The synthesis of RNA is directed by DNA. The base sequence of one strand of DNA is used as a template for RNA synthesis, so that the RNA made is complementary in sequence to the DNA strand, with the exception that in RNA there is uracil (U) instead of thymine (T) and in RNA the sugar is ribose instead of deoxyribose. It is important to realize that although the RNA made is a mirror image of its DNA template, the RNA has the same sequence as the other, non-template strand of DNA. So the information content of DNA is indeed preserved in RNA.

Transcription—the formation of a specific RNA sequence from a specific DNA sequence—requires several components:

Several kinds of RNA are made from DNA templates. The most important from a genetic point of view is mRNA. But transcription also produces tRNA and rRNA, whose roles in protein synthesis will be described below. Like polypeptides, these last two RNAs are encoded by specific genes. Eukaryotes also make many kinds of small RNAs, including small nuclear RNA (snRNA), microRNA (miRNA), and small interfering RNA (siRNA), which are also transcribed. Table 14.1 summarizes some of the RNAs found in eukaryotic cells. We will discuss the roles of miRNA and siRNA in Chapter 16.