5.2 In search of the genetic material

. In search of the genetic material As early as , Wilhelm Hofmeister, a German botanist, had observed that cell nuclei organize themselves into small, rod like bodies during mitosis called chromosomes . In , Friedrich Miescher, a Swiss physician, isolated a substance from the cell nuclei and called it as nuclein . It was renamed as nucleic acid by Altman ( ), and is now known as DNA. By , it became clear that chromosomes are made up of proteins and DNA. Many experiments were carried out to study the actual carriers of genetic information. Griffith's experiment proved that DNA is the genetic material which has been dealt in class XI. Bacterial transformation experiments provided the first proof that DNA is the genetic material. However, he could not understand the cause of bacterial transformation, and the biochemical nature of genetic material was not defined from his experiments. Later, Oswald Avery, Colin Macleod and Maclyn McCarty in repeated Griffith’s experiments in an ‘ in vitro ’ system in order One gene-one enzyme hypothesis The experiments of George Beadle and Edward Tatum in the early ’s on Neurospora crassa (the red bread mould) led them to propose one ­gene- one enzyme hypothesis, which states that one gene controls the production of one enzyme. One gene-one polypeptide hypothesis It was observed that an enzyme may be composed of more than one polypeptide chain and a gene can code for only one polypeptide chain. Thus one gene-one polypeptide hypothesis states that one gene controls the production of only one polypeptide chain of an enzyme molecule. XII Std Zoology Chapter XII Std Zoology Chapter Molecular Genetics to identify the nature of the transforming substance responsible for converting a non- virulent strain into virulent strain. They observed that the DNA, RNA and proteins isolated from the heat-killed S-strain when added to R-strain changed their surface character from rough to smooth and also made them pathogenic (Fig. . ) . But when the extract was treated with DNase (an enzyme which destroys DNA) the transforming ability