A comprehensive series of such temperature-sensitive mutants in Escherichia coli identifies a set of loci called the dna genes. The dna mutants distinguish two stages of replication by their behavior when the temperature is raised

The members of the major class of quick-stop mutants cease replication immediately upon a temperature increase.

The members of the smaller class of slow-stop mutants complete the current round of replication, but cannot start another.

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.

TLS Online TPP Program

#Id: 4045

#Unit 2. Cellular Organization

G-bands are distinguished from interbands by their lower G-C content. This is a peculiar result. If there are 10 bands on a large chromosome with a total content of 100 megabases (Mb), this means that the chromosome is divided into regions of 5 Mb in length that alternate between low G-C (band) and high G-C (interband) content.

There is a tendency for genes to be enriched in the interband regions.

TLS Online TPP Program

#Id: 4046

#Unit 2. Cellular Organization

In the case of human chromosomes, approximately 300 G bands can usually be distinguished during metaphase. A larger number of G bands (in the range of 800) can be observed in prometaphase chromosomes because they are more extended than metaphase chromosomes.