Transcription

Transcription is the process of synthesizing RNA from a DNA template.

Inside the nucleus, transcription begins at a gene, a specific region of DNA that serves as the template for RNA synthesis.

At the beginning of a gene is the core promoter, a regulatory DNA region that includes conserved motifs such as the TATA box. General transcription factors bind to this region and position RNA polymerase near the transcription start site, forming the transcription preinitiation complex.

After the complex forms, a short region of DNA around the transcription start site unwinds and creates a transcription bubble. RNA polymerase then begins to synthesize a complementary RNA strand.

Ribonucleotides are added sequentially in the five-prime to three-prime direction using the template strand. The resulting mRNA sequence is complementary to the template strand and matches the coding strand, except that uracil replaces thymine.

RNA polymerase continues to add nucleotides as it moves along the DNA until it reaches a terminator sequence. At this point, the enzyme releases the newly formed RNA transcript.

In eukaryotes, the RNA transcript is modified during and after transcription. A modified guanine nucleotide, called the 5-prime cap, is added to the beginning of the RNA. Noncoding sequences, called introns, are removed by splicing, and a polyadenylate tail is added to the three-prime end.

These post-transcriptional modifications stabilize the mRNA, protect it from degradation, help it exit the nucleus, and prepare it for translation.