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2. Chromomethylase 3 (CMT3): It is a plant-specific enzyme that

retains asymmetric regions methylated, like centromeric repeats and

transposons (Chan et al., 2005).

3. Domains Rearranged Methylase (DRM): It is associated with the

asymmetric site for de novo methylation (Cao & Jacobsen, 2002).

DRM is categorized into two kinds: DRM1 and DRM2. With the

use of short interfering RNAs (si-RNAs), DRM2 has been revealed

to be concerned with the RNA-directed DNA methylation (RdDM)

process (siRNAs) (Hauser et al., 2011).

Whether active or passive, the reversible nature of methylation is aided

by demethylation. Removal of an ancestral mark or de novo methylation can

be avoided during the passive procedure following the replication phase.

It occurs when DNMT1 fails to methylate freshly produced DNA strands

following several replication cycles after being treated with 5-azacytidine

(Sahu et al., 2013). DEMETER (DME), Demeter-Like 2 (DML2), Demeter-

Like 3 (DML3), as well as Repressor of Silencing 1 (ROS1) are DNA glyco­

sylases and AP lyases involved in the active process (Grativol et al., 2012).

DNA methylation at the promoter of the gene suppresses transcription, but

methylation of the gene body can be beneficial or deleterious on gene expres­

sion. Genes that are expressed seldom are highly methylated, whereas genes

that are expressed frequently are barely methylated (Zemach et al., 2010).

Arabidopsis DRM2 gene gets methylates both – cytosine residues at CCGG

sites, as well as adenines on N6 residues of the GATC locus (Ashapkin et

al., 2002). As a result, it is possible to conclude that two distinct forms of

modification coexist. Although cytosine methylation is thought to influence

adenine methylation, cytosine methylation is thought to influence adenine

methylation though their relationship has yet to be discovered. Methylation

of cytosine has a key function in the existence and maturation of plants

by regulating the transcription procedure, in particular the developmental

procedure and also incidences like vernalization and acclimatization to stress

(Vyse et al., 2020).

12.4.2 HISTONE MODIFICATION

Histone alterations are one of the epigenetic mechanisms that have been

most explored and are the most varied. Nucleosome form a constitutional

component of chromatin is made up of eight molecules of core histones.

Nucleosomes are wrapped with 145–147 bp of double-stranded DNA and