Heterochromatin Knobs Revisited

Abstract

GENETICS Genomic sequencing projects generate vast amounts of data about transcribed genes and euchromatic regions, but this leaves unanswered the mystery of what's going on in heterochromatic regions of chromosomes. Fransz et al. and McCombie et al. present detailed analyses of a portion of chromosome 4 from the small mustard plant, Arabidopsis thaliana , linking physical and cytological markers and delving into the sequence of a large heterochromatic region. Careful mapping studies show how the distribution of transcribed genes and transposons relates to the structure of euchromatic and heterochromatic regions. Analysis through the cell cycle revealed which portions of the chromosome condensed and decondensed during meiosis. Mapping studies indicated that the heterochromatic ‘knob,’ a lump observable by classical cytogenetics, may have evolved from an inversion event that brought heterochromatic sequences normally near the centromere out into the arm of the chromosome. The knob contains largely tandem repeats and retrotransposons with very few transcribed genes, and is susceptible to methylation in the same pattern as are other heterochromatic sequences. With these observations comes greater understanding of the interplay between heterochromatin and the function of chromosomes.— PJH Cell 100 , 367 (2000); Cell 100 , 377 (2000).