lec26 Flashcards
genes that encode enzymes and other proteins that are all involved in the same
process are often clustered in operons and are coordinately controlled by a single promoter
very “economical” for the cell. Why economical?
This kind of gene regulation optimizes resource usage, making bacterial transcription highly efficient.
t or f all genes within an operon are transcribed together onto a single mRNA
t, polycistronic mrna
-each gene on these mRNAs may then be translated independently, so many proteins may
be made from one gene and only few from another
lac operon
lactose present: forms allolactose which is an effector that bidns to repressor lacl, stopping it from binding to the operator. therefore expression of lacz/y/a (b-galactosidase,permease, and acytelase)
no lactose: lacl doesnt have an effector stopping it from being a repressor. it blocks promoter and operator so no gene expression.
whats the inducer of lac operon
allolactose as it just stops the lac repressor from neg reg.
but this is weak it actually needs + regulation to work
🧬 Lac Operon Regulation – 3 Scenarios
- No Lactose Present
Repressor active → operon OFF
No lac mRNA made (even if glucose is low)
- Lactose Present, No Glucose
Allolactose inactivates repressor
High cAMP → CRP binds promoter → High transcription
✅ Operon ON
- Lactose + Glucose Present
Repressor inactivated by allolactose
Low cAMP → CRP inactive → Low transcription, cuz glucose indirectly inhibits production of enzyme (adenylate cyclase) that makes cAMP.
⚠️ Catabolite repression (glucose preferred)
Which of the following is true about the lac operator? a) It binds CRP directly b) It overlaps with the lac promoter c) It is transcribed into mRNA d) It is only present when lactose is available
b ✅ (LacI binds to the operator, physically blocking RNA polymerase from reaching the promoter.)
is rnap a holoenzyme
Once the sigma factor binds, the RNA polymerase holoenzyme is formed, making it fully functional for transcription initiation. so only bacterial rnap!! rmb a holoenzyme is an apoenzyme with its cofactor or coenzyme
Role of Mg²⁺ in RNA Polymerase
🧬 Function: Mg²⁺ ions in the active site of RNA polymerase assist in catalysis during RNA synthesis.
🔬 Key Interactions:
Mg²⁺ interacts with negatively charged oxygens on the α and β phosphoryl groups of incoming nucleoside triphosphates (NTPs).
Mg²⁺ lowers the pKa of the 3′-OH of the last ribose in the existing transcript.
These interactions help position the NTP and assist deprotonation of the 3′-OH, making it a better nucleophile.
how does spliceosome recognize intron?
The U1 and U2 snRNAs in the U1 and U2 snRNPs
recognize and bind the 5′ splice site and branch point
through base pairing.
how can point mutations affect splicing
Point mutations can cause alternative splicing through
elimination of a splice site (mutation in one of the
conserved U1 or U2 recognition sequences) or
introduction of a cryptic 5′ or 3′ splice site, leading to
splicing within an exon.
