Unit 6 Transcription Tap / Space to flip
Unit 6 DNA → mRNA, performed by RNA polymerase. Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).
Unit 6 · 12–16% of exam
Gene Expression & Regulation
Replication, transcription, translation, mutations, operons, and biotech. See the DNA/RNA priority deep dive for the molecular machinery.
Must-know content
- Replication / transcription / translation are covered in depth on the priority page.
- Genetic code: triplet, redundant (degenerate), nearly universal, non-overlapping. Start codon AUG (Met). Stop codons UAA, UAG, UGA.
- Mutations:
- Point — silent (no aa change), missense (different aa), nonsense (premature stop).
- Frameshift — insertion/deletion not a multiple of 3; usually severe.
- Chromosomal — deletion, duplication, inversion, translocation.
- Prokaryotic gene regulation — operons:
- Lac (inducible): off by default. Lactose (allolactose) inactivates the repressor → genes ON. Catabolic.
- Trp (repressible): on by default. Tryptophan binds the repressor → genes OFF. Anabolic.
- Eukaryotic regulation:
- Transcription factors bind enhancers/promoters.
- Chromatin: heterochromatin (condensed, silenced) vs. euchromatin (open, active). Histone acetylation loosens; DNA methylation silences.
- mRNA processing: 5′ cap, 3′ poly-A tail, splicing of introns.
- Alternative splicing → multiple proteins from one gene.
- Post-translational modification (cleavage, phosphorylation, glycosylation).
- Biotechnology:
- PCR — denature → anneal → extend (Taq polymerase). Exponential amplification.
- Gel electrophoresis — smaller fragments travel farther toward + electrode.
- Restriction enzymes & plasmid cloning.
- CRISPR-Cas9 (genome editing).
- DNA sequencing.
- Viruses: lytic vs. lysogenic cycles; retroviruses (RNA → DNA via reverse transcriptase, e.g., HIV).
Example questions
MCQ In the lac operon, when lactose is present: (A) Repressor binds operator and blocks transcription (B) Allolactose binds repressor, which releases from operator, allowing transcription (C) cAMP levels fall and repressor is activated (D) RNA polymerase is degraded
Answer: B. Allolactose (a lactose isomer) binds the repressor, changing its shape so it falls off the operator. RNA polymerase can then transcribe the structural genes (β-galactosidase, permease, transacetylase).
FRQ A point mutation changes the codon UCA (Ser) to UCG (Ser). Predict the effect on the resulting protein and justify your prediction.
Answer: No effect on the protein. This is a silent mutation: due to redundancy of the genetic code, both UCA and UCG code for serine, so the amino-acid sequence — and therefore the protein's structure and function — are unchanged.
MCQ During gel electrophoresis of DNA fragments, which fragments travel farthest from the wells? (A) Largest (B) Smallest (C) Most negatively charged (D) Most A-T rich
Answer: B. All DNA carries roughly the same charge density, so size — not charge — determines migration. Smaller fragments slip through the gel matrix faster.
Drill flashcards
Unit 6 Translation Tap / Space to flip
Unit 6 mRNA → protein, performed by the ribosome in the cytoplasm (or rough ER for membrane/secreted proteins).
Unit 6 Codon Tap / Space to flip
Unit 6 A 3-nucleotide unit on mRNA that specifies one amino acid (or start/stop signal).
Unit 6 Anticodon Tap / Space to flip
Unit 6 A 3-nucleotide sequence on tRNA complementary and antiparallel to its mRNA codon.
Unit 6 Promoter Tap / Space to flip
Unit 6 DNA region where RNA polymerase binds to start transcription (eukaryotes contain a TATA box).
Unit 6 Operon Tap / Space to flip
Unit 6 Cluster of co-regulated prokaryotic genes sharing a promoter and operator. Includes structural genes downstream.
Unit 6 Inducible operon (lac) Tap / Space to flip
Unit 6 Off by default; an inducer (e.g., allolactose) inactivates the repressor → genes on. Used for catabolic pathways.
Unit 6 Repressible operon (trp) Tap / Space to flip
Unit 6 On by default; a corepressor (e.g., tryptophan) activates the repressor → genes off. Used for anabolic pathways.