Unit 6: Gene Expression and Regulation

Heat-killed virulent S strain mixed with live harmless R strain killed mice, because some R cells were transformed into S cells by a heritable 'transforming principle'.

They treated S-strain extract with enzymes; destroying protein or RNA did not stop transformation, but DNase (destroying DNA) did.

Phages labeled with ³²P (DNA) put radioactivity inside infected bacteria, while ³⁵S-labeled protein coats stayed outside after blending.

They showed DNA is a helix with regular repeating dimensions and a backbone on the outside, data Watson and Crick used for the double-helix model (1953).

T = 30% (pairs with A), leaving 40% for G + C, so G = 20% and C = 20% (Chargaff's rules).

Deoxyribose lacks the reactive 2'-OH group, making DNA chemically more stable, and its double strand lets damage on one strand be repaired using the other as a template.