Chapter 15: Genes and Proteins Flashcards
Summarize the central dogma and its biological significance.
DNA → RNA → Protein: Genetic information flows from DNA (transcription) to RNA (translation) to protein.
Significance: Explains how genes direct cellular functions and inheritance. Exceptions include reverse transcription (RNA → DNA in retroviruses).
Describe the steps of transcription in eukaryotes.
Initiation: RNA polymerase binds to a promoter (e.g., TATA box) with transcription factors.
Elongation: RNA polymerase synthesizes mRNA in the 5’→3’ direction using the template DNA strand.
Termination: RNA polymerase detaches at a termination sequence (e.g., poly-A signal in eukaryotes).
How is pre-mRNA modified before export to the cytoplasm?
5’ Cap: 7-methylguanosine added for stability and ribosome recognition.
3’ Poly-A Tail: ~200 adenine nucleotides added for stability.
Splicing: Introns removed by spliceosomes; exons joined (e.g., alternative splicing produces multiple proteins from one gene).
What are the key features of the genetic code?
Triplet code: 3 nucleotides (codon) = 1 amino acid.
Degenerate: Multiple codons code for the same amino acid (e.g., UCU, UCC, UCA all = serine).
Universal: Shared across most organisms (exceptions in mitochondria).
Start codon: AUG (methionine); Stop codons: UAA, UAG, UGA.
Outline the stages of translation.
Initiation: Ribosome (small subunit + tRNAMet) binds mRNA; scans for AUG start codon.
Elongation: tRNAs deliver amino acids via codon-anticodon pairing; peptide bonds form.
Termination: Release factor binds stop codon; ribosome dissociates.
What are the components of a ribosome?
Prokaryotes: 70S (50S + 30S subunits).
Eukaryotes: 80S (60S + 40S subunits).
Functional sites:
A site (aminoacyl): Binds incoming tRNA.
P site (peptidyl): Holds tRNA with growing polypeptide.
E site (exit): Releases empty tRNA.
What is the role of tRNA and aminoacyl-tRNA synthetases?
tRNA: Adaptor molecule with anticodon (matches mRNA codon) and amino acid attachment site.
Aminoacyl-tRNA synthetases: Enzymes that “charge” tRNAs by attaching the correct amino acid (ensures fidelity of translation).
How are proteins modified after translation?
Cleavage: Proinsulin → insulin.
Phosphorylation: Activates enzymes (e.g., kinase signaling).
Glycosylation: Adds sugar groups for cell membrane proteins.
Folding: Chaperonins (e.g., HSP70) assist in proper 3D structure.
Compare transcription/translation in prokaryotes and eukaryotes.
Prokaryotes: No nucleus → coupled transcription/translation.
Eukaryotes: Transcription in nucleus; translation in cytoplasm.
Prokaryotes: Polycistronic mRNA (multiple genes).
Eukaryotes: Monocistronic mRNA (single gene).
Prokaryotes: No RNA splicing.
Eukaryotes: RNA splicing removes introns.
How do mutations affect protein structure?
Silent mutation: No amino acid change (e.g., CGA → CGC = arginine).
Missense mutation: Altered amino acid (e.g., sickle cell: GAG → GTG = glutamic acid → valine).
Nonsense mutation: Premature stop codon (e.g., CGA → TGA = arginine → stop).
Frameshift mutation: Insertion/deletion shifts reading frame (severe impact).
What is the lac operon, and how is it regulated?
Lac operon: Cluster of genes for lactose metabolism in E. coli.
Regulation:
Repressor protein binds operator to block transcription in absence of lactose.
Inducer (allolactose) inactivates repressor when lactose is present.
CAP (catabolite activator protein) enhances transcription when glucose is low.
Name three mechanisms of eukaryotic gene regulation.
Epigenetics: DNA methylation/histone modification (e.g., acetylation opens chromatin).
Transcription factors: Proteins binding promoters/enhancers to activate/suppress transcription.
miRNAs: Bind mRNA to block translation or trigger degradation.
How is CRISPR-Cas9 used to edit genes?
Cas9 enzyme cuts DNA at sequences guided by sgRNA.
Applications: Correct mutations (e.g., cystic fibrosis), study gene function, engineer crops.
What role do chaperones play in protein folding?
Chaperonins (e.g., HSP70) prevent misfolding by binding to nascent polypeptides.
Misfolded proteins are tagged with ubiquitin and degraded by proteasomes.
How are proteins directed to specific organelles?
Signal peptides: Short amino acid sequences (e.g., ER signal sequence recognized by SRP).
Mitochondrial/chloroplast proteins: Imported post-translationally via translocases.
What are prions, and how do they cause disease?
Prions: Misfolded proteins inducing normal proteins to misfold (e.g., mad cow disease).
Mechanism: Accumulation of β-sheet aggregates disrupts cell function.
List RNA types and their roles.
mRNA: Carries genetic code for translation.
tRNA: Delivers amino acids to ribosomes.
rRNA: Structural/functional component of ribosomes.
miRNA/siRNA: Regulate gene expression.
snRNA: Splicing in spliceosomes.
How does alternative splicing increase protein diversity?
Different exons are joined to produce multiple mRNA variants from one gene.
Example: Drosophila DSCAM gene → 38,000+ protein isoforms.
What is codon usage bias?
Organisms prefer certain codons for an amino acid over others.
Influences translation efficiency and gene expression levels.
How is recombinant DNA technology used?
Medicine: Insulin production in bacteria.
Agriculture: Pest-resistant Bt crops.
Research: GFP tagging to study protein localization.
