Biology 1 - Lesson 16: Gene Expression – Transcription

Gene expression begins with transcription—the process of synthesizing RNA from a DNA template. This stage is fundamental to biology because it ensures that the information stored in DNA can be turned into functional molecules, such as proteins (via mRNA) or regulatory RNAs. In prokaryotes, transcription occurs in the cytoplasm, while in eukaryotes, it takes place in the nucleus followed by additional RNA processing steps before the mRNA is exported to the cytoplasm.

General Overview of Transcription

Transcription is carried out by RNA polymerase, which unwinds the DNA double helix locally, reads the template strand in the 3'→5' direction, and synthesizes a complementary RNA strand in the 5'→3' direction. The non-template (coding) DNA strand has the same sequence as the RNA transcript (except for uracil in RNA instead of thymine in DNA).

sequenceDiagram participant DNA_template as DNA Template participant RNA_pol as RNA Polymerase participant mRNA as mRNA Transcript DNA_template->>RNA_pol: Exposes template region RNA_pol->>mRNA: Synthesizes RNA (5'->3') mRNA->>RNA_pol: Grows in length

A simplified Mermaid sequence diagram representing key players during transcription: the DNA template, RNA polymerase, and growing RNA.

Prokaryotic vs. Eukaryotic Transcription

While the core mechanism of RNA synthesis is conserved across life, there are notable differences between prokaryotes and eukaryotes:

Comparison of Prokaryotic and Eukaryotic Transcription

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus
RNA Polymerases	A single RNA polymerase (σ factor aids initiation)	Multiple polymerases (RNA pol I, II, III) for rRNA, mRNA, tRNA
Initiation Factors	σ subunit recognizes -10 / -35 promoter elements	Many transcription factors (TFIID, TFIIB, etc.) binding TATA box or other promoter elements
mRNA Processing	Minimal; often used immediately for translation	Extensive: 5' cap, 3' poly-A tail, splicing of introns

Stages of Transcription

1. Initiation: RNA polymerase (plus any required transcription factors) binds the promoter region upstream of the coding sequence. DNA strands locally unwind to form the transcription bubble.
2. Elongation: RNA polymerase moves along the template strand, adding ribonucleotides that are complementary to the DNA template. The new RNA grows from 5' to 3'.
3. Termination: When RNA polymerase encounters a termination signal, it dissociates from the DNA template, releasing the newly synthesized RNA. In prokaryotes, this often involves a hairpin loop or Rho factor; in eukaryotes, termination mechanisms vary with RNA pol II, often tied to polyadenylation signals.

Eukaryotic mRNA Processing

After transcription by RNA polymerase II, eukaryotic pre-mRNA undergoes several processing steps before exiting the nucleus:

• 5' Capping: A modified guanine (7-methylguanosine) cap is added to the 5' end, protecting RNA from degradation and aiding ribosome binding.
• 3' Polyadenylation: A series of adenine nucleotides (poly-A tail) is added to the 3' end, also enhancing stability and export from the nucleus.
• Splicing: Introns (non-coding regions) are removed, and exons (coding segments) are joined by spliceosomes. Alternative splicing can generate multiple protein isoforms from the same gene.

Example code-based illustration: Eukaryotic pre-mRNA with a 5' cap, exons, introns, and a poly-A tail. Arcs represent introns being removed during splicing.

Examples of Regulatory Elements

In addition to the core promoter, genes often contain upstream enhancers, silencers, or other DNA sequences that influence transcription levels. For instance, an enhancer might bind activator proteins that loop DNA around to RNA polymerase at the promoter, thereby boosting transcription.

Biological Significance

Transcription is the first step in gene expression, dictating which segments of the genome are transcribed into RNA. By controlling transcription initiation and subsequent mRNA processing, cells can finely tune protein production, adapt to environmental changes, and differentiate into diverse cell types. Errors in transcription or splicing can disrupt normal physiology and lead to diseases (e.g., certain forms of β-thalassemia resulting from splicing mutations).

Conclusion

From the moment RNA polymerase binds a promoter to the eventual release of a processed mRNA transcript, transcription serves as the gateway for converting genetic information into actionable molecular forms. Understanding its nuances—promoter recognition, elongation efficiency, termination fidelity, and post-transcriptional modifications—paves the way to grasp complex regulatory networks that govern cell function, development, and responses to environmental stimuli.