During synthesis of RNA, the two strands of the DNA molecule separate temporarily; one of these strands is used as a template for synthesis of an RNA molecule. The code triplets in the DNA cause formation of complementary code triplets (called codons) in the RNA. These codons, in turn, will control the sequence of amino acids in a protein to be synthesized in the cell cytoplasm.
Basic Building Blocks of RNA. The basic building blocks of RNA are almost the same as those of DNA, except for two differences. First, the sugar deoxyribose is not used in the formation of RNA. In its place is another sugar of slightly different composition, ribose, that contains an extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, uracil.
Formation of RNA Nucleotides. The basic building blocks of RNA form RNA nucleotides, exactly as previously described for DNA synthesis. Here again, four separate nucleotides are used in the formation of RNA. These nucleotides contain the bases adenine, guanine, cytosine, and uracil. Note that these bases are the same bases as in DNA, except that uracil in RNA replaces thymine in DNA.
“Activation” of the RNA Nucleotides. The next step in the synthesis of RNA is “activation” of the RNA nucleotides by an enzyme, RNA polymerase. This activation occurs by adding two extra phosphate radicals to each nucleotide to form triphosphates (shown in Figure 1 by the two RNA nucleotides to the far right during RNA chain formation). These last two phosphates are combined with the nucleotide by high-energy phosphate bonds derived from ATP in the cell.
Fig1. 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.
The result of this activation process is that large quantities of ATP energy are made available to each of the nucleotides. This energy is used to promote the chemical reactions that add each new RNA nucleotide at the end of the developing RNA chain.
