Реферат: Dna Four Bases Essay Research Paper In

Dna Four Bases Essay, Research Paper

In the early 1950’s, the structure of DNA had become a crucial puzzle, following the discovery that DNA and not protein was the “transforming” principle. The puzzle was more intriguing because of the challenge of figuring out how a polymer composed of only 4 different “letters” could encode for a polymer such as proteins that are composed of 20 different letters. DNA is an acid, owing to the phosphate groups between each deoxyribose. The “salt” of DNA is the form in which some of the hydrogen ions have disassociated from the phosphate group. The salt then has a net negative charge, specifically at the oxygens of the phosphates. It has been established by chemical studies that DNA is a polymer of nucleotide subunits, each nucleotide comprising a sugar (deoxyribose), phosphate and one of four different bases – the purines, adenine (A) and guanine (G) together with the pyrimidines, thymine (T) and cytosine (C). A most important clue was the discovery in the late 1940s by Erwin Chargaff and his colleagues at Columbia University that the four bases may occur in varying proportions in the DNAs of different organisms, but the number of A residues is always equal to the number of T residues; similarly equal numbers of G and C residues are present. In the double helix the two strands of DNA run in opposite directions and are complementary, being matched by the hydrogen bonds of the A – T and G – C base pairs. This complementary pairing of the bases ensures that, when DNA replicates, an exact duplicate of the parental genetic information is made. The polymerization of a new complementary strand takes place using each of the old strands as a template. For their outstanding work in discovering the double helical structure of DNA, Watson and Crick shared the 1962 Nobel Prize for Physiology and Medicine with Maurice Wilkins.