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
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 ...
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
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 ...
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:
See also
*
Eukaryotic chromosome fine structure
*
Gene mapping
*
Fluorescence in situ hybridization
References
{{reflist
Chromosomes
Staining