Download
the double helix the complementary model n.
Skip this Video
Loading SlideShow in 5 Seconds..
The Double Helix The Complementary Model PowerPoint Presentation
Download Presentation
The Double Helix The Complementary Model

The Double Helix The Complementary Model

173 Vues Download Presentation
Télécharger la présentation

The Double Helix The Complementary Model

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. The Double HelixThe Complementary Model J.D. Watson F.H. Crick

  2. 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.

  3. 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

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. Characteristics of Genetic Material • Must be able to self-replicate. • Must exert a highly specific influence on the cell.

  9. 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?

  10. 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.

  11. DNA Replication Today DNA replication