The mammalian chromosome is longitudinally heterogeneous in structure and function and this is the basis for the specific banding patterns produced by various chromosome staining techniques. The two most frequently used techniques are G, or Giemsa banding and R, or reverse banding. Each type of stained band is characterised by variations in gene density, time of replication, base composition, density of repeat sequences, and chromatin packaging. It is increasingly apparent that R and G bands, which are complementary to each other, (...) represent separate compartments of the euchromatic human genome, with R bands containing the vast majority of genes. R bands are also more GC‐rich, contain a higher density of Alu repeats, and replicate earlier in S phase, than G bands. These properties may be interdependent and may have coevolved. (shrink)

The mammalian chromosome is longitudinally heterogeneous in structure and function and this is the basis for the specific banding patterns produced by various chromosome staining techniques. The two most frequently used techniques are G, or Giemsa banding and R, or reverse banding. Each type of stained band is characterised by variations in gene density, time of replication, base composition, density of repeat sequences, and chromatin packaging. It is increasingly apparent that R and G bands, which are complementary to each other, (...) represent separate compartments of the euchromatic human genome, with R bands containing the vast majority of genes. R bands are also more GC‐rich, contain a higher density of Alu repeats, and replicate earlier in S phase, than G bands. These properties may be interdependent and may have coevolved. (shrink)

Heterochromatin remains condensed throughout the cell cycle, is generally transcriptionally inert and is built and maintainedbygroupsoffactors witheachgroupmember sharing a similar function. In mammals, these groups include sequence-specific transcriptional repressors, functionalRNAandproteinsinvolvedinDNAandhistone methylation. Heterochromatin is cemented together via interactions within and between each protein group and ismaintainedbythecell’sreplicationmachinery.Itcanbe constitutive (permanent) or facultative (developmentally regulated) and be any size, from a gene promotor to a whole genome. By studying the formation of facultative heterochromatin, we have gained information about how heterochromatin is assembled. We have (...) discovered that there are many different architectural plans for the building of heterochromatin, leading to a seemingly never-ending variety of heterochromatic loci, with each built according to a general rule. (shrink)