Another variant, RNA with two complementary strands, occurs in some viruses. Bacterial RNA differs slightly from the RNA of more complex organisms. For example, bacterial mRNA lacks the protective structures at the beginning and end of the molecule.