Human karyotype with bands and sub-bands

G-banding, G banding or Giemsa banding is a technique used in cytogenetics to produce a visible

karyotype A karyotype is the general appearance of the complete set of chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is discerned by de ...

by staining condensed

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

s. It is the most common chromosome banding method. It is useful for identifying genetic diseases (mainly chromosomal abnormalities) through the photographic representation of the entire chromosome complement.Speicher, Michael R. and Nigel P. Carter. "The New Cytogenetics: Blurring the Boundaries with Molecular Biology." ''Nature'' Reviews Genetics, Vol 6. Oct 2005.

Method

The metaphase chromosomes are treated with

trypsin Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the dig ...

(to partially digest the chromosome) and stained with Giemsa stain. Heterochromatic regions, which tend to be rich with

adenine Adenine (, ) (nucleoside#List of nucleosides and corresponding nucleobases, symbol A or Ade) is a purine nucleotide base that is found in DNA, RNA, and Adenosine triphosphate, ATP. Usually a white crystalline subtance. The shape of adenine is ...

and

thymine Thymine () (symbol T or Thy) is one of the four nucleotide bases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine ...

(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 Guanine () (symbol G or Gua) is one of the four main nucleotide bases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside ...

and

cytosine Cytosine () (symbol C or Cyt) is one of the four nucleotide bases found in DNA and RNA, along with adenine, guanine, and thymine ( uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attac ...

( GC-rich) and more transcriptionally active—incorporates less Giemsa stain, and these regions appear as light bands in G-banding. The pattern of bands are numbered on each arm of the

from the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fiber ...

to the

telomere A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes (see #Sequences, Sequences). Telomeres are a widespread genetic feature most commonly found in eukaryotes. In ...

. This numbering system allows any band on the

to be identified and described precisely. The reverse of G‑bands is obtained in R‑banding. Staining with Giemsa confers a purple color to chromosomes, but micrographs are often converted to

grayscale In digital photography, computer-generated imagery, and colorimetry, a greyscale (more common in Commonwealth English) or grayscale (more common in American English) image is one in which the value of each pixel is a single sample (signal), s ...

to facilitate data presentation and make comparisons of results from different laboratories. The less condensed the chromosomes are, the more bands appear when G-banding. This means that the different chromosomes are more distinct in prophase than they are in metaphase. File:Human Chromosomes (crop).jpg, Micrograph of human male chromosomes using Giemsa staining for G banding. File:NHGRI human male karyotype.png, Micrograph of human male chromosomes using Giemsa stain, followed by sorting and grayscaling.

Advantage

It is difficult to identify and group chromosomes based on simple

staining Staining is a technique used to enhance contrast in samples, generally at the Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...

because the uniform colour of the structures makes it difficult to differentiate between the different chromosomes. Therefore, techniques like G‑banding were developed that made "bands" appear on the chromosomes. These bands were the same in appearance on the homologous chromosomes, thus, identification became easier and more accurate.

Types of banding

Other types of cytogenic banding are listed below:

References

{{reflist Chromosomes Staining

Method

Advantage

Types of banding

See also

References