Gene Regulation, Transcription, and Translation Practice Flashcards

Foundations of Gene Expression: Transcription and Translation

  • Transcription: The molecular process of synthesizing RNA from a template DNA strand. Specifically, the RNA polymerase reads the non-coding (template) strand to create an RNA copy. Messenger RNA (mRNA) functions as the vehicle carrying this genetic code from the nucleus to the cytosol for subsequent protein synthesis.
  • Translation: The process of synthesizing a polypeptide chain (protein) using the sequence information contained within the mRNA copy of the original DNA.
  • Genotype vs. Phenotype:
    • Genotype: The complete set of genes an organism possesses; it denotes whether an individual carries specific mutations.
    • Phenotype: The functional and physical appearance of an organism, which is a direct consequence of its genotype.
    • Wild-type: The typical or "normal" form of a species as found in nature.
    • Variants: Deviations from the wild-type DNA sequence; these may or may not result in a change in phenotype.
    • Mutants: Organisms containing one or more mutations, specifically designated as such in laboratory settings.
  • Gene Definition: The entire nucleic acid sequence required for the synthesis of a functional polypeptide or a useful RNA molecule (such as tRNA and rRNA).
  • Eukaryotic Gene Anatomy:
    • Genes are situated within vast stretches of noncoding DNA.
    • Introns: Noncoding regions within a gene sequence.
    • Exons: Coding regions that represent the information for the final product.

Experimental Identification of Genes: The Beadle-Tatum Experiment

  • Objective: To identify specific genes linked to metabolic processes using the "one gene, one enzyme" hypothesis.
  • Methodology (Neurospora crassa):
    1. Grow yeast spores on a minimal medium.
    2. Irradiate spores with X-rays or ultraviolet (UV) light to induce DNA damage and create mutations.
    3. Identify "Loss of Function" mutants: these spores can no longer grow on minimal media without specific supplements (e.g., vitamins, amino acids).
    4. Sub-culture mutants in minimal media supplemented with only one nutrient at a time to isolate the specific metabolic deficiency.
  • Identified Supplements: Specific mutants were found to require supplements such as Pyridoxine, p-Aminobenzoic acid, Choline, Inositol, Folic acid, Pantothenic acid, Niacin, Riboflavin, or Thiamine.
  • Conclusion: Disruption of a single gene causes a specific enzyme deficiency in a metabolic pathway.

Eukaryotic Chromatin Structure and Remodeling

  • Chromatin: The nucleoprotein complex found in the nucleus consisting of DNA and proteins.
  • Euchromatin: A non-condensed, chemically active form of chromatin that allows for transcription.
  • Heterochromatin: A highly condensed form of chromatin that is transcriptionally inactive.
  • Histones: Small, basic proteins rich in Lysine (Lys) and Arginine (Arg) that condense DNA.
    • Core Histones: H2A, H2B, H3, and H4.
    • Linker Histone: H1.
  • Structural Units:
    • Nucleosome: An octamer comprising two units each of the core histones (H2A, H2B, H3, H4) complexed with approximately 146 bp146\,bp of DNA, often described as "beads on a string."
    • Chromatosome: A nucleosome core plus the H1 linker protein.
  • Chromatin Remodeling Enzymes:
    • Histone Acetyltransferase (HAT): Transfers acetyl groups to histones, facilitating the transition to euchromatin (Gene "switched on").
    • Histone Deacetylases (HDAC): Remove acetyl groups, allowing for compaction into heterochromatin (Gene "switched off").
    • DNA Methyltransferase (DMT): Methylates cytosines (CpG islands), which is associated with silent chromatin.
    • SWI/SNF: A chromatin remodeling complex involved in shifting nucleosomes to expose DNA.

