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Biochemistry

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Purines, Pyrimidines and Nucleic Acid Metabolism

Question
38 out of 59
 

Karyotyping under light microscopy: (AIIMS Nov 2009)



A G-banding
B Q BANDING

C C BANDING
D FUdR BANDING

Ans. A G-banding

a. G-banding : It is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes.

b. The metaphase chromosomes are treated with trypsin (to partially digest the protein) and stained with Giemsa.

c. Dark bands that take up the stain are strongly A,T rich (gene poor). The reverse of G-bands is obtained in R-banding.

d. Banding can be used to identify chromosomal abnormalities, such as translocations, because there is a unique pattern of light and dark bands for each chromosome.

e. It is difficult to identify and group chromosomes based on simple staining because the uniform color of the structures makes it difficult to differentiate between the different chromosomes.

f. Therefore, techniques like G-banding were developed that made 'bands' appear on the chromosomes.

g. These bands were same in appearance on the homologous chromosomes, thus, identification became easier and more accurate.

h. The acid/saline/giemsa protocol reveals G-bands. G-banding is a staining technique in which alternating dark and light bands are produced on metaphase chromosomes, with each chromosome displaying a unique banding pattern.

i. The chromosomes can then be counted and characterized to determine the karyotype of a cell. Some limitations of this technique are the ineffectiveness of determining small translocations, detecting microdeletions, and characterizing the chromosomes of cell lines which are complex. However, it is a fast and low-cost technique to determine chromosome number, aneuploidy, large translocations, and macrodeletions.

j. It is a unique chromosome staining technique, used in human cytogenetics to identify individual chromosomes, which produces characteristic bands. It utilises acetic acid fixation, air drying, denaturing chromosomes mildly with proteolytic enzymes, salts, heat, detergents, or urea, and finally Giemsa stain; chromosome bands appear similar to those fluorochromed by Q-banding stain.

Q-banding stain: A fluorescent stain for chromosomes, useful in identifying the Y-chromosome and certain DNA polymorphisms.

Definition of Q banding : A staining technique in which metaphase chromosomes are stained with quinacrine mustard to produce temporary fluorescent Q bands on the chromosomes.

Q-Banding:

Principle: Chromosomes are treated with quinicrine mustard solution, a fluorescent stain, to identify specific chromosomes and structural rearrangements. It is expecially useful for distinguishing the Y chromosome (also Y bodies in interphase nuclei) and various polymorphisms involving satellites and centromeres of specific chromosomes.

C-Banding:

Principle: To specifically stain the centromeric regions and other regions containing constitutive heterochromatin, i.e., the secondary contrictions of human chromsomes 1, 9, 16, and the distal segment of the Y chromosome long arm.

FUdR (fluorodeoxyuridine) is chemical used to induce fragile sites in chromosome.

a. Chromosomal fragile sites are specific sites on chromosomes that undergo breakage at an increased frequency under certain culture conditions.

b. Fragile sites have been recognized on human chromosomes for over 20 years and have been classified into two groups - the rare or heritable fragile sites (h-fra) and the common or constitutive fragile sites (c-fra).

c. A great deal of information is available for the heritable fragile sites 17 h-fra sites were recognized; 16 of these are located on autosomes, and the 17th is on the X chromosome.

d. The common fragile sites can be induced by low folate levels, but their expression is enhanced by chemicals such as aphidicolin, caffeine and fluorodeoxyuridine (FUdR)

Purines, Pyrimidines and Nucleic Acid Metabolism Flashcard List

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