Dr. Israa ayoub alwan Lec – 1-))   AL-Ma’moon University College Medical Laberatory techniques Department Molecular biology/ Second stage Dr. Israa ayoub alwan Lec – 1-))

Describe the Genetic Material “A genetic material must carry out two jobs: duplicate itself and control the development of the rest of the cell in a specific way,” wrote Francis Crick, codiscoverer with James Watson of the three- dimensional structure of DNA in 1953. Only DNA can do this. DNA was first described in the mid-eighteenth century, when Swiss physician and biochemist Friedrich Miescher isolated nuclei from white blood cells in pus on soiled bandages. In the nuclei, he discovered an unusual acidic substance containing nitrogen and phosphorus.

DNA Is the Hereditary Molecule:- In 1928, English microbiologist Frederick Griffith took the first step in identifying DNA as the genetic material. Griffith noticed that mice with a certain form of pneumonia harbored one of two types of Diplococcus pneumoniae bacteria. Type R bacteria were rough in texture. Type S bacteria were smooth because they are enclosed in a polysaccharide (a type of carbohydrate) capsule. Mice injected with type R bacteria did not develop pneumonia, but mice injected with type S did. The polysaccharide coat shielded the bacteria from the mouse immune system, enabling them to cause severe (virulent) infection. When type S bacteria were heated—which killed them but left their DNA intact—they no longer could cause pneumonia in mice. However, when Griffith injected mice with a mixture of type R bacteria plus heat-killed type S bacteria— neither of which, alone, was deadly to the mice—the mice died of pneumonia. Their bodies contained live type S bacteria, encased in polysaccharide. Griffith termed the apparent conversion of one bacterial type into another “transformation.”

How did it happen? What component of the dead, smooth bacteria transformed type R to type S? U.S. physicians Oswald Avery, Colin MacLeod, and Maclyn McCarty hypothesized that a nucleic acid might be the “transforming principle.” They observed that treating broken- open type S bacteria with a protease—an enzyme that dismantles protein—did not prevent the transformation of a nonvirulent to a virulent strain, but treating it with deoxyribonuclease (or DNase), an enzyme that dismantles DNA only, did disrupt transformation. In 1944, they confirmed that DNA transformed the bacteria. They isolated DNA from heat-killed type S bacteria and injected it with type R bacteria into mice. The mice died, and their bodies contained active type S bacteria. The conclusion: DNA passed from type S bacteria into type R, enabling the type R to manufacture the smooth coat necessary for infection.