Messenger RNA molecules are long, single RNA strands that are suspended in the cytoplasm. These molecules are composed of several hundred to several thousand RNA nucleotides in unpaired strands, and they contain codons that are exactly complementary to the code triplets of the DNA genes. Figure 1 shows a small segment of mRNA. Its codons are CCG, UCU, and GAA, which are the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the DNA molecule to the RNA molecule is shown in Figure 2.
Fig1. A portion of an RNA molecule showing three RNA codons—CCG, UCU, and GAA—that control attachment of the three amino acids, proline, serine, and glutamic acid, respectively, to the growing RNA chain.
Fig2. Combination of ribose nucleotides with a strand of DNA to form a molecule of RNA that carries the genetic code from the gene to the cytoplasm. The RNA polymerase enzyme moves along the DNA strand and builds the RNA molecule.
RNA Codons for the Different Amino Acids. Table 1 lists the RNA codons for the 22 common amino acids found in protein molecules. Note that most of the amino acids are represented by more than one codon; also, one codon represents the signal “start manufacturing the protein molecule,” and three codons represent “stop manufacturing the protein molecule.” In Table 1, these two types of codons are designated CI for “chain initiating” or “start” codon and CT for “chain-terminating” or “stop” codon.
Table1. RNA Codons for Amino Acids and for Start and Stop
