| Definition |
The process of copying genetic information from one strand of DNA into RNA. |
| Principle of Complementarity |
Follows the principle of complementarity where adenosine pairs with uracil instead of thymine. |
| Strands Involved |
Only one strand of the DNA (template strand) is copied into RNA. The other strand is the coding strand. |
| Transcription Unit |
Defined by three regions: Promoter, Structural gene, and Terminator. |
| Template Strand |
The DNA strand with 3’→5′ polarity that acts as the template for RNA synthesis. |
| Coding Strand |
The DNA strand with 5’→3′ polarity which has the same sequence as the RNA (except thymine is replaced by uracil). |
| Promoter |
Located upstream of the structural gene, it provides the binding site for RNA polymerase and defines the template and coding strands. In E. coli, it includes consensus sequences at -35 (5′-TTGACA-3′) and -10 (Pribnow box, 5′-TATAAT-3′). |
| Terminator |
Located downstream of the structural gene, it signals the end of transcription. |
| Types of RNA in Bacteria |
mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). |
| RNA Polymerase |
A single DNA-dependent RNA polymerase catalyzes transcription of all types of RNA in bacteria. It binds to the promoter to initiate transcription, facilitates the opening of the helix, and elongates the RNA strand. |
| Initiation |
RNA polymerase holoenzyme (core enzyme + sigma factor) binds to the promoter. The core enzyme includes four polypeptides (two α, one β, and one β’). |
| Elongation |
After initiation, the RNA strand is extended by adding nucleotides in the 5’→3′ direction. The core enzyme moves along the DNA, untwisting the helix and synthesizing RNA. |
| Termination |
Can occur via two mechanisms: Rho-independent (type I) terminators form a hairpin loop in the RNA, and Rho-dependent (type II) terminators require the Rho protein to destabilize the RNA-DNA hybrid. |
| Coupled Transcription-Translation |
In bacteria, transcription and translation can occur simultaneously in the cytoplasm because there is no nuclear compartment. |
| Question: How does RNA polymerase catalyze all three steps of transcription (initiation, elongation, and termination)? |
Answer: RNA polymerase catalyzes the process of elongation. It associates transiently with initiation-factor (σ) and termination-factor (ρ) to initiate and terminate transcription, respectively. Association with these factors alters the specificity of RNA polymerase to either initiate or terminate transcription. |
