Transcription Unit and Process in Prokaryotes, Eukaryotes

Transcription is the process of making an RNA copy from a DNA template. Only one strand of DNA is copied for a gene, and the product in protein-coding genes is messenger RNA.

Studying the transcription unit and how transcription differs in prokaryotes and eukaryotes explains how cells control which genes are on, how fast RNA is made, and why eukaryotic RNA needs extra processing before use.

Key Concepts

• Transcription unit: the stretch of DNA transcribed into one RNA, including a promoter, the structural gene and a terminator.
• Promoter: the DNA region where RNA polymerase binds to begin; in prokaryotes it has -10 and -35 sequences.
• Template strand: the 3 prime to 5 prime DNA strand that is read; the other strand is the coding strand with nearly the same sequence as RNA.
• RNA polymerase: the enzyme that builds RNA 5 prime to 3 prime without needing a primer.
• Terminator: the sequence that signals the end of transcription and release of RNA.

How It Works

1. Initiation: RNA polymerase binds the promoter. In prokaryotes the sigma factor helps it find the promoter; the DNA unwinds into a small bubble.
2. Elongation: polymerase moves along the template, adding ribonucleotides complementary to DNA, with uracil instead of thymine. About 30 to 50 nucleotides are added per second, and the DNA rewinds behind it.
3. Termination: in prokaryotes, a hairpin loop plus a run of uracils (intrinsic termination) or the rho protein ends transcription. In eukaryotes, polymerase continues past a polyadenylation signal, then the RNA is cut.
4. Eukaryotic processing: the primary transcript gets a 5 prime cap, a poly-A tail, and introns are spliced out before the mature mRNA leaves the nucleus.

In prokaryotes there is one RNA polymerase for all RNA types and transcription and translation occur together in the cytoplasm. Eukaryotes have three polymerases: polymerase I makes most rRNA, polymerase II makes mRNA, and polymerase III makes tRNA and 5S rRNA, and transcription occurs inside the nucleus, separate from translation.

Worked Example

Take a prokaryotic coding strand with the sequence 5 prime-ATG GCT TAC-3 prime. The template strand opposite it runs 3 prime-TAC CGA ATG-5 prime. RNA polymerase reads the template 3 prime to 5 prime and builds RNA 5 prime to 3 prime. Following base pairing, with U replacing T: T gives A, A gives U, C gives G for the first triplet, giving AUG; C gives G, G gives C, A gives U giving GCU; A gives U, T gives A, G gives C giving UAC. The mRNA is therefore 5 prime-AUG GCU UAC-3 prime, identical to the coding strand except U replaces T. If this were a eukaryote, any introns inside this region would be removed and a cap and tail added before export.

Common Misconceptions

• Students think both DNA strands are transcribed for one gene. Only the template strand is read for a given gene, though different genes may use opposite strands.
• Many assume transcription needs a primer like replication. RNA polymerase can start on its own at a promoter.
• In eukaryotes, the first RNA is not ready mRNA; capping, tailing and splicing are essential steps, unlike in prokaryotes.

Real-World Applications

Many antibiotics, such as rifampicin used against tuberculosis, block bacterial RNA polymerase without harming human polymerases. mRNA vaccines work because our cells transcribe and then translate the delivered instructions. Studying promoters also lets biotechnologists place useful genes under strong promoters to produce insulin and other proteins in bacteria.

Transcription turns one DNA segment into RNA under tight control at the promoter. Prokaryotes do it fast and coupled with translation, while eukaryotes add processing and nuclear separation for finer regulation.