The Double HelixThe Complementary Model J.D. Watson F.H. Crick
Evidence for Fibrous Nature of DNA: Physico-chemical analysis • DNA is a long asymmetrical chain that consists of a 5-carbon sugar and phosphate backbone joined in a 3`-5` direction by phosphodisester bonds. • Each sugar has 1 of 4 bases attached to it Adenine, Guanine, Cytosine, and Thymine.
DNA Has Two Chemical Chains: X-ray Crystallography • Two alternating forms of DNA exists. • A crystalline form that occurs at 75% humidity (top) with a 2.8 A reflexion about its meridian.. • A para-crystalline form that occurs at higher humidity (bottom) with 3.4 A reflexion about its meridian. • Density data indicate possibly two distinct polynucleotide chains
Hypothesized Structure • DNA is three dimensional. • DNA has two chains that are coiled around a single axis and held together by hydrogen bonds. • Both chains follow right handed helices. • The phosphates and sugar groups are on the outside and the bases on the inside. • A repeat distance of 34A with a reflexion of spacing 3.4A.
Hypothesized Structure • The two chains are held together by hydrogen bonds between the bases. • The base pairing is specific. • Adenine-Thymine Guanine-Cytosine • Each chain complements the other because of the specific base pairing.
Evidence supporting the Complementary Model X-ray pictures suggests that: • DNA’s basic structure is helical. • High concentration of atoms on the circumference of the helix. • The polynucleotide chains are not distinct from each other.
Evidence supporting the Complementary Model Titration curves of DNA suggest: • Hydrogen bond formation is characteristic of DNA structure Analytical data of the bases suggests: • The amount of A – T and C – G is very close.
Characteristics of Genetic Material • Must be able to self-replicate. • Must exert a highly specific influence on the cell.
DNA Replication: The Complementary Model • Of the two characteristics Watson & Crick proposed only a practical mechanism for replication. • Complementary base pairing is the backbone of replication. • But how does the interior of the helix allow itself to be replicated?
DNA Replication: The Complementary Model • Proposed that DNA unwinds by breaking the hydrogen bonds between the strands. • The single strands would then serve as a template to which complementary free nucleotides would attach and form 2 helices where there was originally one. • They were unaware of the numerous protein machinery (helicase, ligase, polymerases, etc.) that facilitated this process.
DNA Replication Today DNA replication