Introduction
Transcription is the process by which genetic information from DNA is copied into RNA. Although the basic idea of transcription is the same across organisms, the way it happens in prokaryotes (like bacteria) and eukaryotes (like human cells) is quite different. In this answer, we will look at the differences in transcription between prokaryotes and eukaryotes, and also explain the structure and function of RNA polymerase in prokaryotic cells.
Part A: Primary Differences Between Prokaryotic and Eukaryotic Transcription
1. Location
- Prokaryotes: Transcription occurs in the cytoplasm because prokaryotic cells do not have a nucleus.
- Eukaryotes: Transcription takes place in the nucleus. After transcription, the RNA moves to the cytoplasm for translation.
2. RNA Polymerase
- Prokaryotes: Have only one type of RNA polymerase that makes all types of RNA (mRNA, tRNA, rRNA).
- Eukaryotes: Have three different RNA polymerases – RNA polymerase I, II, and III – each responsible for different types of RNA.
3. Transcription and Translation
- Prokaryotes: Transcription and translation happen at the same time (coupled process).
- Eukaryotes: Transcription and translation are separate processes. Transcription happens first in the nucleus, then translation happens in the cytoplasm.
4. mRNA Processing
- Prokaryotes: mRNA does not need to be processed and is directly used in translation.
- Eukaryotes: mRNA needs to be processed – a 5’ cap is added, a Poly-A tail is added, and introns are removed through splicing.
5. Promoter Recognition
- Prokaryotes: RNA polymerase recognizes the promoter with the help of sigma factor.
- Eukaryotes: Transcription factors help RNA polymerase recognize and bind to the promoter.
Part B: Structure and Role of RNA Polymerase in Prokaryotic Transcription
Structure of Prokaryotic RNA Polymerase
In prokaryotes, the main RNA polymerase enzyme is made of several subunits that form a large complex:
- Core enzyme: Consists of 4 subunits – 2 alpha (α), 1 beta (β), and 1 beta prime (β’).
- Holoenzyme: The core enzyme plus a sigma (σ) factor. The sigma factor helps the enzyme find the promoter.
Function of RNA Polymerase in Prokaryotic Transcription
The main role of RNA polymerase is to synthesize RNA from a DNA template. Here’s how it works:
- Initiation: The sigma factor helps the RNA polymerase bind to a specific region of DNA called the promoter. This is where transcription begins.
- Elongation: After the RNA strand starts forming, the sigma factor is released. The core enzyme continues to add RNA nucleotides by reading the DNA template strand.
- Termination: Once the RNA polymerase reaches a termination sequence on the DNA, it stops transcribing and releases the newly made RNA.
Importance of RNA Polymerase
- It is essential for gene expression, which is necessary for cell survival and function.
- It ensures that the right genes are turned on or off as per the cell’s needs.
Conclusion
Understanding the differences in transcription between prokaryotes and eukaryotes helps us understand how cells function at a molecular level. Prokaryotic RNA polymerase is a powerful and precise enzyme that plays a central role in gene expression. By knowing how this enzyme works, scientists can develop antibiotics and study bacterial behavior more effectively.