The Double Helix The 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 1. Must be able to self- replicate. 2. 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