Methylation of bases in DNA occurs in both prokaryotes and eukaryotes, although the purpose is generally quite different. [Prokaryotes use methylation to distinguish newly synthesized DNA as discussed in Ch. 14. In eukaryotes, newly synthesized DNA is recognized by other means that are still unclear.] Nonetheless, many eukaryotes do methylate their DNA as a marker for regulating gene expression.
Methylation of DNA is rare in lower eukaryotes. Higher animals methylate up to 10 percent of their cytosines, and higher plants methylate up to 30 percent. In these
FIGURE 10.13 Sequence of Events at the HO Promoter
A) The HO endonuclease gene of yeast is covered by nucleosomes.
B) The transcription factor Swi5p binds to the DNA. C) This is followed by binding of the Swi/Snf complex to Swi5p. D) The remodeled nucleosomes allow binding of an acetyl transferase, SAGA, to the Swi/Snf complex and to a nucleosome. E) As the acetylated histones become less compact, SBF, a transcription factor, binds. F) The transcription apparatus binds.
multi-cellular organisms DNA methylation is often used as a marker for genes whose expression is involved in tissue differentiation. The recognition sequences are extremely short; typically CG for animals and CNG for plants. There are two types of methylases. Maintenance methylases add methyl groups to newly made DNA at locations opposite methyl groups on the old, parental DNA strand. This ensures that the pattern of methylation is inherited during chromosome division. Changing the pattern of methylation involves de novo methylases to add new methyl groups and demethy-lases to remove methyl groups (Fig. 10.14).
Methylation in eukaryotes silences gene expression, as discussed below. In animals, about half the genes are located close to CG-islands (i.e., clusters of CG sequences). Housekeeping genes, which are expressed in all tissues, possess non-methylated CGislands. In contrast, the CG-islands of tissue specific genes are only non-methylated in those particular tissues where the genes are expressed. The maintenance of the pattern of methylation therefore makes sure that the pattern of gene expression stays constant among the cells of a particular tissue. In plants, certain transposable elements may also be inactivated by methylation.
CG-islands Region of DNA in eukaryotes that contains many clustered CG sequences that are used as targets for cytosine methylation de novo methylase An enzyme that adds methyl groups to wholly nonmethylated sites demethylase An enzyme that removes methyl groups housekeeping genes Genes that are switched on all the time because they are needed for essential life functions maintenance methylase Enzyme that adds a second methyl group to the other DNA strand of half-methylated sites
Genetic Imprinting in Eukaryotes Has Its Basis in DNA Methylation Patterns 275
Genes are silenced by methylation of the DNA followed by removal of acetyl groups from the histones.
DNA methylation patterns are reprogrammed when a new zygote is formed.
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