Question
Describe the process of transcription in prokaryotes. Explain the three stages and the role of RNA polymerase, including the significance of the sigma factor and promoter region.
(NEET 2022, similar pattern)
Solution — Step by Step
Transcription is the process of copying genetic information from DNA to mRNA. In prokaryotes, a single enzyme — RNA polymerase — carries out the entire process. Unlike eukaryotes, prokaryotic transcription does not require multiple RNA polymerases.
The DNA strand that serves as the template is called the template strand (3’ to 5’). The other strand is the coding strand (5’ to 3’), which has the same sequence as the mRNA (except T is replaced by U).
- RNA polymerase binds to a specific DNA sequence called the promoter (located upstream of the gene, at the 5’ end of the coding strand).
- The sigma factor () is a subunit of the RNA polymerase holoenzyme that recognises and binds to the promoter. Without sigma, RNA polymerase cannot find the correct starting point.
- Key promoter sequences: the -10 region (Pribnow box, TATAAT) and the -35 region — these are consensus sequences recognised by the sigma factor.
- Once bound, RNA polymerase unwinds a short stretch of DNA, creating a transcription bubble, and begins synthesising RNA.
- After initiation, the sigma factor dissociates. The remaining core enzyme (without sigma) continues RNA synthesis.
- RNA polymerase moves along the template strand in the 3’ to 5’ direction, synthesising mRNA in the 5’ to 3’ direction.
- Ribonucleotides (ATP, UTP, GTP, CTP) are added according to base-pairing rules: A pairs with U, T pairs with A, G pairs with C, C pairs with G.
- No primer is needed — RNA polymerase can initiate synthesis directly (unlike DNA polymerase, which needs a primer).
Transcription stops when RNA polymerase reaches a terminator sequence on the DNA. Two types of termination in prokaryotes:
- Rho-independent (intrinsic) termination: The mRNA forms a GC-rich hairpin loop followed by a poly-U stretch. This destabilises the RNA-DNA hybrid, and the transcript is released.
- Rho-dependent termination: The Rho () protein, a helicase, chases RNA polymerase along the mRNA and unwinds the RNA-DNA hybrid at the termination site.
Why This Works
Transcription is the first step in gene expression — the flow of information from DNA to protein (the central dogma). In prokaryotes, transcription and translation are coupled — ribosomes begin translating the mRNA even before transcription is complete (since there is no nuclear membrane to separate the two processes).
The sigma factor acts as a specificity switch. Different sigma factors recognise different promoters, allowing the cell to transcribe different sets of genes under different conditions (e.g., stress response, heat shock).
NEET commonly tests: “Which strand is used as a template?” Answer: the template strand (also called antisense strand), which runs 3’ to 5’. The mRNA sequence matches the coding strand (sense strand), except U replaces T. Also remember: RNA polymerase does NOT need a primer.
Common Mistake
Students frequently confuse the template strand and coding strand. The template strand is the one actually read by RNA polymerase (3’ to 5’). The coding strand has the same sequence as the mRNA and is NOT used as the template. If a question gives you the coding strand sequence and asks for the mRNA, simply replace T with U.
Another common error: stating that prokaryotes have three RNA polymerases like eukaryotes. Prokaryotes have a single RNA polymerase that transcribes all types of RNA (mRNA, tRNA, rRNA). Eukaryotes have RNA Pol I (rRNA), RNA Pol II (mRNA), and RNA Pol III (tRNA, 5S rRNA).