Chromosome banding helps chromosome deletions, duplications, translocations, inversions, and other less prominent chromosome anomalies to be defined. Chromosome banding, formed after staining with a dye, corresponds to contrasting light and dark regions along the length of a chromosome. A band is characterized as the part of a chromosome that, with the use of one or more banding techniques, is clearly distinguishable from its neighboring segments by appearing darker or lighter.
Methods of Chromosome Banding
- In several distinct species, unique staining methods have unveiled complex collections of bands (transverse stripes) on chromosomes.
- The techniques of chromosome banding are either based on staining chromosomes with a dye or on assaying for a specific function. G-(Giemsa), R-(reverse), C-(centromere), and Q-(quinacrine) banding are the most common methods of dye-based chromosome banding.
- Since their locations and proportions are extremely chromosome-specific, the bands represent valuable landmarks. There are Q bands formed by hydrochloride from quinacrine, G bands produced by stain from Giemsa, and R bands produced by reversed Giemsa. The light and dull fluorescence contrasting bands are called Q-bands. The vivid bands of quinacrine were composed predominantly of DNA rich in adenine and thymine bases.
- On the basis of longitudinal differentiation, chromosome banding is of essential significance for chromosome recognition and is also very useful for elucidating evolutionary relationships in animals.
- It is possible to distinguish four distinct kinds of bands for chromosome characterization.
- Euchromatin, heterochromatin, nucleolar organizer regions, and kinetochores are unique to these bands.
- Throughout the chromosome length in higher vertebrates, euchromatic bands (including Q-, Gand R-bands) form a series of positive (darkly stained or brightly fluorescent) and negative (weakly stained or dimly fluorescent) bands.
- The Q, G, and R bands are likely to represent the degree of DNA compactness and are never related to centromeric heterochromatin.
- At the centromeres or on the ends (telomeres) of chromosomes, darker bands are observed. Black areas of staining are heterochromatic, and light areas of staining are euchromatic.
- In general, in various cells or individuals of a given genus, these regions remain unchanged. Euchromatic areas also undergo a normal contraction and expansion period.
- Heterochromatic bands (C-bands and other more complex types) are heterochromatin-specific and aid in the recognition of chromosomes in most animals without euchromatic bands.
- Heterochromatic bands, typically along the centromeres, are strongly clustered but often appear on the chromosomes elsewhere.
- They refer to heterochromatin that has been classically established, that is, regions of chromosomes that typically remain condensed during the interphase.
- Nucleolar organizer regions (NORs) are regions of chromosomes that are responsible for the creation of nucleoli in interphase nuclei and contain genes for ribosomal RNA.
- Kinetochores are the particular areas in which chromosomes during cell division are bound to the spindle, and this can be illustrated by immunolabelling.
- In terms of DNA base structure, time of DNA replication, and gene material, banding patterns have shown that chromosomes are segmented into a sequence of regions having distinctive properties.
- The chromosome bands are, therefore, in fact, a visible representation of chromosome functional and compositional compartmentalization.
- In polytene chromosomes, normal banding patterns become readily apparent and may act as landmarks such as Drosophila melanogaster in the fruit fly.
- In this genus, the diploid chromosome number is eight (2n=8) and in most of the cells, these eight chromosomes are present.
- However, some curious peculiarities occur in the cells of the special organs comprising the polytene chromosomes. Each chromosome’s banding pattern is unique. The bands do not reflect genes, since there are more genes than the number of polythene bands in any chromosomal area of Drosophila.
Uses of Chromosome Banding
- In humans, to classify chromosome mutations and rearrangements in hereditary disorders and cancers, G-banding is used. For the detection of chromosome rearrangements that have arisen during evolution, banding is often useful.