Discovery of DNA and its 3D Structure

1869 Fredrich Miescher — his proposal for DNA

• studied pus cells

• believed that nuclein (not protein) is the genetic material

1900s Phoebus Levene — his proposal for DNA

• renamed nuclein to DNA/RNA

• discovered nucleotides (sugar, phosphate, and 1 of 4 bases)

• proposed nucleic acids have a tetranucleotide structure (repeating sequence of 4 nucleotides), TOO SIMPLE

• tetranucleotide arrangement made the world shift back to protein idea

1928 Frederick Griffith

• injects rats with r-strain (rough) & s-strain (smooth = virulent)

• r-strain - mice lived, s-strain - died, heat killed s-strain - lived, killed s-strain+ living r-strain - died

• transforming principle — something from the heat-killed bacteria transformed living bacteria (non-pathogenic) to become deadly

1944 Oswald Avery, Macyln McCarty, Colin MacLeod

• continued with pneumonia

• tested for protein, nuclein & DNA

• destroyed DNA and unable to do transforming principle (proved DNA is used in transformation principle)

• not ready to publish

1952 Alfred Hershey, Martha Chase

• blender experiment — radiolabeled viral proteins (s-35) & viral DNA (p-32) and injected into bacteriophage

• DNA has no sulfur, proteins has sulfur, no phosphorus (very clever experiment)

• proved DNA had genetic material

1949 Erwin Chargaff

• DNA structure — % composition and base pair rules (A + T, C + G)

1953 Rosalind Franklin

• 3D DNA structure —> X-Ray diffraction

• double helix w/ repeating pattern every 3.4nm & 0.34nm

1954 Watson & Crick

• used Chargaff’s rule and Franklin’s findings

• shows H-bonds, double helix, turn clockwise every 10 nucleotides, 5’ — 3’ direction, antiparallel

how DNA is wrapped EUKARYOTES

• wrapped around histones (proteins) for protection & reduce volume

• nucleosome — group of 8 histones

• chromatin — nucleosome arranged in loops (scaffold)

• in mitosis — chromosomes (visible)

how DNA is wrapped PROKARYOTES

• DNA resides in nucleoid — no nucleus

• eubacterial chromosomes are found in loops that join together, making a ring, can supercoil

• could have plasmids (accessory loops)

• archaea have circular chromosomes associated with histones