9.4
Transcription
La transcription est la synthèse d'ARN à partir d'une séquence d'ADN par l'ARN polymérase. C'est la première étape dans la production d'une protéine à…
La transcription est le processus de synthèse de l’ARN à partir d’une matrice d’ADN.
Sur l’ADN modèle, le complexe de pré-initiation de la transcription s’assemble autour du promoteur central d’un gène. Le promoteur comprend la boîte TATA et une séquence Initiator. Les facteurs de transcription généraux se lient au niveau de la boîte TATA, tandis que la séquence initiatrice contient le site de début de la transcription.
Une fois que les composants nécessaires sont liés, le complexe de pré-initiation déroule une courte portion de l’ADN en amont du site de début de la transcription. Après cela, les facteurs de transcription généraux se dissocient du brin et l’ARN polymérase commence à produire un nouveau brin d’ARNm.
Les nucléotides sont ajoutés un par un, et la synthèse de l’ARNm se produit dans une direction de cinq à trois premiers ordres, en lisant à partir du brin matrice, sauf que les thymidines sont remplacées par des uridines.
Ce brin d’ARNm nouvellement créé représente une copie de l’information contenue dans le brin codant. La synthèse se poursuivra jusqu’à ce qu’une séquence de terminaison soit rencontrée, qui libérera l’ARNm nouvellement fabriqué.
View the full transcript and gain access to JoVE Core videos
Q1: What is the role of the promoter in transcription initiation?
The promoter is a DNA region where the transcription pre-initiation complex assembles to begin RNA synthesis. It contains the TATA box, where general transcription factors bind, and an Initiator sequence that marks the transcription start site. These elements position RNA polymerase correctly to begin mRNA synthesis from the DNA template.
Q2: How does RNA polymerase synthesize mRNA from the DNA template?
RNA polymerase reads the template DNA strand and adds nucleotides one by one in the five prime to three prime direction. Thymidines in the DNA template are replaced by uridines in the new mRNA strand. This process continues until a termination sequence is encountered, which releases the completed mRNA molecule.
Q3: What happens after the preinitiation complex assembles at the promoter?
After the preinitiation complex assembles, it unwinds a short stretch of DNA upstream of the transcription start site. The general transcription factors then dissociate from the DNA strand, allowing RNA polymerase to begin synthesizing the new mRNA strand from the exposed template.
Q4: What types of RNA molecules can be produced by transcription?
Transcription produces messenger RNA (mRNA) that codes for proteins, as well as non-coding RNA molecules including transfer RNA (tRNA), ribosomal RNA (rRNA), and microRNAs (miRNAs). In eukaryotes, pre-mRNA is first transcribed and then processed into mature mRNA. These different RNA types all play essential roles in protein synthesis.
Q5: How does transcriptional regulation control cellular differentiation?
Transcription factors are DNA-binding proteins that control which genes are turned on or off in different cell types. During development, transcription factors activated by induction signals regulate gene expression to transform precursor cells into specialized cells like muscle or nerve cells. This selective gene regulation orchestrates the differentiation process.
Q6: Why is transcription considered the first step in protein production?
Transcription synthesizes RNA from a DNA template, creating the genetic instructions needed for protein synthesis. The mRNA produced during transcription serves as the template for translation into polypeptides. Without transcription, cells cannot access the genetic information stored in DNA to produce functional proteins.
Q7: What is the difference between transcription in prokaryotes and eukaryotes?
In prokaryotes, translation into polypeptides can begin while transcription is still ongoing because there is no nuclear membrane separating these processes. In eukaryotes, DNA is first transcribed into pre-mRNA, which undergoes pre-mRNA processing modification of pre-mRNA ends before becoming mature mRNA that exits the nucleus for translation.