BIO273 Biology: The Structure of DNA PDF

BIO273 Biology The Structure of DNA DNA Structure Erwin Schrodinger (1944)  Theoretical Physicist (have you heard of his cat) Wrote a book “What is Life?”  The storing & passing of biological information Chromosomes was where information was stored Information was compres...

BIO273 Biology The Structure of DNA DNA Structure Erwin Schrodinger (1944)  Theoretical Physicist (have you heard of his cat) Wrote a book “What is Life?”  The storing & passing of biological information Chromosomes was where information was stored Information was compressed into a code  To understand life Must understand the DNA molecule  Who would be the first to crack the code? SCHOOL OF BIOLOGICAL SCIENCES 2 AND APPLIED CHEMISTRY DNA Structure Erwin Chargaff (1949)  Chargaff’s Rules Amount of Adenine (A) in DNA = Thymine (T) Amount of Guanine (G) in DNA = Cytosine (C)  Ratio of A : G in DNA differs from between species SCHOOL OF BIOLOGICAL SCIENCES 3 AND APPLIED CHEMISTRY DNA Structure Linus Pauling (1951)  Discovered α-helix secondary structure of Protein  Set sites on the structure of DNA  Started to create 3D models to determine DNA structure  Theory of a triple helix with a Sugar – Phosphate core  Incorrect But competition helped the Cavendish Laboratory SCHOOL OF BIOLOGICAL SCIENCES 4 AND APPLIED CHEMISTRY Structure of DNA James Watson, Francis Crick & Rosalind Franklin (1953)  Cavendish Laboratory Director was disappointed Pauling beat the team to protein α-helix  Started using Pauling method for the structure of DNA Building 3D models (Watson & Crick @ Cavendish) X-ray Crystallography (Franklin & King’s College)  Watson & Crick were shown Franklin’s work prior to publication Used some of her unpublished data without her consent Her work indicated that the Sugar-Phosphate backbone must be on the outside of the molecule & two DNA strands were antiparallel  Watson & Crick got all the credit (until recently) SCHOOL OF BIOLOGICAL SCIENCES 5 AND APPLIED CHEMISTRY Watson & Crick (1953) Put all the pieces together  Chargaff A=T, C=G  Franklin Backbone on outside & antiparallel strands  Pauling Bond angles & molecular distances SCHOOL OF BIOLOGICAL SCIENCES 6 AND APPLIED CHEMISTRY Structure of DNA Double Stranded Helix held together by Hydrogen bonding  Sugar - Phosphate backbone on outside Deoxyribose Sugar Phosphodiester Covalent bonds  Complementary base pairing Nitrogenous Bases on the inside Hydrogen bonding - A with T; C with G  Antiparallel Strands run in opposite directions  Outer edge of bases exposed for outside bonding Access for proteins involved in gene expression & DNA replication SCHOOL OF BIOLOGICAL SCIENCES 7 AND APPLIED CHEMISTRY Sugar – Phosphate Backbone  Sugar-Phosphate backbone outside  Phosphodiester covalent bond  Sugar is Deoxyribose  DNA is negatively charged because of the Phosphate Remember this! We can use this fact in the Lab SCHOOL OF BIOLOGICAL SCIENCES 8 AND APPLIED CHEMISTRY DNA is Double Stranded  Two separate strands of DNA joined together by Hydrogen bonds  Nitrogenous Bases on the inside Contains Nitrogen  Complementary Base Pairing Adenine has 2 Hydrogen-bonds with Thymine Guanine has 3 Hydrogen-bonds with Cytosine Purine (double ring) always bonds to a Pyrimidine (single ring) SCHOOL OF BIOLOGICAL SCIENCES 9 AND APPLIED CHEMISTRY Antiparallel Strands  Two separate strands run in opposite directions  Use the outside Carbon on the Sugar for which end is which SCHOOL OF BIOLOGICAL SCIENCES 10 AND APPLIED CHEMISTRY Structure of DNA SCHOOL OF BIOLOGICAL SCIENCES 11 AND APPLIED CHEMISTRY Summary  The work of many Scientists, over the course of many years, discovered the structure of DNA  The constancy in DNA’s bonding pattern is the basis for life’s unity  The variation in DNA’s base sequences is the basis for life’s diversity SCHOOL OF BIOLOGICAL SCIENCES 12 AND APPLIED CHEMISTRY