Q.15 Which one of the following modifications occurs both on DNA and protein?
(A) ADP-ribosylation (B) Methylation
(C) Sumoylation (D) Ubiquitination
Methylation occurs on both DNA and proteins, making it the correct answer. This post-translational modification on proteins and epigenetic mark on DNA regulates key cellular processes like gene expression and chromatin structure. Understanding these modifications is crucial for CSIR NET Life Sciences preparation.
Option Analysis
ADP-ribosylation adds ADP-ribose from NAD+ to targets. It modifies proteins, especially in DNA damage response via PARPs targeting serine residues, and DNA at phosphate groups or bases during repair. Thus, it occurs on both.
Methylation adds methyl groups to cytosine in DNA (5mC) by DNMTs, repressing transcription, and to lysine/arginine in histones by methyltransferases, altering chromatin. It clearly occurs on both DNA and proteins.
Sumoylation conjugates SUMO proteins to lysine on target proteins, regulating DNA repair, transcription, and chromatin via nuclear proteins. No evidence shows direct SUMO attachment to DNA itself.
Ubiquitination attaches ubiquitin to lysine on proteins, signaling degradation or DNA repair pathways like Fanconi anemia. It targets histone proteins near DNA but not DNA molecules directly.
Correct Answer Explanation
Option (B) Methylation is the standard answer in CSIR NET contexts, as DNA methylation at CpG sites is a core epigenetic mechanism, while protein methylation (especially histones) is equally well-established. Other options like ADP-ribosylation occur on both but less commonly emphasized; sumoylation and ubiquitination remain protein-specific.
Biological Significance
These modifications fine-tune gene expression without sequence changes. DNA methylation silences genes via promoter hypermethylation, while protein methylation recruits readers like chromodomains. Dysregulation links to cancer, making them exam hotspots for molecular biology.
