methylation

(noun)

the addition of a methyl group to cytosine and adenine residues in DNA that leads to the epigenetic modification of DNA and the reduction of gene expression and protein production

Related Terms

Examples of methylation in the following topics:

  • Regulating Protein Activity and Longevity

    • Proteins can be chemically modified with the addition of methyl, phosphate, acetyl, and ubiquitin groups.
    • Methyl groups are added to proteins via the process of methylation; this is the most common form of post-translational modification.
    • Methylation on side chain nitrogens is considered largely irreversible while methylation of the carboxyl groups is potentially reversible.
    • Methylation in the proteins negates the negative charge on it and increases the hydrophobicity of the protein.
    • Methylation on carboxylate side chains covers up a negative charge and adds hydrophobicity.

  • Block Copolymerization

    • This is illustrated for methyl methacrylate in the following diagram.

  • Epigenetic Alterations in Cancer

    • Mechanisms of epigenetic silencing of tumor suppressor genes and activation of oncogenes include: alteration in CpG island methylation patterns, histone modifications, and dysregulation of DNA binding proteins.
    • In cancer cells, the DNA in the promoter region of silenced genes is methylated on cytosine DNA residues in CpG islands, genomic regions that contain a high frequency of CpG sites, where a cytosine nucleotide occurs next to a guanine nucleotide .
    • This combination of DNA methylation and histone deacetylation (epigenetic modifications that lead to gene silencing) is commonly found in cancer.
    • Because these changes are temporary and can be reversed (for example, by preventing the action of the histone deacetylase protein that removes acetyl groups, or by DNA methyl transferase enzymes that add methyl groups to cytosines in DNA) it is possible to design new drugs and new therapies to take advantage of the reversible nature of these processes.

  • Radical Recombination Reactions

    • To this end, a clever reaction sequence, resulting in intramolecular functionalization of an isolated methyl group, was designed by Derek Barton, and has been named the "Barton reaction".
    • Photolysis generates an oxy radical that is located close to the 18-methyl group.
    • From models it appears that the axial-11-hydroxyl group is equally close to both the 18- and 19-methyl groups.
    • However, the two double bonds in ring A slightly change the orientation of the 19-methyl group, and hydrogen atom abstraction from that location does not occur.
    • If the double bond between carbons 1 & 2 is removed the orientation of the the 19-methyl group improves, and hydrogen atom abstraction from both methyl sites takes place.

  • Alkyl Halide Occurrence

    • The ocean is the largest known source for atmospheric methyl bromide and methyl iodide.
    • Furthermore, the ocean is also estimated to supply 10-20% of atmospheric methyl chloride, with other significant contributions coming from biomass burning, salt marshes and wood-rotting fungi.

  • Addition Copolymerization

    • The copolymerization of styrene with methyl methacrylate, for example, proceeds differently depending on the mechanism.
    • However, the product of cationic polymerization is largely polystyrene, and anionic polymerization favors formation of poly(methyl methacrylate).

  • Acid-Base Indicators

    • In the case of the indicator methyl orange, the HIn is colored red and the ionized In- form is yellow.
    • For methyl orange, Ka = 1.6 X 10-4 and pKa = 3.8.
    • Below pH 2.8, a solution containing methyl orange is red; above approximately 4.8, it is clearly yellow.
    • Both methyl orange and bromocresol green change color in an acidic pH range, while phenolphtalein changes in a basic pH.
    • The molecule methyl orange is commonly used as an indicator in acid-base equilibrium reactions.

  • Hydroboration Reactions and Oxidations

    • The bulky hydroboration reagent needed for this strategy is prepared by reaction of diborane with 2-methyl-2-butene, a highly branched alkene.
    • Thus, by the proper choice of reagents, terminal alkynes may be converted either to methyl ketones (mercuric ion catalyzed hydration) or aldehydes (hydroboration followed by oxidation).

  • Nucleophilicity

    • If we use a common alkyl halide, such as methyl bromide, and a common solvent, ethanol, we can examine the rate at which various nucleophiles substitute the methyl carbon.
    • Note that by using methyl bromide as the reference substrate, the complication of competing elimination reactions is avoided.

  • Alcohol Nomenclature

    • Halogens, on the other hand, do not have a suffix and are named as substituents, for example: (CH3)2C=CHCHClCH3 is 4-chloro-2-methyl-2-pentene.
    • For example: (CH3)2C=CHCH(OH)CH3 is 4-methyl-3-penten-2-ol.
    • The IUPAC name of (CH3)3C–SH is 2-methyl-2-propanethiol, commonly called tert-butyl mercaptan.