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M03.05.02 Chromatin Remodeling in Eukaryotic Gene Expression
Understanding transcription and RNA processing is crucial for comprehending gene expression and regulation. This section provides a detailed overview with tables and side notes to enhance retention.
Transcription
Transcription is the process of synthesizing RNA from a DNA template. It consists of three main stages: initiation, elongation, and termination.
Stages of Transcription:
- Initiation:
- Promoter Recognition: RNA polymerase binds to the promoter region upstream of the gene.
- DNA Unwinding: Helicase unwinds the DNA double helix.
- Formation of Transcription Bubble: A region of single-stranded DNA is exposed.
- Elongation:
- RNA Synthesis: RNA polymerase adds ribonucleotides complementary to the DNA template strand.
- Direction: Synthesis occurs in the 5′ to 3′ direction.
- Termination:
- Signal Recognition: Specific sequences in the DNA signal the end of transcription.
- Release of RNA Transcript: The RNA polymerase releases the newly synthesized RNA molecule.
Table 1: Comparison of Prokaryotic and Eukaryotic Transcription
| Feature | Prokaryotes | Eukaryotes |
|---|---|---|
| RNA Polymerases | Single RNA polymerase | Three RNA polymerases (I, II, III) |
| Promoter Elements | -10 and -35 elements | TATA box, enhancers |
| Transcription Factors | Sigma factor | General transcription factors (GTFs) |
| RNA Processing | Minimal | Extensive (capping, splicing, polyadenylation) |
| Location | Cytoplasm | Nucleus |
RNA Processing
In eukaryotes, the primary RNA transcript (pre-mRNA) undergoes several processing steps before becoming mature mRNA.
Steps of RNA Processing:
- Capping:
- Addition of 5′ Cap: A modified guanine nucleotide is added to the 5′ end of the pre-mRNA.
- Function: Protects RNA from degradation and assists in ribosome binding during translation.
- Splicing:
- Removal of Introns: Non-coding regions (introns) are removed from the pre-mRNA.
- Joining of Exons: Coding regions (exons) are joined to form the mature mRNA.
- Spliceosome: A complex of small nuclear RNAs (snRNAs) and proteins facilitate splicing.
- Polyadenylation:
- Addition of Poly(A) Tail: A sequence of adenine nucleotides is added to the 3′ end of the mRNA.
- Function: Enhances stability and export of the mRNA from the nucleus to the cytoplasm.
Table 2: Key Points of RNA Processing
| Step | Description | Function |
|---|---|---|
| 5′ Capping | Addition of a modified guanine | Protects RNA, aids in translation initiation |
| Splicing | Removal of introns, joining of exons | Produces continuous coding sequence |
| Polyadenylation | Addition of poly(A) tail at 3′ end | Increases stability, facilitates nuclear export |
Flowchart: RNA Processing Steps
- Pre-mRNA Transcription
- ↓
- 5′ Capping
- ↓
- Splicing
- Introns removed, exons joined
- ↓
- Polyadenylation
- ↓
- Mature mRNA
- Export to the cytoplasm for translation
Side Notes: Key Points to Memorize
- Transcription Factors: Essential for the initiation of transcription by assisting RNA polymerase in binding to DNA.
- Spliceosome: Complex responsible for accurately removing introns and splicing exons together.
- 5′ Cap and Poly(A) Tail: Both modifications are critical for mRNA stability and translation efficiency.
- Introns vs. Exons: Introns are non-coding regions removed during splicing, while exons are coding regions that remain in mature mRNA.
Summary
Transcription and RNA processing are vital processes that convert genetic information in DNA into functional RNA molecules. Understanding these processes is fundamental for medical students as they provide the basis for gene expression and regulation, impacting numerous biological functions and disease mechanisms.
References for Further Reading:
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular Biology of the Cell (4th ed.). Garland Science.
- Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., & Ploegh, H. (2007). Molecular Cell Biology (6th ed.). W. H. Freeman.