VL 5 Epigenetics (ii) Flashcards
What is genomic imprinting?
Genomic imprinting is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner.
This regulation is often controlled by DNA methylation and histone modifications, leading to the silencing of one allele based on its parental origin.
- The zygote receives two copies of each gene, one from the mother’s genome and one from the father’s.
- At most loci, both copies are active.
- Some loci, imprinted loci, show a “parent of origin effect”.
- Expression of these loci is controlled by epigenetic factors.
What did the nuclear transplant experiments demonstrate about genomic imprinting?
Nuclear transplant experiments showed that zygotes with only maternal or only paternal nuclei do not survive, indicating that the two parental genomes are not equivalent and that imprinting requires contributions from both parents.
Flashcard 4
How does DNA methylation contribute to imprinting?
DNA methylation is responsible for imprinting by inactivating genes.
Paternal and maternal alleles may have different methylation patterns, and the survival of the embryo depends on having at least one functional, unmethylated allele from either parent.
*Imprinted genes occur in clusters and may depend on a local control site where de novo methylation occurs unless specifically prevented.
What are imprinting control regions (ICRs)?
ICRs are cis-acting sites that control imprinted genes through methylation.
Methylation at these regions can either inactivate or activate genes, depending on the specific genomic context.
only activation in special occasions. normally i
nactivation
What is genomic imprinting in angiosperms, and how does it differ from animals?
Genomic Imprinting in Angiosperms:
Reproductive Complexity:
Angiosperms (flowering plants) show genomic imprinting, but their reproduction is more complex than that of animals.
Double Fertilization: Involves two fertilization events - one sperm nucleus fertilizes the egg cell to produce a diploid embryo, and the other fertilizes the 2 polar nuclei to produce a triploid endosperm. (nahrungsquelle für embryo)
Imprinting in Seeds
Angiosperms:
* Parent-specific gene expression in the endosperm.
* Controlled by differential methylation of ICRs.
* Ensures balanced nutrient allocation to the embryo.
Animals:
* Parent-specific gene expression in multiple tissues.
* Imprinting marks established during gametogenesis.
* Regulates growth, development, and metabolism.
What is the role of the MEDEA (MEA) gene in Arabidopsis, and how is it regulated?
MEA (MEDEA) Gene:
Function: MEA is a Polycomb group gene that regulates seed development by controlling gene expression through histone modification.
Imprinting: MEA is imprinted, meaning it is expressed from the maternal allele while the paternal allele is silenced.
Regulation by Methylation:
The paternal MEA allele is methylated by MET1 in vegetative tissues and the male gametophyte, leading to its silencing.
In the female gametophyte, MEA is demethylated by DME1, allowing its expression.
-> Importance in Seed Viability
(In crosses where the maternal plant is heterozygous for the mea mutant allele (MEA/mea), 50% of the seeds abort, indicating the critical role of the maternal MEA allele for seed viability. The paternal allele remains silent and does not compensate for the loss.)
What is paramutation?
Paramutation is the interaction between two alleles of the same locus that results in a heritable change of one allele induced by the other. This process violates Mendel’s segregation rule and involves siRNA acting in trans.
The maize b1 locus has a paramutagenic allele.
Einfacher Satz: Paramutation ist ein Phänomen, bei dem ein mutiertes Allel ein normales Allel inaktiviert, sodass auch Nachkommen mit nur normalen Allelen trotzdem die veränderte Eigenschaft zeigen.
What is vernalization, and how does it affect flowering time?
Vernalization is the process by which prolonged exposure to cold (winter) accelerates flowering. It involves the epigenetic silencing of the FLC gene, allowing the activation of flowering promoters like FT.
It is an adaptation that has evolved (several times independently) to align flowering with the favourable conditions of spring and prevent flowering in autumn just before the onset of winter
How does FLC regulate flowering in Arabidopsis?
FLC (Flowerng Locus C) is a repressor of flowering that binds to the FT gene.
Vernalization silences FLC epigenetically -> removing its inhibitory effect on FT and enabling flowering.
FLC silencing:
* deacetylation and methylation (H3K9me and H3K27me)
* VIN3 is needed for methylation (recruites PRC2, which contains a H3K27methyltransferase)
* VIN3 is also induced by vernalization
H3K27me (fakultative heterochromatin) and H3K9me (Heterochromatin)
What role do long noncoding RNAs (lncRNAs) play in FLC regulation?
lncRNAs such as COOLAIR and COLDAIR are involved in the epigenetic silencing of FLC. They are induced by cold and help recruit the PRC2 complex to the FLC locus to establish repressive histone marks.
COOLAIR:
* Antisense lncRNA
* Recruits PRC2
* Leads to H3K27me3 and FLC repression.
COLDAIR:
* Intronic lncRNA (next to FLC gene)
* Guides PRC2 to FLC.
* Promotes H3K27me3 and silencing.
How is the vernalization-induced silencing of FLC maintained?
The silencing of FLC induced by vernalization is maintained by PRC1-like complexes, including LHP1. These complexes help maintain the repressive chromatin state at the FLC locus after the return to warm temperatures.
-> in wild-type plants, FLC levels remain low after returning plants to warm temperatures.
What are the key components involved in the vernalization process?
Key components include:
* the FLC gene (repressor of flowering),
* VIN3 (required for the initiation of FLC silencing by recruiting PRC2),
* PRC2 complex (mediates H3K27me3 modification),
* PRC1-like complexes (maintain silencing) -> LHP1
What is the role of DNA methylation in transposon silencing?
DNA methylation silences transposons by condensing chromatin structure and preventing transcription.
-> crucial for maintaining genome stability and **preventing the mobilization of transposable elements.
What happens in plants with reduced DNA methylation (like met1 or ddm1 mutants)?
In mutants with reduced DNA methylation, transposons become hypomethylated and active, leading to increased mutagenic activity and developmental abnormalities over generations.
Summary vernalisation
- During vernalization, epigenetic silencing of FLC allows expression of FT and other flowering promoters
- VIN3 and the PRC2 complex silence FLC epigenetically.
- Maintaining the silenced state requires PRC1-like complex which includes LHP1.
- Induction of lncRNAs (COOLAIR & COLDAIR) are early events of vernalization and function in targeting the PRC2 complex.
