Potential for secondary (hairpin) structure is found to be greater in introns than exons

TLS Online TPP Program

#Id: 4023

#Unit 2. Cellular Organization

In most regions, the fibers are much less densely packed than in the mitotic chromosome. This material is called euchromatin. It has a relatively dispersed appearance in the nucleus and occupies most of the nuclear region.

TLS Online TPP Program

#Id: 4024

#Unit 2. Cellular Organization

Some regions of chromatin are very densely packed with fibers, displaying a condition comparable to that of the chromosome at mitosis. This material is called heterochromatin.

TLS Online TPP Program

#Id: 4033

#Unit 2. Cellular Organization

In some cases, the various heterochromatic regions, especially those associated with centromeres, aggregate into a densely staining chromocenter.

The common form of heterochromatin that always remains heterochromatic is called constitutive heterochromatin. 

It is permanently condensed. 

It replicates late in S phase and has a reduced frequency of genetic recombination relative to euchromatic gene rich areas of the genome.

TLS Online TPP Program

#Id: 4042

#Unit 2. Cellular Organization

In contrast, there is another category of heterochromatin, called facultative heterochromatin, in which regions of euchromatin are converted to a heterochromatic state.

TLS Online TPP Program

#Id: 4043

#Unit 2. Cellular Organization

When mitotic chromosomes are subjected to proteolytic enzyme (trypsin) treatment followed by staining with the chemical dye Giemsa, they generate distinct chromosome-specific patterns called G-bands.

p (short; after the French petit) or q (long; for queue, the French word for tail)

TLS Online TPP Program

#Id: 4044

#Unit 2. Cellular Organization

In G-banding, chromosomes are first gently heated and then exposed to Giemsa stain; this DNA dye preferentially darkens certain regions to produce alternating dark and light G bands. Each G band is a very large segment of DNA from 1 to 10 Mb in length, containing many loops. With high-resolution G-banding techniques, a standard diploid human karyotype of 46 chromosomes is seen to contain hundreds of dark and light G bands. Not used in plants.