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 by staining condensed

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

s. It is the most common chromosome banding method. It is useful for identifying genetic diseases 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. The

metaphase Metaphase ( and ) is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed in anaphase). These chromosomes, carrying genetic information, a ...

chromosomes are treated with trypsin (to partially digest the chromosome) and

stained A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials. Accidental staining may make materials app ...

with Giemsa stain. Heterochromatic regions, which tend to be rich with

adenine Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its deriv ...

and

thymine Thymine () ( symbol T or Thy) is one of the four nucleobases 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 pyrimidin ...

(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 nucleobases 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 is ...

and

cytosine Cytosine () ( symbol C or Cyt) is one of the four nucleobases 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 attached ( ...

( 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 fibers ...

to the telomere. This numbering system allows any band on the

to be identified and described precisely. The reverse of G‑bands is obtained in

R‑banding R-banding is a cytogenetics technique that produces the reverse of the G-band stain on chromosomes. R-banding is obtained by incubating the slides in hot phosphate buffer, then a subsequent treatment of giemsa dye. Resulting chromosome patterns s ...

. Banding can be used to identify chromosomal abnormalities, such as

translocations In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal translo ...

, because there is a unique pattern of light and dark bands for each chromosome. Staining with Giemsa confers a purple color to chromosomes, but micrographs are often converted to grayscale to facilitate data presentation and make comparisons of results from different laboratories. 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. 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 microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissues), in cytology (microscopic study of cells), and in th ...

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. 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. Other types of cytogenic banding are listed below:

References

{{reflist Chromosomes Staining

See also

References