L6-DNA methylation and how does it regulate gene expression Flashcards
(5 cards)
❓ What is DNA methylation?
DNA methylation is an epigenetic modification—a chemical change to DNA that does not alter the DNA sequence.
It involves the addition of a methyl group (–CH₃) to the 5th carbon of the cytosine base in DNA.
This process occurs mostly at Cp(phosphate)G sites is immediately followed by a guanine (G).
The result is 5-methylcytosine (5mC), a stable and inheritable mark.
It is catalyzed by DNA methyltransferases (DNMTs) using the methyl donor SAM (S-adenosyl methionine).
❓ Where in the genome does methylation happen, and what does it affect?
DNA methylation commonly occurs in promoter regions, especially within CpG islands (stretches rich in CpG sites).
When methylation happens in these regions, it typically leads to gene repression—meaning the gene is turned off.
Methylation in other regions (like gene bodies) has variable effects—it can either enhance or silence expression depending on context.
Methylation is particularly important for genomic imprinting, X-chromosome inactivation, and early development.
It is a key way that cells establish and maintain cell identity across divisions.
❓ How does DNA methylation repress gene expression physically?
Methyl groups added to DNA can block transcription factors from binding to the DNA.
This prevents the transcription machinery from accessing the gene, so it cannot be transcribed into mRNA.
This repression is direct—it physically stops proteins from reading the gene.
As a result, the gene stays silent, and the corresponding protein is not produced.
This mechanism is crucial in turning off unneeded or harmful genes in specific cell types.
❓ How does DNA methylation repress gene expression indirectly?
Methylated DNA attracts special proteins called MBDs (methyl-CpG-binding domain proteins).
MBDs recruit other silencing complexes, especially HDACs (histone deacetylases).
These enzymes modify the chromatin, turning it into heterochromatin—a tightly packed, inaccessible form of DNA.
Heterochromatin is transcriptionally silent, meaning genes in these regions are not expressed.
This indirect pathway strengthens and stabilizes the gene silencing effect of methylation.
❓ Can DNA methylation be reversed? What is its biological significance?
Yes—demethylation is possible through enzymes like TETs (ten-eleven translocation enzymes).
TETs convert 5mC into 5-hydroxymethylcytosine (5hmC) as part of a potential reversal process.
Methylation is essential for development, long-term gene silencing, and cell memory.
It plays a major role in brain plasticity, especially in response to stress, learning, and early-life experiences.
Abnormal methylation patterns are associated with mental health disorders, cancer, and neurological diseases.
