Transcription — Revision Notes

⚡ 30-Second Revision

Definition: — DNA $\rightarrow$ RNA (gene expression first step).
Enzyme: — RNA Polymerase (no primer needed).
Directionality: — Template DNA read $3' \rightarrow 5'$, RNA synthesized $5' \rightarrow 3'$.
Prokaryotes: — One RNA Pol, $\sigma$ factor for promoter recognition, coupled transcription-translation, Rho-dependent/independent termination, no introns.
Eukaryotes: — Pol I (rRNA), Pol II (mRNA/hnRNA), Pol III (tRNA, 5S rRNA). Nuclear process, uncoupled from translation.
Eukaryotic mRNA processing:

- **$5'$ Capping: 7-methylguanosine, protects, aids export/translation. - $3'$ Polyadenylation: Poly-A tail, protects, aids export/translation. - Splicing:** Intron removal, exon joining (by spliceosome/snRNPs).

2-Minute Revision

Transcription is the process of synthesizing RNA from a DNA template, catalyzed by RNA polymerase. It's the first step in gene expression, converting genetic information from DNA into an RNA molecule. RNA polymerase reads the DNA template strand in the $3' \rightarrow 5'$ direction and synthesizes RNA in the $5' \rightarrow 3'$ direction, using ribonucleotides (A, U, G, C). The RNA sequence is complementary to the template strand and identical to the coding strand (with U instead of T).

In prokaryotes, a single RNA polymerase handles all transcription, with the sigma factor crucial for promoter recognition. Transcription and translation are coupled. Termination is either Rho-dependent or Rho-independent.

Eukaryotes have three distinct RNA polymerases (Pol I for rRNA, Pol II for mRNA/hnRNA, Pol III for tRNA/5S rRNA) and transcription occurs in the nucleus, separate from cytoplasmic translation. Eukaryotic pre-mRNA (hnRNA) undergoes essential post-transcriptional modifications: $5'$ capping (7-methylguanosine), $3'$ polyadenylation (poly-A tail), and splicing (removal of introns and joining of exons by the spliceosome).

These modifications are vital for mRNA stability, nuclear export, and efficient translation.

5-Minute Revision

Transcription is the process where genetic information from a DNA segment (a gene) is copied into an RNA molecule. This is the initial stage of gene expression, following the Central Dogma (DNA $\rightarrow$ RNA $\rightarrow$ Protein).

The key enzyme is RNA polymerase, which, unlike DNA polymerase, does not require a primer to initiate synthesis. It unwinds the DNA helix locally, reads the template strand in the $3' \rightarrow 5'$ direction, and synthesizes a complementary RNA strand in the $5' \rightarrow 3'$ direction, incorporating Uracil (U) instead of Thymine (T).

The process unfolds in three stages: Initiation, where RNA polymerase binds to a specific DNA sequence called the promoter; Elongation, where the polymerase moves along the DNA, synthesizing the RNA chain; and Termination, where the polymerase dissociates from the DNA and releases the RNA transcript at specific terminator sequences.

Prokaryotic transcription involves a single RNA polymerase holoenzyme (core enzyme + sigma factor). The sigma factor is crucial for specific promoter recognition (e.g., $-35$ and $-10$ sequences). Transcription and translation are coupled, meaning they can occur simultaneously. Termination can be Rho-independent (hairpin loop followed by U-rich region) or Rho-dependent (involving the Rho protein).

Eukaryotic transcription is more complex and occurs in the nucleus. It employs three distinct RNA polymerases: RNA Pol I synthesizes most rRNA, RNA Pol II synthesizes precursor mRNA (hnRNA) and some snRNAs, and RNA Pol III synthesizes tRNA and 5S rRNA.

Eukaryotic promoters are more elaborate, often including a TATA box, and require multiple general transcription factors for RNA polymerase recruitment. A hallmark of eukaryotic transcription is post-transcriptional modification of hnRNA to mature mRNA: $5'$ capping (addition of 7-methylguanosine) for protection, nuclear export, and translation initiation; $3'$ polyadenylation (addition of a poly-A tail) for stability and export; and splicing (removal of non-coding introns and joining of coding exons) by the spliceosome.

Alternative splicing allows a single gene to produce multiple protein products. Understanding these differences and the specific roles of each component is vital for NEET.

Prelims Revision Notes

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Central Dogma: — DNA $\rightarrow$ RNA $\rightarrow$ Protein. Transcription is the DNA $\rightarrow$ RNA step.
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RNA Polymerase: — Enzyme for RNA synthesis. Does not require a primer. Reads DNA template $3' \rightarrow 5'$, synthesizes RNA $5' \rightarrow 3'$.
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DNA Strands:

* Template Strand (Antisense): Read by RNA Pol. RNA is complementary to this strand. * Coding Strand (Sense): Sequence identical to RNA (except T in DNA, U in RNA).

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Stages of Transcription:

* Initiation: RNA Pol binds to promoter. * Elongation: RNA synthesis. * Termination: RNA Pol dissociates at terminator.

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Prokaryotic Transcription:

* RNA Pol: One type, holoenzyme ($\alpha_2\beta\beta' + \sigma$ factor). * Sigma factor: Essential for promoter recognition (e.g., $-35$ TTGACA, $-10$ TATAAT/Pribnow box). * Coupling: Transcription and translation are coupled (simultaneous). * Introns: Generally absent. * Termination: Rho-independent (hairpin + U-rich) or Rho-dependent (Rho protein). * mRNA: Polycistronic (encodes multiple proteins).

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Eukaryotic Transcription:

* Location: Nucleus. * RNA Polymerases: * Pol I: Synthesizes 28S, 18S, 5.8S rRNA. * Pol II: Synthesizes hnRNA (pre-mRNA), snRNA. Has C-terminal domain (CTD). * Pol III: Synthesizes tRNA, 5S rRNA, other small RNAs.

* Promoters: More complex (TATA box, GC box, CAAT box, enhancers). Requires general transcription factors (GTFs). * Coupling: Not coupled (transcription in nucleus, translation in cytoplasm).

* mRNA: Monocistronic (encodes one protein). * Post-transcriptional Modifications (hnRNA processing): * **$5'$ Capping:** Addition of 7-methylguanosine. Functions: protection from degradation, nuclear export, translation initiation.

* **$3'$ Polyadenylation:** Addition of poly-A tail (50-250 A's). Functions: protection, nuclear export, translation efficiency. * Splicing: Removal of non-coding introns; joining of coding exons.

Catalyzed by spliceosome (snRNPs + proteins). Alternative splicing increases protein diversity.

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Types of RNA:

* mRNA: Carries genetic code for protein synthesis. * tRNA: Adaptor molecule, carries amino acids to ribosome. * rRNA: Structural and catalytic component of ribosomes. * snRNA: Involved in splicing (part of spliceosome). * hnRNA: Precursor to mRNA in eukaryotes, undergoes processing.

Vyyuha Quick Recall

Transcription: Template Three to Five, RNA Reaches Five to Three.

Template: The DNA strand that is read.
Three to Five: The direction in which the template strand is read ($3' \rightarrow 5'$).
RNA: The newly synthesized molecule.
Reaches Five to Three: The direction in which RNA is synthesized ($5' \rightarrow 3'$).

For Eukaryotic mRNA processing: Cap, Splice, Poly-A Tail.

Cap: $5'$ Capping
Splice: Splicing (Intron removal)
Poly-A Tail: $3'$ Polyadenylation