Essential Transcriptional Proteins and Gene Layout

  • Transcription Factors: Proteins that recognize and bind specific DNA sequences to influence transcription.
    • Basal Transcription Factors (TFIIs): Required for the assembly of the transcription complex.
    • Upstream Transcription Factors: Bind to consensus sequences or enhancer elements to stimulate transcription.
    • Inducible Factors: Regulated by specific inducers.
    • Mediator: A large multiprotein complex that serves as a bridge/regulator between activators, coactivators, and the TFIID/basal transcription complex. It contains Head, Middle, and Tail modules, as well as a CDK kinase domain (Cdk8, Med12, Med13, CycC).
    • Activators and Co-activators: Alter chromatin structure or interact with the Mediator.
    • Repressors and Corepressors: Block transcription.
    • Methyl-binding Proteins: Bind to methylated CpG islands to facilitate chromosome compaction.
  • Functional DNA Elements:
    • Promoter: Site for transcription factor binding. The TATA Box is a key component located 10–30 bp10\text{--}30\,bp upstream (−34-34 to −26-26 range) of the start site with a specific consensus sequence.
    • Enhancers: Sites that can be located upstream or downstream (sometimes >3 kb>3\,kb away) that help recruit factors to the promoter.
    • CpG Islands: Regions rich in CG repeats. Methylation of cytosine here inhibits expression.
    • Silencers: Interact with repressors to block the initiation complex.
    • Insulators: Act as boundaries to prevent cross-talk between adjacent genes, ensuring the correct gene is transcribed.
  • Helix-Loop-Helix Motif: A structural motif found in transcription factors featuring DNA binding and protein interaction domains; it uses a negative charge to attract Ca2+Ca^{2+}.

Epigenetic Regulation

  • Definition: Heritable gene regulation that does not involve changes to the underlying DNA sequence.
  • DNA Modification: Primarily CpG methylation.
  • Histone Modifications: Includes Acetylation, Ubiquitination, Alkylation, Phosphorylation, and Sumoylation.
  • Environmental Influences: Epigenetic patterns are affected by diet (specifically Folic Acid), environmental pollution, and childhood environment (nurturing vs. aggressive).
  • Inheritance: Methylation patterns are inherited and can be expressed through the 3rd generation (grandchildren).

The Mechanism of Eukaryotic Transcription

  • RNA Polymerases:
    • RNA Pol I: Transcribes Class I genes (rRNA).
    • RNA Pol II: Transcribes Class II genes (protein-coding genes to produce mRNA).
    • RNA Pol III: Transcribes tRNA, small rRNA, and small viral RNAs.
  • Transcription Orientation: The enzyme reads the DNA template in the 3′→5′3' \rightarrow 5' direction, while the nascent RNA strand is synthesized in the 5′→3′5' \rightarrow 3' direction. Energy is derived from linking the PO3−C5PO_3-C5 to the nascent HO−C3HO-C3.
  • Transcription Unit Structure:
    • Prokaryotic: Often polycistronic (one promoter controls multiple genes, e.g., the trp operon).
    • Eukaryotic: Generally monocistronic (one promoter per gene). Eukaryotes also utilize alternative splicing (alternative 3′3' exons or internal exons) to produce different mRNA variants from a single gene.
  • Pre-initiation Complex (PIC) Assembly (Unidirectional):
    1. TFIID (comprising TBP [TATA Binding Protein] and TAFs [TBP-Associating Factors]) binds the TATA box, causing the DNA to bend.
    2. TFIIA and TFIIB bind to the TFIID-DNA complex.
    3. RNA Pol II and TFIIF bind to the complex.
    4. TFIIE and TFIIH join the complex.
    5. TFIIH (acting as a helicase) separates the DNA duplex and phosphorylates the carboxyl-terminal domain (COO−COO^-) of RNA Pol II to initiate elongation.
  • Termination: A poorly understood process where RNA Pol II pauses. Stop signals include specific DNA sequences, secondary RNA structures, termination factors, or Adenine/Thymine-rich regions.

mRNA Processing and Stability

  • Modifications:
    • 5' Capping: Addition of a methyl-guanine (mG) cap.
    • 3' Polyadenylation: Addition of a Poly-A tail following nuclease cleavage.
    • Splicing: Removal of introns from the hnRNA (heterogeneous nuclear RNA) to create mature mRNA.
  • Stability and Localization:
    • Nuclear Pore: Structures through which mRNA exits to the cytoplasm.
    • Half-life: The time required for half of the produced message to be degraded.
    • Stability Elements: The 3′3' end controls stability. Elements like AUUUA signify an unstable message. Secondary structures and protective proteins also influence lifespan.

