How can transcription be terminated?

In the genetic code, a stop codon (or termination codon) is a nucleotide triplet or codon within messenger RNA that signals a termination of the translation of a protein.

Stop codons signal the termination of this process by binding to release factors, which cause the ribosomal subunits to disassociate, releasing the amino acid chain.

These are the codons found in RNA: UAG, UAA and UGA.

The start codon is AUG and the stop is UAG. You can see the protein growing and then being released by the stop codon.

Transcription termination in both prokaryotes and eukaryotes can occur through different mechanisms: ### In Prokaryotes: 1. **Rho-dependent termination**: Requires the Rho protein, which binds to a specific RNA sequence and moves toward the RNA polymerase. When Rho catches up to the RNA polymerase at a pause site, it causes the polymerase to dissociate from the DNA, ending transcription. 2. **Rho-independent termination (Intrinsic termination)**: Involves a GC-rich region of RNA forming a hairpin loop structure followed by a series of uracil (U) bases. The hairpin structure causes the RNA polymerase to slow down and the U-rich sequence facilitates the dissociation of the RNA-DNA hybrid, leading to termination of transcription. ### In Eukaryotes: 1. **Polyadenylation signal-dependent termination**: Involves a sequence in the pre-mRNA (typically AAUAAA) that signals for cleavage of the new RNA molecule a bit downstream of the signal, followed by addition of a poly(A) tail. The RNA polymerase II continues to transcribe for a short distance beyond this cleavage site. Various factors involved in this process contribute to the eventual termination of transcription. 2. **Torpedo model**: After the pre-mRNA is cleaved, the continued transcription by RNA polymerase II generates a 5' to 3' exonuclease vulnerable RNA that is targeted by the exonuclease. The exonuclease degrades this RNA in the 5' to 3' direction and eventually catches up to the RNA polymerase, leading to its termination. 3. **Allosteric model**: Suggests that the process of cleaving the pre-mRNA and adding a poly(A) tail alters the composition and conformation of the transcription complex, making it less stable and leading to termination. These mechanisms ensure that transcription is stopped at the appropriate time, allowing the cell to regulate gene expression effectively.