Term
| How are GAL (galactose) genes regulated so that they are only expressed when glucose absent and galactose present? |
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Definition
In the absence of galactose – Gal80 forms a dimer and blocks C term of Gal4 =can’t activate transcription.
In presence of galactose, the galactose interacts with Gla3, which enters the nucleus and interacts with Gal80. Gal80 stays bound to Gal4 but changes conformation, revealing the Gal4 activation domain so transcription is possible.
When Glucose is present it is preferentially used, so there is no more Gal3 interaction so Gal80 stays bound to Gal 4 on the GAL UAS = no transcription. |
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Term
| What is the TUP1-Ssn6 complex and how does it affect galactose production? |
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Definition
| TUP1-Ssn6 binds onto Mig1 repressor protein upstream of the GAL UAS to repress the gene. TUP1-Ssn6 also alters the structure of chromatin to inhibit gene transcription – creates a loop to lock the mediator complex and RNAp2 to inhibit transcription of GAL. Also recruits HDACs to remove acetyl groups from histones and may coat the chromatin fibre via interactions with N term tails of H3 and H4. Mig1 localisation is controlled by glucose – high glucose = Mig1 enters nucleus and repressed GAL genes. Low glucose = Mig1 is phosphorylated and transported out of the nucleus so GAL genes can transcribe if galactose (GAL4) is present. |
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Term
| What are the differences between repression and silencing? |
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Definition
Repression is short range - individual genes
Silencing is long range - multiple genes |
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Term
| How does the Rb protein regulate entry into the S phase of the cell cycle? |
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Definition
| it interacts directly with activators of E2F family and represses expression of proliferative genes (activators still bind to target sites but have repressor so isn’t expressed). Mitogens drive cell cycle progression by induction of cyclinD and inactivation of the retinoblastoma (Rb) protein – when it is unphosphorylated it can interact/bind with E2F – thus blocking E2F from functioning, when it is phosphorylated by CDK4, 6, or 2 then theE2F is free to dissociates and is free for DNA synthesis – forms a dimerization partner. Rb has the ability to bind in the activator domain (acidic, like GAL4) Bound (directly) Rb masks activation of E2F – acts as a repressor. Recruits a histone deacetylase (HDAC) – a co-repressor and also recruits SUV39H1 (targets K9 H3) and HP1 (Heterochromatin protein 1 - targets histone 1 and causes further coiling). |
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Term
| What do Sir proteins do and how do they work? |
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Definition
Sir2, Sir3 and Sir4 spread from the silencer into adjacent chromatin
Interact with N terminal tails of H3 and H4 and tighten up chromatin
Spreading requires deacetylation
Sir2 is a histone deacetylase
Deacetylates along DNA until stopped by signal/boundary - silencing mating type alleles
To switch between types, the allele is copied without the silencers, and the previously active type must be silenced |
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Term
| What causes the white/red variegated eye phenotype in drosophila? |
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Definition
| A rare inversion places the white gene near heterochromatin during development - heterochromatin spread silences white gene, but only in some cells leading to eyes with mutant sectors |
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Term
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Definition
| heterochromatin protein 1 - found in repressive heterochromatin, alpha beta and gamma forms - likely interacts with histones. HP1 beta complexes with SUV39H1, methylating H3K9, and allowing Hp1 to interact and spread along the chromatin |
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Term
| What inactivates CpG Islands and what enzymes do it> |
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Definition
methylation,
dnmt3a and dnmt3b methylate new sites,
dnmt1 maintains methylation after DNA replication |
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Term
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Definition
Xlinked - males normally die around or before birth, predominantly affects females
Affected individuals regress after about 18 months
Head growth slows, speech affected, repetitive hand movements, mental retardation
Condition stabilises, survival to adult possible |
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Term
| What causes Rett syndrome? |
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Definition
| Affected females are heterozygous for the mutant allele MeCP2 - in about half their cells the normal allele is turned off by X inactivation. Methyl C normally recruits HDACs - especially in the brain - loss leads to expression of 'wrong' genes |
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Term
| What is fragile X syndrome? |
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Definition
Commonest inherited mental impairment syndrome
X linked dominant with more severe effects in males
Caused by triplet repeat (CGG) expansion in the 5’UTR of the FMR-1 gene
DNA containing the expanded repeat is methylated at CpGs causing the gene to be turned off |
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Term
| Which is the most active RNAP? |
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Definition
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Term
| What processes rRNA genes? |
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Definition
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Term
| What silences half of the rDNA genes in a cell? |
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Definition
| the NoRC chromatin remodelling complex, which is targeted to silence rDNA genes by a pRNA transcribed from the spacer region, by base pairing with the upstream terminator sequence. After that, NoRC moves a nucleosome to block the transcription start site and HDACs and HMTs form heterochromatin |
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Term
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Definition
| small RNAs such as tRNAs, telomerase RNA components, snRNAs and 5S RNAs |
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Term
| What TFs target RNAPIII to the ICR of a 5S rRNA gene? |
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Definition
| TFIIIA, which contains nine zn fingers which recognise boxes A and C, TFIIIC, which is needed to recruit TFIIIB, whcih recruits RNAPIII |
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