Process of Transcription — Revision Notes

⚡ 30-Second Revision

Transcription: — DNA $\rightarrow$ RNA.
Enzyme: — RNA Polymerase (DNA-dependent RNA polymerase).
Direction: — DNA template read 3' $\rightarrow$ 5'; RNA synthesized 5' $\rightarrow$ 3'.
Transcription Unit: — Promoter, Structural Gene, Terminator.
Prokaryotes: — Single RNA Pol, sigma factor (initiation), Rho-dependent/independent termination. Coupled transcription-translation.
Eukaryotes:

- Pol I: rRNA (18S, 28S, 5.8S) - Pol II: mRNA (hnRNA), some snRNA - Pol III: tRNA, 5S rRNA, other small RNAs - Post-transcriptional modifications (pre-mRNA $\rightarrow$ mRNA): - 5' Capping: 7-methylguanosine (protection, ribosome binding). - Splicing: Intron removal, exon joining (spliceosome). - 3' Polyadenylation: Poly-A tail (stability, export).

2-Minute Revision

Transcription is the process of synthesizing RNA from a DNA template, the first step in gene expression. RNA polymerase is the key enzyme, reading the DNA template in the 3' to 5' direction and synthesizing RNA in the 5' to 3' direction.

A transcription unit includes a promoter (RNA Pol binding), a structural gene (coding region), and a terminator (signals end). In prokaryotes, a single RNA polymerase, aided by the sigma factor for initiation, synthesizes all RNA types.

Termination can be rho-dependent (requires Rho protein) or rho-independent (involves hairpin loop and U-rich sequence). Transcription and translation are coupled. Eukaryotes have three distinct RNA polymerases: Pol I for most rRNAs, Pol II for mRNA, and Pol III for tRNA and 5S rRNA.

Eukaryotic transcription occurs in the nucleus and is followed by crucial post-transcriptional modifications of pre-mRNA: 5' capping (7-methylguanosine for protection and ribosome binding), splicing (removal of non-coding introns and joining of coding exons by the spliceosome), and 3' polyadenylation (poly-A tail for stability and nuclear export).

These modifications are essential for producing functional mRNA.

5-Minute Revision

Transcription is the fundamental process of converting genetic information from DNA into RNA. It's catalyzed by DNA-dependent RNA polymerase, which unwinds a segment of DNA and synthesizes a complementary RNA strand using one DNA strand (the template strand) as a guide. The RNA is synthesized in the 5' to 3' direction, while the DNA template is read 3' to 5'.

The process is divided into three stages: Initiation, Elongation, and Termination.

Prokaryotic Transcription:

1
Initiation: — RNA polymerase holoenzyme (core enzyme + sigma factor) recognizes and binds to specific promoter sequences (-35 and -10 regions). The sigma factor is crucial for promoter recognition and dissociates after initiation.
2
Elongation: — The core RNA polymerase moves along the DNA, unwinding it and synthesizing RNA. The transcription bubble moves with the polymerase.
3
Termination: — Two main types:

* Rho-dependent: Rho protein binds to the RNA, moves towards the polymerase, and unwinds the RNA-DNA hybrid, causing dissociation. * Rho-independent: Involves a hairpin loop structure in the nascent RNA followed by a U-rich region, leading to polymerase pausing and dissociation. Prokaryotic transcription and translation are coupled, occurring simultaneously in the cytoplasm.

Eukaryotic Transcription:

1
RNA Polymerases: — Three types:

* Pol I: Synthesizes 18S, 5.8S, and 28S rRNA. * Pol II: Synthesizes pre-mRNA (hnRNA) and some snRNAs. This is the polymerase for protein-coding genes. * Pol III: Synthesizes tRNA, 5S rRNA, and other small RNAs.

