FIGURE 5-10 RNA is synthesized 5′→3′. (a) Polymerization of ribonucleotides by RNA polymerase during transcription. The ribonucleotide to be added at the 3′ end of a growing RNA strand is specified by base pairing between the next base in the template DNA strand and the complementary incoming ribonucleoside triphosphate (rNTP). A phosphodiester bond is formed when RNA polymerase catalyzes a reaction between the 3′ oxygen of the growing strand and the α phosphate of a correctly base-paired rNTP. RNA strands are always synthesized in the 5′→3′ direction and are opposite in polarity to their template DNA strands. (b) Conventions for describing RNA transcription. Top: The DNA nucleotide where RNA polymerase begins transcription is designated +1. The direction the polymerase travels on the DNA is “downstream,” and downstream bases are marked with positive numbers. The opposite direction is “upstream,” and upstream bases are marked with negative numbers. Some important gene features lie upstream of the transcription start site, including the promoter sequence that localizes RNA polymerase to the gene. Bottom: The DNA strand that is being transcribed is the template strand; its complement is the nontemplate strand. The RNA being synthesized is complementary to the template strand and is therefore identical with the nontemplate strand sequence, except with uracil in place of thymine.