8.6
Transcription
Transcription is the process of synthesizing RNA from a DNA sequence by RNA polymerase. It is the first step in producing a protein from a gene sequen…
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.
View the full transcript and gain access to JoVE Core videos
Q1: What is the central process that converts DNA information into RNA?
Transcription is the process by which DNA is converted into RNA through the action of RNA polymerase enzymes. This enzyme reads the DNA template strand and synthesizes a complementary RNA molecule. Transcription initiation and transcription factors regulate where and when this process begins, ensuring accurate gene expression in cells.
Q2: How do eukaryotic cells differ from bacteria in their transcription machinery?
Eukaryotic cells employ three distinct RNA polymerases, each transcribing different gene types, whereas bacteria use a single RNA polymerase. The three eukaryotic RNA polymerases are specialized for messenger RNA, ribosomal RNA, and transfer RNA synthesis. This compartmentalization allows eukaryotes to regulate gene expression more precisely than bacteria.
Q3: What happens to RNA after it is initially transcribed in eukaryotes?
After transcription, eukaryotic RNA undergoes extensive processing before becoming mature mRNA. This includes 5' capping, 3' polyadenylation, and removal of introns through alternative RNA splicing regulated splicing of exons and introns. These modifications prepare the RNA for translation and export from the nucleus.
Q4: Where in the nucleus does ribosomal RNA synthesis occur?
Ribosomal RNA synthesis occurs in the nucleolus, a specialized subnuclear compartment. The nucleolus contains ribosomal RNA synthesis ribosomal proteins and assembly factors that work together to produce ribosomal subunits. This dedicated organelle ensures efficient production of the ribosomes needed for protein synthesis.
Q5: How does chromatin structure influence the transcription process?
Chromatin structure regulates pre-mRNA processing by controlling RNA polymerase access to DNA. Tightly packed chromatin prevents transcription, while loosened chromatin permits it. This epigenetic regulation allows cells to selectively express genes without changing DNA sequences, enabling cell-type-specific gene expression patterns.
Q6: What role do transcription elongation factors play during RNA synthesis?
Transcription elongation factors pausing of RNA polymerase regulate the rate at which RNA polymerase moves along the DNA template. These factors can cause temporary pausing, allowing for proper RNA folding and quality control. They also facilitate the transition from initiation to productive elongation, ensuring efficient and accurate transcription.
Q7: What modifications are made to transfer RNA after transcription?
Transfer RNA undergoes extensive post-transcriptional modifications including base modifications that alter its structure and function. These chemical modifications enhance tRNA stability, improve codon recognition, and ensure accurate translation. Transfer RNA synthesis and modified bases are critical for proper protein synthesis and cellular function.