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Questions and Answers
What is the primary function of RNA polymerase in transcription?
What is the primary function of RNA polymerase in transcription?
Which option correctly describes the sense and anti-sense strands of DNA during transcription?
Which option correctly describes the sense and anti-sense strands of DNA during transcription?
At which location does RNA polymerase bind to initiate transcription?
At which location does RNA polymerase bind to initiate transcription?
Which sequence is NOT involved in the initiation of transcription?
Which sequence is NOT involved in the initiation of transcription?
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What is the direction of RNA synthesis by RNA polymerase?
What is the direction of RNA synthesis by RNA polymerase?
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What is the primary role of RNA polymerase II in eukaryotic transcription?
What is the primary role of RNA polymerase II in eukaryotic transcription?
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Which of the following is a characteristic of rho-independent transcription termination?
Which of the following is a characteristic of rho-independent transcription termination?
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Which post-transcriptional modification is NOT performed in eukaryotic cells?
Which post-transcriptional modification is NOT performed in eukaryotic cells?
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What is required for transcription initiation in eukaryotic genes?
What is required for transcription initiation in eukaryotic genes?
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What distinguishes eukaryotic transcription termination from prokaryotic termination?
What distinguishes eukaryotic transcription termination from prokaryotic termination?
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What is the effect of rifampicin in bacterial transcription?
What is the effect of rifampicin in bacterial transcription?
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Which sequence is commonly recognized as a promoter in eukaryotic genes?
Which sequence is commonly recognized as a promoter in eukaryotic genes?
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What type of RNA is synthesized by RNA polymerase III?
What type of RNA is synthesized by RNA polymerase III?
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Study Notes
Transcription Overview
- Powerpoint slides are supplementary material, not a substitute for textbooks
- Textbooks are essential for complete understanding and exam preparation
- Transcription is the synthesis of mRNA from a DNA gene
- The enzyme responsible for this process is RNA polymerase
- mRNA is a messenger RNA, carrying the genetic code from DNA to the ribosomes for protein synthesis.
Learning Objectives
- Define translation
- Describe transcription initiation, elongation, and termination
- Compare and contrast transcription in eukaryotes and prokaryotes
- List transcription inhibitors
- Describe post-transcriptional modification process
Transcription
- RNA polymerase binds to the promoter region of a gene, a specific sequence on the DNA template that signals the start of transcription
- Only one DNA strand is transcribed (template strand)
- The transcribed strand creates a complementary strand of mRNA
- RNA polymerase synthesizes RNA by attaching nucleotides in the 5' to 3' direction.
Transcription Steps (Prokaryotes)
- Chain initiation
- Chain elongation
- Chain termination
Chain Initiation
- RNA synthesis is initiated at specific sites on the DNA called promoter regions (often containing Pribnow box and -35 sequence)
- The DNA strand that gives rise to mRNA is the template/anti-sense/non-coding strand
- The other (non-template) strand has a sequence identical to the mRNA (coding strand).
Chain Elongation
- DNA unwinds at the origin, creating a transcription bubble
- RNA polymerase synthesizes RNA within the bubble
- The RNA-DNA hybrid is formed during elongation.
Chain Termination
- Occurs at a termination site
- Two types of termination:
- Rho dependent (requires rho protein)
- Rho independent (intrinsic)
- Rho independent terminators have a sequence of 4-10 AT base pairs after a GC rich region
Transcription in Eukaryotes
- Eukaryotes have multiple RNA polymerases (I, II, III) each responsible for different types of RNA synthesis:
- RNA polymerase I: produces rRNA
- RNA polymerase II: produces mRNA
- RNA polymerase III: produces tRNA and some rRNA
- Eukaryotic promoters contain TATA box and CCAAT box, and enhancer sequences.
- Transcription factors (proteins) are required to recognize and bind to the promoter sequences
- Eukaryotic genes lack specific termination sequences
Inhibitors of Transcription
- Rifampicin is an antibiotic used against tuberculosis that binds to prokaryotic RNA polymerase and inhibits chain elongation.
- α-amanitin is a potent toxin from mushrooms that binds tightly to eukaryotic RNA polymerase II, inhibiting its function
Post-Transcriptional Modifications (RNA Processing)
- In prokaryotes, mRNA is directly ready to synthesize protein
- In eukaryotes, nascent mRNA is processed before becoming mature mRNA. This includes:
- mRNA Capping: adding a methylated guanine nucleotide to the 5' end (protection from degradation)
- Polyadenylation: adding a poly-A tail to the 3' end (protection from degradation, ribosomal RNA recognition)
- Splicing: removal of introns (non-coding regions) and rejoining exons (coding regions)
Eukaryotes vs. Prokaryotes (Transcription Differences)
- Overall the process is similar, but differences include:
- Prokaryotic mRNA is often polycistronic (multiple genes on one mRNA) and translated immediately, while eukaryotic mRNA is monocistronic, processed, and translated in separate compartments
- Eukaryotic mRNA has introns that are removed post-transcription, unlike prokaryotic mRNA, which does not
- Eukaryotic transcription occurs in the nucleus, while prokaryotic transcription occurs in the cytoplasm:
- Eukaryotes have multiple RNA polymerases each for different transcripts, while prokaryotes have only one
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Description
This quiz focuses on the process of transcription, detailing the synthesis of mRNA from DNA. It covers essential concepts such as initiation, elongation, termination, and the differences in transcription between eukaryotes and prokaryotes. Additionally, it includes the roles of RNA polymerase and post-transcriptional modifications.
