Chapter 11 BSC 1005 Mr Tidwell DNA The Molecule of Heredity In 1933 J L Alloway showed that transformation occurred readily in culture dishes In the 1940s Oswald Avery Collin MacLeod and Maclyn McCarty isolated DNA from killed S strain bacteria mixed it with R strain bacteria and produced live S strain bacteria o This suggested that the transforming molecule from the S strain was DNA o To rule out the possibility that a protein contaminant was actually causing the transformation Avery treated samples with protein destroying enzymes and still induced transformation o When DNA destroying enzymes were added to the samples transformation did not occur The secrets of DNA and heredity are found in the 3D structure of the DNA molecule In the 1940s biochemist E Chargraff determined that the amount of A in a DNA molecule equaled the amount of T and the amount of C equaled the amount of G o This finding is referred to as Chargraff s Rule From x ray patterns they determined DNA is long and thin and has a uniform diameter of 2 nanometers The two strands in a DNA double helix are said to be antiparallel oriented in opposite directions Adenine A bonds with thymine T and guanine G bonds with cytosine C o Bases that bond with each other are called complementary base pairs Adenine and guanine Purines are large molecules thymine and cytosine Pyrimidines are relatively smaller o Because base pairing always places a large molecule with a small one the diameter of the double helix remains constant The key to the diverse amount of information DNA codes for lies in the sequence not the number of subunits o Within DNA strand the four types of bases can be arranged in any linear order and this sequence is what encodes genetic information o An organism has millions bacteria to billions plants and animals of nucleotide bases Page 1 of 1