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