The History of DNA
DNA• Deoxyribonucleic acid (DNA) • Found in nucleus of all
organisms (within chromosomes) • DNA only molecule capable of
replicating itself • Contains instructions that ensure
continuity of life - coded within chemical messages of DNA- regulates the production of proteins
• Ability to change due to mutations and new combinations of genes
Frederich Miescher• 1869 – extracted viscous white substance
from bandages of wounded soldiers- slightly acidic, phosphorus & nitrogen rich- called it nuclein
• Nuclein composed of acidic portion (nucleic acid) and alkaline portion (protein)
• Single nucleic acid was later shown to be 2 nucleic acids- deoxyribonucleic acid (DNA) - ribonucleic acid (RNA)
• DNA material of heredity: early focus was on proteins
Joachim Hammerling
• Acetabularia: green algae, 3 distinct regions (cap, stalk, foot)
• Nucleus in foot: cut off cap and new cap regenerated, cut off foot, no new foot regeneration
• Suggested hereditary material located in nucleus
Frederick Griffith• Lab Exercise pg 644 • Streptococcus pneumoniae – 2 forms
- virulent: S-form (coated)- harmless: R-form
• S-form cells heated and killed, injected into mice and they lived
• Heated cells mixed with R-form cells, killed mice
• Concluded there must be something chemical altering the living cells: transformation - transformed into virulent cells
Avery, McCarty, MacLeod• Lab Exercise pg 645 • 1944 – experiments with Streptococcus
pneumoniae in test tubes • Treated heat-killed virulent bacteria with
a protein-destroying enzyme: transformation still occurred
• Treated heat-killed virulent bacteria with DNA-destroying enzyme: transformation DID NOT occur
• Concluded DNA was “transforming principle” - likely source of hereditary information
Chargraff’s Ratio
• http://www.hhmi.org/biointeractive/chargaffs-ratio
Alfred Hershey & Martha Chase
• 1952 – used bacteriophages (virus) that infect bacterial host (2 components: DNA and protein coat)
• Infects by injecting DNA into it, virus multiplies within and then bursts out, killing the cell
• Hershey & Chase concluded that only the DNA, not protein coat, enters bacteria- tagged viral proteins with isotope of sulfur (not component of DNA)- tagged viral DNA with isotope of phosphorus (component of DNA)
• Allowed tagged bacteriophage to infect bacterial cell • Cells blended to remove protein coats and centrifuged
to isolate virus from bacteria
• Bacterial cells found to contain isotope of phosphorus, not isotope of sulfur
• Isotope of sulfur found in culture medium • Conclusion! DNA was hereditary material
James Watson & Francis Crick• Known that DNA comprised of chains of nucleotides
- consist of 5-carbon cyclic ring: deoxyribose sugar - one of 4 nitrogenous bases attached to 1’ carbon - phosphate group attached to 5’ carbon
• 4 bases: adenine (A), guanine (G), thymine (T), cytosine (C)- A & G: purines (double ring) - C & T: pyrimidines (single ring)
• Evidence from Edwin Chargaff: calculated that amount of adenine always equal to amount of thymine (same for guanine and cytosine). Observed for almost all species
• Evidence from Rosalind Franklin: x-ray diffraction, photograph taken - shows that DNA was a helix, likely double-stranded
James Watson & Francis Crick• All the evidence compiled,
Watson & Crick created a 3D model
• Portrayed relationship between bases as well as bond angles and spacing of atoms- consistent with observations from other researchers to that point
• Won Nobel Prize in 1962 along with Maurice Wilkins (researcher in charge of Rosalind Franklin’s work)
• Rosalind Franklin left out – she died prior to 1962
http://www.hhmi.org/biointeractive/watson-constructing-base-pair-models
• Do the Structure of DNA notes/ppt, then watch The Double Helix from the Howard Hughs Medical Institute website (and do handout)
• http://www.hhmi.org/biointeractive/great-discoveries-science-double-helix