Exam Focus: The major difference between prokaryotic and eukaryotic transcription (single vs. multiple RNA polymerases, presence of transcription factors/enhancers in eukaryotes) is crucial. Know the specific roles of the **Sigma factor** and the TATA box/Promoter in initiating prokaryotic transcription.
RNA (Ribonucleic Acid) is generally single-stranded, uses **ribose** sugar (instead of deoxyribose), and contains **Uracil (U)** instead of Thymine (T).
Carries the genetic information transcribed from DNA to the ribosome for protein synthesis.
A major component of the ribosomes, the cellular machinery that catalyzes protein synthesis (translation).
Acts as an adaptor molecule, linking specific amino acids to their corresponding mRNA codons during translation. It has a characteristic clover-leaf secondary structure. [Image of tRNA structure]
Transcription is the synthesis of RNA from a DNA template.
Prokaryotes (like E. coli) have **only one type of RNA polymerase** responsible for transcribing all three types of RNA (mRNA, tRNA, rRNA).
The **Sigma (σ) factor** is essential for **initiation**. It recognizes and binds specifically to the promoter region on the DNA, ensuring that the polymerase starts transcription at the correct site.
A specific DNA sequence upstream of the gene where RNA polymerase binds to initiate transcription. In prokaryotes, key sequences include the -35 region and the -10 region (**Pribnow box**).
Eukaryotic transcription is more complex, involving multiple RNA polymerases and accessory factors.
Eukaryotes have three main RNA polymerases, each responsible for transcribing different genes:
Proteins required for RNA polymerase (especially Pol II) to recognize and initiate transcription at the promoter. **General TFs** (e.g., TFII D) form the pre-initiation complex at the promoter; **Specific TFs** bind to regulatory sequences (enhancers/silencers) to control gene expression levels.
Contain specific conserved sequences where TFs and RNA Pol bind. For Pol II, the common core promoter element is the **TATA box**.
DNA sequences, often located far away from the gene, that bind specific TFs. When bound, enhancers greatly **increase the rate of transcription** by looping the DNA to interact with the promoter/Pol II complex.
Involves the assembly of the pre-initiation complex at the promoter, recruitment of RNA Pol, unwinding of DNA, and synthesis of the pre-mRNA molecule.