BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 10. NUCLEIC ACID METABOLISM

10.6. DNA Biosynthesis (Replication)

10.6.3. DNA Modification

Following the completion of the DNA Synthesis cycle, certain purine and pyrimidine bases within the molecule may undergo chemical modification. Notably, the degree and pattern of modifications in parental DNA molecules are precisely restored in newly synthesized molecules and stably inherited.

In prokaryotic organisms, nitrogenous bases within DNA undergo chemical modification in the form of methylated, hydroxymethylated, or glucosylated derivatives.

Each bacterial Cell contains one or more Enzymes that determine the specific type of DNA modification. Post-replicative DNA Methylation is a particularly significant marking process, occurring at specific sites (recognition sites) through the action of enzymes known as DNA methyltransferases (DNA methylases). These enzymes catalyze The transfer of methyl groups from the active form of Methionine, S-adenosylmethionine, to adenine (yielding N-6-methyladenine) or cytosine (yielding 5-methylcytosine) residues within the recognition site.

Each bacterial species features a distinct distribution pattern of methylated bases along the DNA, which differs from that of other species. This variation in methylation sites ensures the susceptibility of foreign DNA to specific endonucleases (restriction enzymes). These enzymes recognize the absence of methyl groups at respective sites and cleave such unfamiliar DNA, even though it may be methylated at other locations.

Thus, DNA methylases responsible for specific modification recognize the same nucleotide sequences (4-6 nucleotide pairs) as restriction enzymes. This restriction-modification system is widespread in Bacteria and serves a protective function against intracellular Restriction Endonucleases, which would otherwise cleave DNA at these sites in the absence of the corresponding site-specific modification.

Unlike prokaryotes, the functional role of DNA methylation in eukaryotes is not associated with the restriction of unmodified bases.

Introduction/5.html">Eukaryotic Cell DNA methylases are localized in The Nucleus and Mitochondria, catalyzing the transfer of methyl groups to all adenine residues within -GATC- sequences (yielding N-6-methyladenine) and cytosine residues within -GC- sequences (yielding 5-methylcytosine). The enzyme utilizes S-adenosylmethionine as a source of methyl groups.

The exact functional role of DNA methylation in eukaryotes remains incompletely understood. The complexity of this issue is likely due to the multifunctional Nature of DNA methylation, meaning it is involved in A wide variety of cellular processes. One prominent hypothesis regarding The Significance of DNA methylation in eukaryotes is the established positive correlation between cell functional activity and 5-methylcytosine content—cellular Aging is accompanied by a decline in DNA 5-methylcytosine levels.

It is also possible that the presence of methyl groups in DNA strands is essential for Chromosome Structure formation. From an organic chemistry perspective, the methylation of cytosine within DNA enhances its interaction with the complementary base, guanine. This stabilizes the respective DNA region, as CH₃ groups increase the hydrophobic Properties of the bases.

Notably, for a short period following Replication, the -GATC- nucleotide sequences in the DNA molecule are methylated at adenine only in the template strand, not in the newly synthesized strand. This Asymmetry is utilized by repair enzymes to correct errors that may arise during METABOLISM/36.html">DNA replication.

Finally, studies have shown that the degree of DNA methylation varies across different Regions of the molecule and correlates with Gene Expression—inactive genes are generally more heavily methylated than the DNA of actively expressed genes. This relationship can be viewed as a potential mechanism ensuring the direct inheritance of functional gene status in Eukaryotic Cells.