Transcription

Transcription copies information from one DNA strand into RNA. Pairing follows complementarity, except that adenine pairs with uracil in RNA.

Only one strand is copied. If both were used, the two RNAs would be complementary, pair into double-stranded RNA and not be translated, and the gene segment would give two different proteins, making the process futile and complicating the genetic machinery.

A transcription unit has a promoter, a structural gene and a terminator. The template strand has 3′→5′ polarity. The other strand, the coding strand, has 5′→3′ polarity and the same sequence as the RNA, with T in place of U; it is displaced during transcription.

By convention, the promoter lies upstream at the 5′ end of the structural gene, taken relative to the coding strand. It is where RNA polymerase binds. The terminator lies downstream at the 3′ end and marks where transcription stops.

A cistron is a DNA segment coding for one polypeptide. Structural genes are mostly monocistronic in eukaryotes and polycistronic in bacteria.

Eukaryotic genes are split: coding exons, which appear in mature RNA, alternate with non-coding introns, which do not.

Bacteria have three main RNAs: mRNA gives the template, tRNA brings amino acids and reads the code, and rRNA has structural and catalytic roles in translation. One DNA-dependent RNA polymerase transcribes all of them.

RNA polymerase binds the promoter and uses nucleoside triphosphates to elongate RNA. It associates briefly with the sigma (σ) factor to initiate and with the rho (ρ) factor to terminate. In bacteria mRNA needs no processing, and translation can start before transcription is finished since both occur in the cytoplasm.

Eukaryotes have three nuclear RNA polymerases: RNA polymerase I makes rRNAs (28S, 18S, 5.8S), RNA polymerase III makes tRNA, 5S rRNA and snRNAs, and RNA polymerase II makes hnRNA, the precursor of mRNA.

hnRNA is processed: introns are removed by splicing; capping adds methyl guanosine triphosphate at the 5′ end; tailing adds 200–300 adenylate residues at the 3′ end without a template. The processed mRNA then leaves the nucleus. Split genes and splicing are thought to reflect the ancient RNA world.