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M03.03.01 Transcription
Transcription is the first stage of gene expression, where the genetic information encoded in the base sequence of double-stranded DNA is copied into a single-stranded RNA molecule. This process is crucial for synthesizing proteins and other functional molecules necessary for cellular processes.
Key Steps in Transcription
- Initiation:
- RNA polymerase binds to the promoter region of the DNA template.
- The DNA strands unwind, creating a transcription bubble, allowing RNA polymerase access to the template strand.
- Elongation:
- RNA polymerase moves along the template strand in the 3′ to 5′ direction, synthesizing RNA in the 5′ to 3′ direction.
- The RNA strand is complementary and antiparallel to the DNA template strand.
- Termination:
- Transcription ends when RNA polymerase encounters a terminator sequence.
- The RNA polymerase releases the newly formed RNA strand and detaches it from the DNA template.
Template vs. Coding Strand
| Feature | Template Strand | Coding Strand |
|---|---|---|
| Direction | 3′ to 5′ | 5′ to 3′ |
| Role | Serves as the template for RNA synthesis | Has the same sequence as the RNA (except T is replaced by U in RNA) |
| RNA Complementarity | Antiparallel and complementary to RNA | Identical in sequence to the newly formed RNA (except for U replacing T) |
Promoters and Terminators
- Promoters: Specific DNA sequences that serve as binding sites for RNA polymerase, signaling the start of transcription.
- Terminators: DNA sequences that signal the end of transcription, allowing RNA polymerase to detach and release the RNA transcript.
Important Points to Remember
- Transcription is unidirectional, meaning RNA is synthesized in the 5′ to 3′ direction while RNA polymerase moves along the template DNA strand in the 3′ to 5′ direction.
- Only the template strand of DNA is transcribed, while the coding strand remains untranscribed but has a sequence similar to the RNA product.
- RNA polymerase recognizes both start (promoter) and stop (terminator) signals, ensuring the correct transcription of genes.
- Eukaryotic transcription differs from prokaryotic transcription in the complexity of promoters, additional regulatory sequences, and post-transcriptional modifications like splicing, capping, and polyadenylation.
Bibliography
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2014). Molecular Biology of the Cell (6th ed.). Garland Science.
- Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Bretscher, A., Ploegh, H., & Amon, A. (2016). Molecular Cell Biology (8th ed.). W. H. Freeman.
- Watson, J. D., Baker, T. A., Bell, S. P., Gann, A., Levine, M., & Losick, R. (2013). Molecular Biology of the Gene (7th ed.). Cold Spring Harbor Laboratory Press.