Regulation of Gene Expression Flashcards
What is gene regulation and expression and types of genes
- Growth and division genes of bacteria are regulated by genes
- Controlled by the needs of cell as it responds to environment with goal of increasing in mass and dividing
- Constitutive Genes: Continuously expressed (housekeeping genes)
- Prokaryotic Genes: Organised into operons that are co-transcribed
What is an operon
- A group of bacterial structural genes that are transcribed together (along with their promoter and additional sequences that control transcription), inducible
- Control Region: Promoter and operator
- Structural Genes: Coding sequence for induction, transcription and translation
- Regulation: Dependent on presence of repressor
- Repressor: Functional (binds downstream of promoter region)
What is the LAC operon
- Lactose: Disaccharide (glucose-1-galactose), lactose is E. coli’s sole carbon source
- β-galactosidase: Gene, two functions, breaking lactose into glucose and galactose and converting lactose to allolactose (an isomerisation)
- Lactose Permease: Gene, M protein, required for transport of lactose across the cytoplasmic membrane
- Transacetylase: Gene, poorly understood
What is an inducible vs repressible operon and examples
- Inducible: Always off, turned on when small molecule is present (removes repressor) LAC
- Repressible: Always on, turned off when small molecule is present (adds repressor), TRP
What occurs in translation of LAC operon
- Wild Type: β-galactosidase produced, ribosome slides to lacY+, permease produced, ribosome slides to lacA+, transacetylase produced and stop codon
- lacZ-: Nonsense mutation, incomplete β-galactosidase protein release, ribosomal subunits fall of mRNA
What occurs in presence / absence of lactose and glucose in LAC operon
Absence:
- CAP site (LacI) undergoes transcription to produce LacI protein (diffusible regulatory protein)
- RNA polymerase binds but is unable to act due to presence of LaI at regulatory regions
- No transcript and therefore no proteins
Presence:
- CAP site (LacI) undergoes transcription to produce LacI protein
- Lactose binds with LacI protein and undergoes conformational change
- Prevents LacI from binding to regulatory region
- RNA polymerase binds promoter and transcribe genes
How is the LAC operon regulated
- The regulatory gene lacI produces an mRNA that produces a Lac repressor protein, which can bind to the operator of the lac operon
- Prevents RNA polymerase from transcribing structural genes
- Lac repressor inhibits transcription of lac operon
What is the TRP operon and its function
- If amino acids are available in medium, E. coli will import them rather than make them
- Genes for amino acid biosynthesis are repressed
- When amino acids are absent, genes are expressed and biosynthesis occurs
- Repressible (always on), generally
- Anabolic pathways are repressed when end product is available
What occurs in presence / absence of tryptophan in TRP operon
Presence:
- High concentration of TRP binds to repressor inducing a conformational change
- Allowing it to bind to operator (regulatory regions) and preventing RNA polymerase from binding and synthesis
Absence:
- TRP doesn’t bind with repressor
- Repressor is unable to bind regulatory regions
- Instead RNA polymerase binds regulatory genes and synthesis proceeds
How is TRP regulated
Repressor / Operator Mechanism:
- When tryptophan is present, it binds to an apo-repressor protein (trpR gene product)
- Active repressor (apo-repressor + tryptophan) binds trp operator, and prevents transcription initiation
Attenuation:
- Premature termination of transcription, produces short transcripts that do not encode structural proteins
- TRP-Starved: 2 and 3 form hairpin
- Non-Starved: 3 and 4 form hairpin
List the mechanisms of regulating gene expression in eukaryotes
- DNA methylation
- Histone modifications
- Gene amplification
- Transcription initiation
How does DNA methylation regulate gene expression
- Addition of methyl groups to heterochromatin (tightly packaged DNA)
- Occurs most often in symmetrical CG sequences
- Transcriptionally active genes possess significantly lower levels of methylated DNA than inactive genes
- Fragile X Syndrome: Result of methylation, FMR-1 gene is silenced by CGG repeats
How does histone modifications regulate gene expression
- Histones package and organise DNA
- Acetylation by HATs / co-activators leads to euchromatin (allows gene expression)
- Methylation by HDACs / co-repressors leads to heterochromatin formation (inhibits gene expression)
How does gene amplification regulate gene expression
- Repeated rounds of DNA replication, multiple copies of particular chromosomal region
- Increased levels of resulting protein, can confer resistance / susceptibility to certain diseases / cancers
- EGFR Up-Regulation: Associated with cancers, increased cell proliferation
- Spinal Muscular Atrophy: Copies of SMN2 directly correlate to severity of juvenile motor neuron disease
- Transgenic Animals: Use PrP promoter to over-express SMN
How does transcription initiation regulate gene expression
- By using different sequences (promoter, enhancer or silencer sequences) and factors
- Rate of transcription of a gene is controlled
