AP Biology: Transcription – RNA Polymerase, Promoter, mRNA Processing (5? cap, poly?A tail, splicing)

Transcription – RNA Polymerase, Promoter, mRNA Processing (5? cap, poly?A tail, splicing)

Concept Summary

• Transcription: DNA-directed synthesis of RNA by RNA polymerase, producing an RNA transcript complementary to the template DNA strand.
• RNA polymerase: Enzyme that catalyzes RNA synthesis; in eukaryotes, RNA Pol II transcribes mRNA.
• Promoter: DNA sequence upstream of a gene where RNA polymerase binds to initiate transcription; includes TATA box in eukaryotes.
• 5? cap: Modified guanine nucleotide added to the 5? end of pre-mRNA, protecting it from degradation and facilitating ribosome binding.
• Poly-A tail: String of adenine nucleotides added to the 3? end of pre-mRNA, enhancing stability and export from the nucleus.
• Splicing: Removal of introns and joining of exons in pre-mRNA to form mature mRNA; performed by the spliceosome.

Core Questions

WHAT (definitional)

Q: What is a promoter? A: A specific DNA sequence upstream of a gene that recruits RNA polymerase and transcription factors to initiate transcription. Trap/Clarification: Promoters are not transcribed; they are regulatory sequences that determine where transcription starts.

Q: What is the 5? cap? A: A 7-methylguanosine nucleotide added to the 5? end of pre-mRNA via a 5?-5? triphosphate linkage. Trap/Clarification: The cap is not encoded by DNA; it is added post-transcriptionally by capping enzymes.

WHY (causal/explanatory)

Q: Why is the poly-A tail important? A: It stabilizes mRNA, facilitates nuclear export, and enhances translation efficiency by protecting the 3? end from exonucleases. Trap/Clarification: The poly-A tail is not encoded by DNA; it is added by polyadenylate polymerase after transcription.

Q: Why does splicing occur? A: To remove non-coding introns and join coding exons, generating a continuous open reading frame (ORF) for protein synthesis. Trap/Clarification: Splicing is not random; it follows consensus sequences (e.g., GU-AG rule) recognized by the spliceosome.

HOW (process/application)

Q: How does RNA polymerase initiate transcription in eukaryotes? A: Transcription factors (e.g., TFIID) bind the TATA box in the promoter, recruiting RNA Pol II to form the pre-initiation complex. Trap/Clarification: RNA Pol II cannot bind promoters alone; it requires general transcription factors (GTFs) for assembly.

Q: How is the 5? cap added? A: Capping enzymes remove the 5? phosphate, add GMP via a 5?-5? linkage, and methylate the guanine base. Trap/Clarification: The cap is added co-transcriptionally (during transcription), not after completion.

Q: How does splicing work? A: The spliceosome (snRNPs + proteins) recognizes intron-exon boundaries, cuts at the 5? splice site, forms a lariat, and ligates exons. Trap/Clarification: Splicing is not performed by RNA Pol II; it is a separate post-transcriptional process.

CAN (conditions/possibilities)

Q: Can transcription occur without a promoter? A: No; promoters are essential for RNA polymerase binding and transcription initiation in all domains of life. Trap/Clarification: Some viruses use alternative mechanisms (e.g., internal ribosome entry sites), but cellular transcription requires promoters.

Q: Under what conditions does alternative splicing occur? A: When splice sites are differentially recognized, allowing a single pre-mRNA to produce multiple mature mRNA isoforms (e.g., tissue-specific proteins). Trap/Clarification: Alternative splicing is regulated (e.g., by splicing enhancers/silencers), not random.

Quick Facts & Traps

• Fact: RNA Pol II synthesizes mRNA in eukaryotes; RNA Pol I (rRNA) and III (tRNA/snRNA) transcribe other RNAs.
• Trap: "RNA polymerase proofreads like DNA polymerase."-Reality: RNA Pol lacks 35? exonuclease activity; errors are tolerated in RNA.
• Fact: TATA box is a core promoter element in eukaryotes (~25–35 bp upstream of the start site).
• Trap: "All genes have a TATA box."-Reality: Many eukaryotic genes use TATA-less promoters (e.g., GC-rich or initiator sequences).
• Fact: Spliceosome is a ribonucleoprotein complex (snRNPs U1, U2, U4/U6, U5) that catalyzes splicing.
• Trap: "Introns are junk DNA."-Reality: Introns can contain regulatory elements (e.g., enhancers) or encode non-coding RNAs.

Rapid-Fire True/False

• Statement: The poly-A tail is encoded by a stretch of thymine nucleotides in the DNA template. Answer: FALSE Why the common mistake happens: Students confuse the poly-A tail with poly-T sequences in DNA; the tail is added post-transcriptionally.

• Statement: RNA polymerase reads the template DNA strand in the 35? direction to synthesize RNA in the 53? direction. Answer: TRUE Why the common mistake happens: Students often reverse the directionality, assuming RNA Pol reads 53? like ribosomes.

• Statement: The 5? cap and poly-A tail are added to prokaryotic mRNA. Answer: FALSE Why the common mistake happens: Students overgeneralize eukaryotic mRNA processing to prokaryotes, which lack nuclei and post-transcriptional modifications.