The Genetic Code and Translation Mechanics

  • The Codon: A triad of three nucleotides. Since 42=164^2 = 16 is insufficient for 20 amino acids, 43=644^3 = 64 possibilities are used.
  • Deciphering Examples:
    • UUU=PhenylalanineUUU = \text{Phenylalanine}
    • CCC=ProlineCCC = \text{Proline}
    • AAA=LysineAAA = \text{Lysine}
    • GGG=GlycinGGG = \text{Glycin}
    • Repeats like ACACAC...ACACAC... alternating codons CACCAC and ACAACA produce alternating Threonine and Histidine.
  • Features of the Code:
    • Start Codon: AUG (Methionine).
    • Stop Codons: UAA, UAG, UGA.
    • Non-overlapping: Each nucleotide is part of only one codon.
    • Redundant (Degenerate): Multiple codons can specify the same amino acid (e.g., Serine has 6 codons).
    • Wobble Hypothesis: The first two nucleotides of a codon pair tightly with the anticodon, but the 3rd position (the "wobble base" at the 5′5' end of the tRNA anticodon) is looser, allowing one tRNA to recognize multiple codons.
  • Translational Machinery:
    • Ribosome: The site of peptide bond synthesis.
    • tRNA: Cloverleaf secondary structure with four areas of complementary base pairing. Carries amino acids to the ribosome via its anticodon.
    • Aminoacyl-tRNA Synthetase: Attaches the correct amino acid to the tRNA acceptor stem using ATP.

Ribosome Structure and Initiation

  • Sedimentation (S): Refers to mass and shape, not just size.
  • Prokaryotic Ribosome (70S):
    • Small Subunit (30S): Contains 16S rRNA16S\,rRNA (binds the Shine-Dalgarno sequence) and 21 proteins.
    • Large Subunit (50S): Contains 23S23S and 5S rRNA5S\,rRNA and 31 proteins.
    • Initiation: Uses N-formylmethionine (fMet). Initiation factors IF1, IF2-GTP, and IF3 coordinate assembly.
  • Eukaryotic Ribosome (80S):
    • Small Subunit (40S): Contains 18S rRNA18S\,rRNA and 33 proteins.
    • Large Subunit (60S): Contains 28S,5.8S, and 5S rRNAs28S, 5.8S, \text{ and } 5S\,rRNAs and 49 proteins.
    • Initiation: mRNA is recognized by the 5' mG cap (by eIF4E) and Poly-A tail (by PABP). The Kozak sequence (GCCGCC(A/G)CCACCAUGGGCCGCC(A/G)CCACCAUGG) identifies the correct start codon. Initiation uses Methionine (Met).
  • Eukaryotic Initiation Steps:
    1. 40S preinitiation complex forms with eIF1A and eIF3.
    2. Ternary Complex forms (Met-tRNA + eIF2 + GTP).
    3. 43S subunit forms (40S + Ternary complex).
    4. 48S complex forms as the 43S unit binds the mRNA (scanned by eIF4A helicase).
    5. 80S complex forms upon addition of the 60S subunit and hydrolysis of GTP to GDP.

