9.4
Transcrição
A transcrição é a síntese de RNA a partir de uma sequência de DNA pela RNA polimerase. Esta é a primeira etapa na produção de uma proteína a partir de…
A transcrição é o processo de síntese de RNA a partir de um molde de DNA.
No DNA molde, o complexo de pré-iniciação da transcrição se reúne em torno do promotor central de um gene. O promotor inclui a caixa TATA e uma sequência de iniciador. Os fatores gerais de transcrição se ligam na caixa TATA, enquanto a sequência iniciadora contém o local de início da transcrição.
Uma vez que os componentes necessários são ligados, o complexo de pré-iniciação desenrola um pequeno trecho do DNA a montante do local de início da transcrição. Depois disso, os fatores gerais de transcrição são dissociados da fita e a RNA polimerase começa a produzir uma nova fita de mRNA.
Os nucleotídeos são adicionados um a um e a síntese do mRNA ocorre em uma direção de cinco primos a três primos, lendo a partir da fita molde, exceto que as timidinas são substituídas por uridinas.
Esta fita de mRNA recém-criada representa uma cópia da informação na fita codificante. A síntese continuará até que uma sequência de terminação seja encontrada, o que liberará o mRNA recém-produzido.
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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.