Text Books
<p><b>An Introduction to Genetic Analysis. 7th edition.</b>
<b>Transcription and RNA polymerase</b>
As described earlier, transcription relies on the complementary pairing of bases. The two strands of the double helix separate locally, and one of the separated strands acts as a template. Next, free nucleotides are aligned on the DNA template by their complementary bases in the template. The free ribonucleotide A aligns with T in the DNA, G with C, C with G, and U with A. The process is catalyzed by the enzyme RNA polymerase, which attaches and moves along the DNA adding ribonucleotides in the growing RNA as shown in Figure 10-6a. Hence, already we see the two principles of base complementarity and binding proteins (in this case, the RNA polymerase) in action.
Figure 10-6. Transcription of two genes.
 
<img src="/3eca30ef-72b9-5b90-85b7-331c2431120e.png"/>
 
<b>Transcription of two genes.</b> (a) RNA polymerase moves from the 3′ end of the template strand, creating an RNA strand that grows in a 5′ → 3′ direction (because it must be antiparallel to the template strand). (more...)
RNA growth is always in the 5′ → 3′ direction: in other words, nucleotides are always added at a 3′ growing tip, as shown in Figure 10-6b. Because of the antiparallel nature of the nucleotide pairing, the fact that RNA is synthesized 5′ → 3′ means that the template strand must be oriented 3′ → 5′.</p>