DNA as Genetic Material-Experimental Proof Part II- The Chemical Experiments- Dr. C R Meera

 DNA as Genetic Material-Experimental Proof                                                         Part II- The Chemical Experiments- Dr. C R Meera                                                       

Chemical experiments to prove DNA as genetic material was put forward by Chargaff & collegues in 1940s.

In 1930s, existed the Tetranucleotide hypothesis by Phoebus Levene suggested that DNA is composed of repeating sequences of four nucleotides. The hypothesis suggested that DNA contained equimolar quantities of Adenine, Thymine, Cytosine, and Guanine. There were two reasons behind the development of such a hypothesis.                                                                     1.           Techniques used to separate bases did not resolve them very well, so quantitative analysis was not perfect.

2.           DNA analyzed was isolated from mainly eukaryotes in which four bases are equimolar or from bacteria in which bases were almost equimolar.

The most important clue to the chemical structure of DNA came from the work of Erwin Chargaff and colleagues in the 1940s.  Using the DNA of a wide variety of organisms, Chargaff applied new separation and analytical techniques and showed that molar concentrations of bases could vary widely. Thus, DNA could have variable composition, a primary requirement for genetic material. They found that 4 nucleotide bases of DNA occur in different ratios in the DNA of different organisms. They also found that the amounts of certain bases are closely related. Data collected from so many different species lead Chargaff to the following conclusions known as “Chargaff’s Rules”.

1.           The base composition of DNA generally varies from one species to another.

2.           DNA specimens from different tissues of the same species have the same base composition

3.           The base composition of DNA in a given species does not change with organism’s age, nutritional state, or changing environment

4.           In all cellular DNAs,   regardless of the species, the number of adenosine residues is equal to the number of thymidine residues (A=T) and the number of guanosine residues is equal to cytidine residues (C=G). Thus, the sum of purine residues = sum of pyrimidine residues.

ie;   (A + G)  = (C + T)

Upon publication of Chargaff’s results, Levene’s tetranucleotide hypothesis quietly died and the idea of DNA as the genetic material began to catch on.

Shortly after, researchers in several laboratories found that, for a wide variety of organisms, somatic cells have twice the DNA content than germ cells, a characteristic expected of the genetic material.

Thus, objections to the work of Avery, McLeod, and McCarty were no longer heard and the hereditary nature of DNA rapidly became the acceptable idea.