3.4 Transcription and Gene Activation

A gene is a heritable DNA sequence that contributes to a functional product, which may be a polypeptide or a functional RNA. To express a protein-coding gene, a cell first transcribes its information into RNA. RNA polymerase locally separates the DNA strands, reads the template strand 3′→5′ and links ribonucleotides so the RNA grows 5′→3′. Only one strand of a particular gene is used as the template.

RNA nucleotides contain ribose, and uracil replaces thymine. During transcription, DNA adenine pairs with RNA uracil, while DNA thymine pairs with RNA adenine. Cytosine and guanine remain complementary. The RNA transcript detaches as polymerase advances and the DNA behind it re-forms a duplex. Transcription copies a selected region rather than the whole chromosome.

The coding DNA strand is not used as the template for that transcript. Its 5′→3′ base order matches the RNA product except that DNA contains T where RNA contains U. This relationship is useful for checking answers. If a template is written 3′-TAC GGA-5′, the mRNA is 5′-AUG CCU-3′; a code table then reads AUG and CCU in that displayed order.