Feulgen stain is a staining technique discovered by Robert Feulgen

Robert Feulgen

Robert Feulgen was a German chemist who, in 1914, developed a method for staining DNA and who also discovered that DNA is located in the chromosomes....

 and used in histology

Histology

Histology is the study of the microscopic anatomy of cells and tissues of plants and animals. It is performed by examining cells and tissues commonly by sectioning and staining; followed by examination under a light microscope or electron microscope...

 to identify chromosomal material

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

 or DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 in cell specimens. It depends on acid hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 of DNA, therefore fixating agents using strong acid

Strong acid

A strong acid is an acid that ionizes completely in an aqueous solution by losing one proton, according to the equationFor sulfuric acid which is diprotic, the "strong acid" designation refers only to dissociation of the first protonMore precisely, the acid must be stronger in aqueous solution than...

s should be avoided.

The specimen is subjected to warm (60 °C) hydrochloric acid

Hydrochloric acid

Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

, then to Schiff reagent. In the past, a sulfite

Potassium metabisulfite

Potassium metabisulfite, K2S2O5, also known as potassium pyrosulfite, is a white crystalline powder with a pungent sulfur odour. The main use for the chemical is as an antioxidant or chemical sterilant. It is a disulfite and is chemically very similar to sodium metabisulfite, with which it is...

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

Counterstain

A counterstain is a stain with color contrasting to the principal stain, making the stained structure more easily visible.An example is the malachite green counterstain to the fuchsine stain in the Gimenez staining technique....

ed with Light Green SF yellowish

Light Green SF yellowish

Light Green SF yellowish, or Light Green, Acid Green, Lissamine green SF, Acid Green 5, Food Green 2, FD&C Green no. 2, Green No. 205, Acid Brilliant Green 5, Pencil Green SF, or C.I. 42095, is a green triarylmethane dye. It is used in histology for staining collagen; for that purpose it is a...

. Finally, it is dehydrated with ethanol

Ethanol

Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, cleared with xylene

Xylene

Xylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...

, and mounted in a resin

Resin

Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

ous medium.

DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 should be stained red. The background, if counterstained, is green.

The Feulgen reaction is a semi-quantitative technique. If the only aldehydes remaining in the cell are those produced from the hydrolysis

Hydrolysis

Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

 of DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

, then the technique is quantitative for DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

. It is possible to use an instrument known as a microdensitometer or microspectrophotometer to actually measure the intensity of the pink Feulgen reaction for a given organelle

Organelle

In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

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

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 and those with tetraploid DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 (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

Cell cycle

The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...

to G1, S, and G2 phases based on the synthesis of that extra DNA.http://homepages.gac.edu/~cellab/chpts/chpt2/ex2-5.html