Update on Heterochromatin Heterochromatin in Animals and Plants. Similarities and Differences

Gene function is subjected to the effects of sur-rounding chromatin. The nature of these effects may be epigenetic occurring in some cells, but not others, of the same genetic background. Epigenetic regula-tory mechanisms remain an enigma, but recent stud-ies have provided informative insights into the mo-lecular basis underlying them (for review, see Henikoff et al., 2001; Martienssen and Colot, 2001; Reik et al., 2001). Inside the nucleus, the three levels of structural compaction of DNA are seen as 11-, 30-, and 300-nm fibers, the latter representing the folding of the 30-nm chromatin fiber into loops attached to the nuclear matrix. In addition, regions of dense heterochromatin masses scattered throughout the interphase nucleu