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The following points highlight the top five techniques of chromosome banding. The techniques are: 1. C-Bands 2. G.Bands 3. Q-Bands 4. N-Bands 5. Other Techniques of Chromosome Banding.
Chromosome Banding: Technique # 1. C-Bands:
The technique of C-banding originated after the work of Pardue and Gall who reported that constitutive heterochromatin can be stained specifically by Giemsa-solution. Each chromosome possesses a different degree of constitutive heterochromatin which enables the identification of individual chromosomes.
Constitutive heterochromatin is located near the centromere, at telomeres and in the nucleolar organizer regions; it is composed of highly repetitive DNA. C-banding represents the constitutive heterochromatin, and the banding is caused by differential staining reactions of the DNA of heterochromatin and euchromatin.
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The banding method is a complex technique that involves several treatments with acid, alkali or increased temperature. Denaturation of DNA is caused by these treatments. Subsequently, DNA renaturation occurs in treatments with sodium-citrate at 60°C.
By these treatments, the repetitive DNA (heterochromatin) re-natures but low repetitive and unique DNAs do not re-nature. This results in differential staining of the specific chromosome regions. Giemsa-C-banding technique has been used to identify chromosomes of various plant and animal species including human. The Y chromosome of mammals is mostly heterochromatic and therefore, the technique of C-banding is quite useful for its identification.
In barley chromosomes, Linde-Laursen in 1978, divided the C-bands into the following classes based on their position:
(i) Centromeric bands situated at one or both sides of the centromere,
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(ii) Intercalary bands,
(iii) Telomeric bands and
(iv) Bands beside the secondary constriction in the short arm of satellited chromosomes.
He observed polymorphism in C-banding pattern in different barley lines, Giemsa-C-banding patterns may also be used to identify the extra-chromosomes of trisomies and telotrisomics.
Chromosome Banding: Technique # 2. G-Bands:
The technique of G-banding involves Giemsa staining following pretreatment with weak trypsin solution, urea or protease. It provides greater detail than C-banding. It was first used for human chromosomes by Summer et al. in 1971. G-bands may reflect a stronger chromatin condensation. However, this technique is not suitable for plant chromosomes.
Chromosome Banding: Technique # 3. Q-Bands:
The method of Q-banding was developed by Caspersson et al. in 1968. The chromosomes stained with Quinacrine mustard show bright and dark zones under UV light. This technique is used to identify human and mice chromosomes.
Chromosome Banding: Technique # 4. N-Bands:
The technique of N-banding was originally described by Matsui and Sasaki in 1973. Briefly, air-dried chromosomes slides are stained for 90 minutes with Giemsa (diluted 1 : 10 in 1/15 M phosphate buffer at pH 7.0) following extraction with 5% trichloroacetic acid at 95°C for 30 minutes and then 0.1 NHCl at 60″C for 30 minutes.
The N-bands are generally located at the secondary constriction, satellites, centromeres, telomeres and heterochromatic segments. It is suggested that the N-bands represent certain structural non-histone proteins specifically linked to the nucleolar organizer region of the eukaryotic chromosomes.
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The N- banding patterns have been used for the location of nucleolar regions in the different organisms, such as, mammals, birds, amphibians, fishes, insects and plants. N-banding patterns differ in the chromosomes of different species.
In 1980, Islam used this method to identify the barley chromosomes from those of wheat in the reciprocal wheat-barely F, hybrids, and to detect translocations between the wheat and barley chromosomes. He also used this technique to isolate lines possessing a pair of barely chromosomes substituted for particular pair of wheat chromosomes.
A modified Giemsa-N-banding technique was developed by Singh and Tsuchiya in 1982 for the identification of barley chromosomes. This method is a combination of acetocarmine staining and Giemsa-N-banding. After processing according to this method, the centromeric region looks like a “diamond-shaped” structure; this is not seen in other techniques.
Early metaphase or prometaphase chromosomes are more suitable for this staining as they show better banding pattern than the chromosomes at mid-metaphase in somatic cells.
Chromosome Banding: Technique # 5. Other Techniques of Chromosome Banding:
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Besides the above, there are other techniques for chromosome banding, e.g., R-banding (Reverse Giemsa banding). H-banding, and T-banding (Terminal banding). Chromosome banding patterns can be used not only for the identification of individual chromosomes of an organism but also to establish evolutionary relationships between different species.
Banding patterns in human, chimpanzee, gorilla and orangutan have indicated that the evolutionary relationship between human and chimpanzee is closer than that between human and gorilla. It has further indicated that humans have a more distant evolutionary relationship with orangutans.