Elongation and Termination of Translation

  • Ribosomal Domains:
    • A (Aminoacyl) Site: Where the incoming aminoacyl-tRNA binds (escorted by EF-Tu in prokaryotes).
    • P (Peptidyl) Site: Where the growing polypeptide chain is held.
    • E (Exit) Site: Where deacylated tRNA leaves the ribosome.
  • Translocation: Driven by EF-G-GTP (prokaryotes) or EF-2 (eukaryotes). The ribosome moves one codon forward.
  • Energy Cost: 2 GTPs and 1 ATP are consumed per amino acid added.
  • Termination: Triggered by stop codons. Release Factors (RF1/RF2 in bacteria; eRF1 in eukaryotes) mimic tRNA, causing hydrolysis of the peptide from the P site and disassembly of the ribosome.
  • Reading Frame and Mutations:
    • Frameshift: Insertion or deletion of bases shifts the frame, often leading to truncations or misfolded, non-functional proteins.

Antibiotics Targeting Translation

  • Chloramphenicol: Binds to the 50S subunit and inhibits peptide bond formation.
  • Erythromycin: Binds to the 50S subunit and prevents translocation movement.
  • Tetracyclines: Interfere with the attachment of tRNA to the mRNA-ribosome complex.
  • Streptomycin: Changes the shape of the 30S subunit, causing the mRNA code to be read incorrectly.

Protein Biosynthesis and Trafficking

  • Endoplasmic Reticulum (ER):
    • Smooth ER: Involved in steroid hormone synthesis, detoxification, glucose conversion, and Ca2+Ca^{2+} storage. Lacks ribosomes.
    • Rough ER (RER): Site of synthesis for secreted proteins, integral membrane proteins, and lysosomal proteins. Features ribosomes on the cytosolic face.
  • Targeting Signals:
    • ER Signal: N-terminal, 6–126\text{--}12 hydrophobic amino acids.
    • Mitochondrial Signal: N-terminal, 3–53\text{--}5 nonconsecutive Arg/Lys residues, rich in Ser/Thr.
    • Chloroplast Signal: N-terminal, rich in Ser/Thr and small hydrophobic residues.
    • Peroxisome Signal: C-terminal (usually Ser-Lys-Leu).
    • Nuclear Signal (NLS): Internal, cluster of 5 basic amino acids.
  • The Secretory Pathway:
    1. Signal Recognition Particle (SRP) binds the N-terminal signal of the nascent peptide, pausing translation.
    2. The complex docks at a Translocon on the RER membrane (GTP-dependent).
    3. Signal Peptidase cleaves the signal peptide.
    4. Chaperones (Heat Shock Proteins) assist in folding.
  • Post-Translational Modifications:
    • N-glycosylation: Oligosaccharide added to Asparagine (Asn) via dolichol carrier in the RER.
    • O-glycosylation: Carbohydrates linked to Threonine (Thr) or Serine (Ser).
    • GPI Anchor: Transamidase attaches the protein to a lipid anchor near the C-terminus.
    • Protein Disulfide Isomerase: Catalyzes disulfide bond formation.

Vesicle Trafficking and Organelle Targeting

  • Golgi Complex: Organized into Cis, Medial, and Trans cisternae. Involved in modification and sorting.
  • Vesicle Coats:
    • COPII: Anterograde transport (ER to Golgi).
    • COPI: Retrograde transport (Golgi back to ER).
    • Clathrin: Forms triskelion lattices for endocytosis and transport between the Golgi and plasma membrane.
  • Membrane Fusion: Mediated by V-SNARES (on vesicle) and T-SNARES (on target membrane).
  • Lysosomal Targeting: Proteins are tagged with Mannose 6-phosphate (M6P) in the Golgi, recognized by M6P receptors, and transported via clathrin-coated vesicles.
  • Mitochondrial Import: Requires MSF (Matrix Stimulating Factor) or Hsc70, which delivers unfolded peptides to TOM (Transporter Outer Membrane) and TIM (Transport Inner Membrane) protein complexes.
  • Nuclear Import/Export: Regulated by Importins and Exportins through the Nuclear Pore Complex. The directionality is determined by the Ran-GTP/Ran-GDP gradient.