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Cell bio exam 2
| Question | Answer |
|---|---|
| What is the directionality of DNA strand synthesis, and what holds complementary bases together? | DNA strands are synthesized in a 5' to 3' orientation. Complementary base pairs are held together by noncovalent hydrogen bonds: Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds, and Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds |
| Where does RNA polymerase bind to initiate transcription in prokaryotes versus eukaryotes? | In prokaryotes, consensus promoter sequences located at -35 and -10 relative to the start site position RNA polymerase. In eukaryotes, a TATA box upstream of the start site localizes general transcription factors and RNA polymerase II |
| What three RNA processing steps convert eukaryotic pre-mRNA into mature mRNA? | Eukaryotic pre-mRNA undergoes 5' capping, RNA splicing of noncoding introns by small nuclear ribonucleoproteins ('snurps'), and 3' polyadenylation before export through nuclear pores to the cytoplasm |
| How is the genetic code structured, and what specifies an amino acid or translation stop? | The genetic code is formulated in triplicate nucleotide sequences called codons. Each of the 64 possible triplicate codons designates either a specific amino acid or a translation stop signal |
| How are proteins targeted for post-translational degradation in the cell? | Specific amino acid sequences signal the post-translational attachment of ubiquitin to proteins, which flags them for destruction by the proteasome |
| What is the primary control point for regulating gene expression in cells? | The primary mechanism controlling gene expression is at the level of transcription |
| How do transcriptional activators, repressors, and Mediator regulate eukaryotic transcription? | Transcriptional activators bind enhancers to turn up transcription, while repressors bind silencers to suppress it. The Mediator protein complex links these regulatory factors to RNA polymerase and general transcription factors to coordinate transcription |
| How do histone acetylation and DNA methylation affect chromatin state and gene expression? | Histone acetylation neutralizes positive histone charges, opening chromatin structure to promote transcription. DNA methylation (cytosine modification) dampens or silences gene expression and can be inherited by daughter cells as epigenetic memory |
| How do microRNAs (miRNAs) regulate gene expression post-transcriptionally? | Small non-coding miRNAs (~22 nucleotides) processed by Dicer bind to the RISC complex and base-pair with complementary 3' UTR sequences in mRNAs, targeting them for rapid degradation or translational repression |
| What are the three primary lipid classes in biological membranes, and why do they form bilayers? | Membranes are composed of phospholipids, glycolipids, and sterols. Because these molecules are amphipathic (hydrophilic polar heads and hydrophobic nonpolar tails), forming a bilayer in an aqueous environment is energetically favorable |
| What three lipid composition factors increase the fluidity of a membrane bilayer? | Membrane fluidity increases with shorter fatty acid tails, greater unsaturation (double bonds creating kinks that loosen packing), and lower sterol (cholesterol) content |
| What roles do scramblases, flippases, and floppases play in membrane lipid movement? | Scramblases indiscriminately catalyze lipid flip-flop between leaflets in the ER where lipids are synthesized. Flippases and floppases selectively move specific lipids to the inner or outer leaflet of the plasma membrane |
| How do integral membrane proteins differ structurally from peripheral membrane proteins? | Integral membrane proteins pass through or embed within the lipid bilayer. Peripheral membrane proteins associate noncovalently with the membrane surface or with integral proteins without entering the hydrophobic core |
| What three fundamental components are required for protein sorting to organelles? | Protein sorting requires (1) a signal sequence within the protein's primary structure, (2) a specific receptor for that signal sequence, and (3) a translocation system to transport the protein across membranes |
| How does nuclear import occur via importin and Ran-GTP? | Cargo proteins with a Nuclear Localization Signal (NLS) bind importin receptors. Importin moves along hydrophobic -FG- repeat fibrils lining the nuclear pore, and Ran-GTP binding inside the nucleus releases the cargo |
| What are the 5 core components of co-translational ER translocation? | Translocation into the ER requires: (1) the nascent protein with a hydrophobic ER signal sequence, (2) Signal Recognition Particle (SRP), (3) ER membrane SRP receptor, (4) Sec61 translocation channel, and (5) signal peptidase in the ER lumen |
| What functions do coat proteins, adaptins, and dynamin perform during vesicle budding? | Coat proteins (clathrin, COPI, COPII) and adaptins recruit specific cargo and force the donor membrane into a sphere. Dynamin GTPase constricts the bud neck to perform membrane fission |
| How do Rab GTPases and SNARE proteins ensure high-fidelity vesicle targeting and fusion? | Rab GTPases associate with tethering factors to dock vesicles at target membranes. Complementary v-SNAREs (on the vesicle) and t-SNAREs (on the target membrane) wrap tightly together to pull the bilayers close and catalyze fusion. |
| How are resident ER proteins distinguished from proteins destined for export to the Golgi? | Soluble cargo destined for the Golgi carry ER exit signals recognized by COPII coat receptors. Resident ER proteins contain ER retention signals (-KDEL-C) and are retrieved from the Golgi back to the ER via retrograde COPI vesicles |
| How are lysosomal enzymes tagged in the Golgi for transport to the lysosome? | GlcNAc phosphotransferase adds a mannose-6-phosphate (M6P) tag to glycosylated lysosomal enzymes in the Golgi. M6P receptors then bind the tagged proteins and recruit clathrin/adaptin coats to form vesicles bound for endosomes and lysosomes. |
Created by:
bbrdd