Question

Explain how DNA methylation could be used to regulate gene expression in a tissue-specific way. When and where would de novo methylation occur, and when would demethylaiton occur? What would occur in the cells that give rise to eggs and sperm (germ-line cells)

Answer 1

Addition of methyl groups to the bases of DNA is called DNA methylation. Methylation doesn't change the sequence of DNA but it changes the expression of DNA. DNA methylation represses the transcription of DNA generally if methylation occurs in the promoter sequences of DNA.
The denovo methylation occurs in germ cells or at the early cells of embryonic development. The denovo methylation is active at these cells. But some amount of denovo methylation occurs even in somatic cells also.
The CpG sites or CG sites are the DNA sequence regions, where the cytosine nucleotide is followed by Guanine nucleotide in the 5'--->3' direction of linear sequence.
The CpG sites are the sites most of which will get methylated. 70 - 80% of CpG sites are methylated in mammals. Methylation of this dinucleotide is associated with the repression of transcription of this sequence.
Reprogramming of methylation in the primordial germ cells establishes monoallelic expression of imprinted genes. monoallelic expression is exhibited by the imprinting genes through out the life of the Organisms.
Maintenance of the inactivation of retrotransposons and inactivation of one of the two X chromosomes is achieved by the monoallelic expression, which in turn is achieved by the DNA methylation of the CpG regions.
Genomic stability is achieved by the suppression of recombination. Suppression of recombination is caused by the DNA methylation.