EPIGENETICS Flashcards
LEARNING OBJECTIVES
- explain significance of epigenetic marks in context of gene expression and phenotypes
- compare and contrast different types of epigenetic marks
- discuss mitotic and meiotic mechanisms of epigenetic inheritance
definition
Epigenetics: The study of heritable changes in gene expression and gene function that are not explained by changes in DNA sequence.
Labels DNA and proteins with chemical markers, influencing whether specific genes are read or not.
epigenetic marks and cell function
Each cell has a unique epigenome, allowing for differentiation into different types of cells and forming organs.
Epigenetic marks provide an additional layer of information that influences how identical genomes are expressed differently in various cell types.
DNA methylation
Occurs in both prokaryotes and eukaryotes.
Methyl group is transferred to cytosine nucleotides (often CG sequences) by specific writer enzymes and removed by eraser proteins.
Methylation in promoter regions inhibits transcription factor binding, silencing genes.
Leads to differences in gene expression between cells.
example of epigenetics in twins
Despite having identical DNA, twins have unique epigenomes influenced by their environment and lifestyle choices.
As a result, twins can develop different phenotypes and even live different lives.
maintenance during DNA replication
Methylation needs to be re-established on newly synthesized strands.
Hemi-methylated DNA is recognized by specific writer/reader proteins, which methylate the new DNA strand.
mitotic inheritance
epigenetic marks are maintained during cell division
daughter cells retain same epigenetic patterns as the parent cell, ensuring cell identity and gene expression patterns are preserved
important as if replacing a certain cell, must need to express same things
histone code
Histones are proteins that DNA wraps around, forming nucleosomes.
Four main types of histones: H2A, H2B, H3, H4.
Histones can be modified by acetylation, methylation, phosphorylation, and ubiquitylation, impacting DNA accessibility.
Histone modifications create a unique “code” that influences gene expression.
Histone Acetylation (H3K9ac)
H3K9ac: Histone 3, lysine residue at position 9, acetylated.
Acetylation neutralizes the positive charge on histones, loosening the DNA and making it more accessible to transcription machinery.
Increases transcription by promoting gene expression.
Histone Methylation (HEK27me3)
HEK27me3: Histone 3, lysine residue at position 27, tri-methylated.
Causes gene silencing by affecting nucleosome positioning and condensation.
Methylation at other histone positions may promote transcription, depending on the context.
Maintenance During DNA Replication (Nucleosomes)
Nucleosomes disassemble ahead of replication, and histones are re-deposited after replication.
Parental histones are shared between daughter strands, and new histones are added from the cytosol.
Marks on parental histones recruit writers to deposit similar marks on newly formed histones.
Maintenance During Meiosis
For epigenetic marks to be inherited across generations, they must be maintained in the germline.
In animal gametes, epigenetic marks are often removed by eraser proteins.
If erasure is incomplete or marks are re-established, this can lead to transgenerational inheritance.
This phenomenon is more commonly recognized in plants and remains contentious in animals.
Regulation of Flowering Time in Plants (Autumn)
Autumn: Germination occurs followed by vegetative growth.
Flowering is repressed by the expression of Flowering Locus C (FLC), which encodes a transcription factor repressing floral transition.
Regulation of Flowering Time in Plants (Winter)
Winter: Vernalization (prolonged cold) leads to histone modification.
This causes repression of FLC transcription and induces stable epigenetic silencing.
Regulation of Flowering Time in Plants (Spring)
Spring: Warmer temperatures maintain the epigenetic silencing of FLC.
The plant enters the flowering stage due to this “memory of the cold.”
