Chromosome Banding

Introduction - the earliest techniques stained chromosomes uniformly, and only allowed a few chromosomes of unusual size or shape to be identified unequivocally. - it is not possible to distinguish between the p and q arms of metacentric chromosome. Mitotic chromosomes (metaphase) of incense cedar (Calocedrus decurrens) stained by aceto-carmine

Chromatin - Chromosomes are made of chromatin. - there are two forms of chromatin: heterochromatin euchromatin - these two forms of chromatin can be distinguished by intensity of staining.

Heterochromatin - condensed chromatin and genetically inactive (limited transcription) during interphase. - it consists of repetitive DNA sequences which are relatively rich in AT base pairs and is late replicating in the cell cycle. - heterochromatic regions of chromosomes are generally referred to as positively heteropycnotic (pycnotic = dense) by cytologist. - is darkly stained.

Heterochromatin - heterochromatin is usually localized to the periphery of the nucleus.

Heterochromatin - Heterochromatin is believed to serve several functions, from gene regulation to the protection of the integrity of chromosomes. - there are two types of heterochromatin: constitutive heterochromatin and facultative heterochromatin.

Constitutive Heterochromatin - constitutive heterochromatin is present at identical positions on all chromosomes in all cell types of an organism. - any genes contained within the constitutive heterochromatin will be poorly expressed. - all human chromosomes 1, 9, 16, and the Y chromosome contain large regions of constitutive heterochromatin. - in most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres.

Facultative Heterochromatin - facultative heterochromatin is variable in its expression. It varies with the cell type and may be manifested as condensed, or heavily stained, chromatin in only certain differentiated somatic cells in the same organism. - an example of facultative heterochromatin is X- chromosome inactivation in female mammals: one X chromosome is packaged in facultative heterochromatin and silenced, while the other X chromosome is packaged in euchromatin and expressed.

Euchromatin - chromatin that is not belonged to heterochromatin is euchromatin. - is lightly stained due to the less compact structure. - early-replicating and GC rich region. - it should be noted that in prokaryotes, euchromatin is the only form of chromatin present. - euchromatin participates in the active transcription of DNA to mRNA products.

Chromosome Banding - chromosome banding is developed based on the presence of heterochromatin and euchromatin. - heterochromatin is darkly stained whereas euchromatin is lightly stained during chromosome staining. - a band is defined as that part of a chromosome which is clearly distinguishable from its adjacent segments by appearing darker or brighter with one or more banding techniques. - there are a few types of chromosome banding: G- banding, C-banding, Q-banding, R-banding etc.

G-Banding - G-banding is obtained with Giemsa stain following digestion of chromosomes with enzyme trypsin. - Giemsa stain, named after Gustav Giemsa, an early malariologist, is used for the histopathological diagnosis of malaria and other parasites. It is a mixture of methylene blue and eosin. It is specific for the phosphate groups of DNA and attaches itself to regions of DNA where there are high amounts of adenine-thymine bonding. - it yields a series of lightly and darkly stained bands - the dark regions tend to be heterochromatic, late-replicating and AT rich. The light regions tend to be euchromatic, early-replicating and GC rich.

G-Banding G-banding of human female metaphase chromosomes

Q-Banding - Q-banding is a fluorescent pattern obtained using quinacrine for staining. The pattern of bands is very similar to that seen in G-banding. - Quinacrine banding (Q-banding) was the first staining method used to produce specific banding patterns for mammalian chromosomes. - It is especially useful for distinguishing the Y chromosome. - This method requires a fluorescence microscope (quinacrine fluoresces strongly in the ultraviolet) and is no longer as widely used as G-banding.

Q-Banding Q-banding of human male metaphase chromosomes

R-Banding - R-banding is the reverse of G-banding (the R stands for "reverse"). - the dark regions are euchromatic (guanine-cytosine rich regions) and the bright regions are heterochromatic (thymine-adenine rich regions). - telomeres are stained well by this procedure. - Reverse banding (R-banding) requires heat treatment and reverses the usual white and black pattern that is seen in G-bands and Q-bands.

R-Banding R-banding of human female metaphase chromosomes

C-Banding - C-banding stains the constitutive heterochromatin, which usually lies near the centromere. Chromosomes of mouse Chromosomes of human female