Feulgen stain is a

staining Staining is a technique used to enhance contrast in samples, generally at the Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...

technique discovered by

Robert Feulgen Joachim Wilhelm Robert Feulgen (2 September 1884 – 24 October 1955) was a German physician and biochemist who, in 1914, developed a method for staining DNA (now known as the Feulgen stain) and who also discovered plant and animal nuclear DNA (''" ...

and used in

histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...

to identify chromosomal material or

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

in cell specimens. It is darkly stained. It depends on acid

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of DNA, therefore fixating agents using

strong acid Acid strength is the tendency of an acid, symbolised by the chemical formula , to dissociate into a hydron (chemistry), proton, , and an anion, . The Dissociation (chemistry), dissociation or ionization of a strong acid in solution is effectivel ...

s should be avoided. The specimen is subjected to warm (60 °C)

hydrochloric acid Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...

, then to

Schiff reagent : The Schiff test is an early organic chemistry named reaction developed by Hugo Schiff, and is a relatively general chemical test for detection of many organic aldehydes that has also found use in the staining of biological tissues. The Schiff ...

. In the past, a

sulfite Sulfites or sulphites are compounds that contain the sulfite ion (systematic name: sulfate(IV) ion), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are widely used. Sulfites are ...

rinse followed, but this is now considered unnecessary. Optionally, the sample can be

counterstain A counterstain is a stain with colour contrasting to the principal stain, making the stained structure easily visible using a microscope. Examples include the malachite green counterstain to the fuchsine stain in the Gimenez staining techniqu ...

ed with

Light Green SF yellowish Light, visible light, or visible radiation is electromagnetic radiation that can be perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400–700 nanometres (nm) ...

. Finally, it is dehydrated with

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

, cleared with

xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are su ...

, and mounted in a

resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...

ous medium. DNA should be stained red. The background, if counterstained, is green. This reaction is also specific to DNA since RNA is characterized by a ribose sugar, instead of deoxyribose, preventing acid hydrolysis. This impedes purine detachment and thus do not allow the staining. The Feulgen reaction is a semi-quantitative technique. If the only aldehydes remaining in the cell are those produced from the

of DNA, then the technique is quantitative for DNA. It is possible to use an instrument known as a

microdensitometer A microdensitometer is an optical instrument used to measure optical density, optical densities in the microscopic domain.Christopher Dainty, J. C. Dainty and R. Shaw, ''Image Science'' (Academic, New york, 1974).T. H. James, ''The Theory of the P ...

or microspectrophotometer to actually measure the intensity of the pink Feulgen reaction for a given

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

. Using this procedure, it was early determined that interphase cells were composed of two populations, those with

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

DNA and those with

tetraploid Polyploidy is a condition in which the cells of an organism have more than two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...

DNA (two complete genomes). The nuclei looked identical, but one contained twice as much DNA. This gave rise to the division of the interphase period of the

cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...

to G1, S, and G2 phases based on the synthesis of that extra DNA.

References

{{Reflist Biochemistry detection reactions Histotechnology Staining dyes