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.

TLS Online TPP Program

#Id: 4047

#Unit 2. Cellular Organization

Heterochromatic regions, which tend to be rich with adenine and thymine (AT-rich) DNA and relatively gene-poor, stain more darkly in G-banding.

In contrast, less condensed chromatin (Euchromatin)—which tends to be rich with guanine and cytosine (GC-rich) and more transcriptionally active—incorporates less Giemsa stain, and these regions appear as light bands in G-banding.

TLS Online TPP Program

#Id: 4048

#Unit 2. Cellular Organization

Chromatin Immunoprecipitation (ChIP) is same as Chromosome Conformation Capture.

3C assays involve three basic steps:

(1) interacting chromosome segments are cross-linked with formaldehyde, 

(2) the DNA is digested, and

(3) cross-linked DNA fragments are ligated to produce products that are amplified and can be further analyzed.