RNAi Mechanism. The Central Dogma DNA (double-stranded) RNA (single-stranded) Protein.
How can a sense transgene generate double stranded RNA?
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How can a sense transgene generate double stranded RNA?
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Transgenes insert into chromosomal DNA randomly
or
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Transgenes often integrate in complexes
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Transcriptional gene silencing is initiated by RNA directed methylation of promoter regions
dsRNA homologous to promoters leads to methylation and inactivation by recruitment of chromatin remodeling enzymes.
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Transcriptional Gene silencing in Arabidopsis requiresdsRNA, DNA methyltransferases, histone methylation and histone deacetylation.
RNA directed DNA methylation
DNA methylases associate with histone modification enzymes leading to changes in histone methlyation patterns and histone deacetylation.Condensed chromatin results.
Histone methylation
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Maternal Inheritance
Trait is controlled by genes that are not in the nucleus,Cytoplasmic inheritance.
Eg. mitochondrial or chloroplast genome.
female femalemale male
progeny
parents
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Maternally inherited human diseases:
Leber’s hereditary optic atrophy
Kearns-Sayre Syndrome - eye muscle atrophy
Progressive external opthalmoplegia
MELAS mitochondrial encephalomyopathy lactic acidosis and strokes
Myoclonus epilepsy – muscle disorder
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Maternal effects are traits that are expresseddifferently if inherited from mother or father
Tissue in next generation is really derived from mother. Seed coat controlled by mother’s genome. Trait is not related to embryo’s genome.What would happen in next generation?
female male
parents
progeny
female male
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Maternal effect mutations in Drosophila affect embryo polarityDevelopmental axes of embryo affected by maternal RNAs
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Imprinting causes alleles to be expresseddifferently if they come from the female or the male parent.
Imprinting tends to be important for placental development:Turn off cellular markers from father’s genome in placentaLimit growth of fetus in utero.
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Imprinting is another form of epigenetic gene regulation
ICR – imprintingControl region
Differential methylationleads to differential expression of Maternal and paternal alleles
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Establishment of differential methylation
CTCF zinc finger proteinProtects ICR in femalegermline from denovo methylases
Igf2 insulin-related growth factorRepressed in extraembryonic tissuesand in some embryo tissues
Imprinting
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RNA can mediate differential expression of genes on same chromosome
Igf2r and distant, linked genes: Maternal ICR is methylated,Paternal alleles are not expressed.Small non-translated RNAExpressed from paternal Allele. Initiation or maintenance?
X-innactivation:Paternal X chromosome Not expressed early in embryoAnd in extraembryonic tissuesXist and Tsix expressed on innactive X.
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RNA world
We are currently discovering that RNA has many roles in regulation of gene expression. It is not just an intermediate in protein synthesis anymore
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