1
Initiation: — More complex, requiring multiple general transcription factors (GTFs) to bind to the promoter (e.g., TATA box for Pol II) and recruit the specific RNA polymerase.
2
Elongation: — Similar to prokaryotes, RNA polymerase moves along the template, synthesizing RNA.
3
Termination: — Varies by polymerase. For Pol II, it's often linked to post-transcriptional processing, involving a polyadenylation signal and subsequent cleavage.

Post-transcriptional Modifications (Eukaryotes Only): Pre-mRNA undergoes extensive processing to become mature mRNA:

1
5' Capping: — A 7-methylguanosine cap is added to the 5' end. This protects mRNA from degradation, aids nuclear export, and is essential for ribosome binding.
2
Splicing: — Non-coding introns are removed, and coding exons are precisely joined together by a spliceosome (composed of snRNPs). This allows for alternative splicing, generating multiple proteins from one gene.
3
3' Polyadenylation: — A poly-A tail (50-250 adenine nucleotides) is added to the 3' end. This enhances mRNA stability, facilitates nuclear export, and plays a role in translation.

These modifications are crucial for the stability, transport, and translation of eukaryotic mRNA, ensuring proper gene expression.

Prelims Revision Notes

Central Dogma: — DNA $\rightarrow$ RNA $\rightarrow$ Protein. Transcription is the first step.
RNA Polymerase: — DNA-dependent RNA polymerase. Synthesizes RNA from DNA template. Does not require a primer.
Template Strand (Antisense): — DNA strand read by RNA Pol (3' $\rightarrow$ 5').
Coding Strand (Sense/Non-template): — DNA strand with sequence similar to mRNA (except T for U).
Transcription Unit: — Promoter (RNA Pol binding), Structural Gene (coding), Terminator (stop signal).
Prokaryotic Transcription:

- Location: Cytoplasm. - RNA Pol: One type, with sigma ($\sigma$) factor for promoter recognition (-35, -10 sequences). - Initiation: Sigma factor binds promoter, forms open complex, then dissociates.

- Elongation: Core enzyme synthesizes RNA. - Termination: - Rho-dependent: Requires Rho protein, binds RNA, unwinds RNA-DNA hybrid. - Rho-independent (Intrinsic): Hairpin loop in RNA followed by U-rich sequence causes dissociation.

- Coupling: Transcription and translation are coupled. - mRNA: Polycistronic (codes for multiple proteins) is common. No introns.

Eukaryotic Transcription:

- Location: Nucleus. - RNA Polymerases: - Pol I: Synthesizes 18S, 5.8S, 28S rRNA. - Pol II: Synthesizes pre-mRNA (hnRNA), some snRNA. Inhibited by $\alpha$-amanitin. - Pol III: Synthesizes tRNA, 5S rRNA, other small RNAs.

- Initiation: Requires General Transcription Factors (GTFs) to recruit RNA Pol to promoter (e.g., TATA box). - Termination: Complex, often coupled with 3' end processing for Pol II. - mRNA: Monocistronic (codes for one protein) is common.

- **Post-transcriptional Modifications (pre-mRNA $\rightarrow$ mature mRNA): - 5' Capping:** Addition of 7-methylguanosine cap (5'-5' linkage). Functions: protection from degradation, nuclear export, ribosome binding.

- Splicing: Removal of non-coding introns and joining of coding exons. Catalyzed by spliceosome (snRNPs). Allows alternative splicing. - 3' Polyadenylation: Addition of a poly-A tail (50-250 A's) to the 3' end.

Functions: stability, nuclear export, translation efficiency.

Key Differences: — Compartmentalization, number of RNA Pol, presence of introns, post-transcriptional modifications, coupling of processes.

Vyyuha Quick Recall

For Eukaryotic RNA Polymerases and their products: ReMeT

RNA Pol I $\rightarrow$ RRNA (except 5S)
RNA Pol II $\rightarrow$ MRNA (and some snRNA)
RNA Pol III $\rightarrow$ TRNA (and 5S rRNA, other small RNAs)