Gene expression as RNA and protein Flashcards
Describe the initiation phase of transcription in eukaryotes
Initiation begins with RNA polymerase II binding to the promoter region, specifically the TATA box and Initiator (Inr). General transcription factors (GTFs) like TBP, TFIIB, TFIIF, TFIIE, and TFIIH assemble into the pre-initiation complex (PIC) to position RNAP correctly and start transcription.
How does transcription initiation differ between prokaryotes and eukaryotes?
In prokaryotes, RNA polymerase, aided by a sigma factor, directly binds to -10 (Pribnow box) and -35 promoter elements. Eukaryotes require multiple GTFs and enhancers, with transcription occurring in the nucleus and involving chromatin remodeling.
What role does the sigma factor play in bacterial transcription?
The sigma factor directs RNA polymerase to the promoter by recognizing specific DNA sequences (-10 and -35 regions), enhancing promoter specificity and ensuring transcription starts at the correct site.
Differentiate intrinsic and rho-dependent termination in bacteria.
Intrinsic termination involves RNA forming a stem-loop structure followed by a U-rich region, causing dissociation. Rho-dependent termination requires the rho protein, which binds to the RNA and uses ATP to unwind the RNA-DNA hybrid, halting transcription.
What is the function of the CAAT box in eukaryotic transcription?
The CAAT box, located upstream of the TATA box, is a promoter element that enhances the efficiency of transcription initiation by serving as a binding site for transcription factors.
Describe the three major modifications of eukaryotic pre-mRNA.
These include 5’ capping with 7-methylguanylate, 3’ polyadenylation with a poly-A tail, and intron removal via splicing. These processes protect mRNA and facilitate translation.
How does splicing occur, and what role do snRNAs play?
Splicing removes introns and joins exons via the spliceosome, which consists of snRNAs (U1, U2, U4, U5, U6) and proteins forming snRNPs. snRNAs recognize splice sites and catalyze the removal of intron
What is alternative splicing and why is it important?
Alternative splicing generates multiple mRNA variants from a single gene, leading to different protein products and increasing protein diversity. It’s crucial for development and tissue-specific expression.
Explain the concept of RNA editing and give an example.
RNA editing involves enzymatic modification of RNA bases post-transcription, such as changing cytidine to uridine (C→U). This alters the protein-coding potential without changing the DNA.
How does the 5’ cap protect mRNA?
The 5’ cap structure resists exonuclease degradation and facilitates ribosome binding during translation, enhancing mRNA stability and translation efficiency.
What are the roles of the A, P, and E sites on the ribosome?
The A site binds incoming aminoacyl-tRNA, the P site holds the growing peptide chain, and the E site releases deacylated tRNA. These sites coordinate peptide elongation during translation.
Outline the steps of translation initiation in eukaryotes.
The small ribosomal subunit binds the mRNA’s 5’ cap and scans for the start codon (AUG). Initiator tRNA binds AUG, then the large subunit assembles, forming the complete translation complex.
The small ribosomal subunit binds the mRNA’s 5’ cap and scans for the start codon (AUG). Initiator tRNA binds AUG, then the large subunit assembles, forming the complete translation complex.
Each tRNA has an anticodon complementary to an mRNA codon and is charged with a specific amino acid by its corresponding aminoacyl-tRNA synthetase, ensuring codon-anticodon specificity.
What signals the end of translation?
Stop codons (UAA, UAG, UGA) in mRNA signal translation termination. Release factors bind the A site, prompting polypeptide release and ribosome dissociation.
Stop codons (UAA, UAG, UGA) in mRNA signal translation termination. Release factors bind the A site, prompting polypeptide release and ribosome dissociation.
Degeneracy means multiple codons can encode the same amino acid, which allows for some mutations without changing the protein product, providing genetic robustness.
What is the purpose of post-translational modifications (PTMs)?
PTMs like phosphorylation, glycosylation, and ubiquitination alter protein function, localization, stability, and interactions, fine-tuning protein activity after translation.
