Decoding Life - Discovery of DNA

Griffith experiment

Studied Pneumonia in mice

• 2 strains of bacteris:

  • Smooth strain —> virulent (kills mice)

  • Rough Strain —> nonvirulent (does not kill mice)

• Live Smooth Strain injected into mice —> mouse dies

• Live Rough Strain injected into mice —> mouse lives

• Heat-killed Smooth strain injected into mice —> mouse lives

• Heat-killed Smooth strain + Live Rough strain injected into mice —> mouse dies

  • Live S strain bacteria recvered from dead mouse

• CONCLUSION:

  • A substance from the dead Smooth strain transformed live Rough strain into smooth strain —> transformation

  • Depicted that genetic information CAN be transferred through bacteria

Avery, MacLeod + McCarty Experiment

• Followed up on Griffiths experiment, goal was to identify the TRANSFORMING FACTOR/PRINCIPLE

• Used same pneumonia bacteria

  • Smooth, virulent strain and Rough, nonvirulent strain

  • Heat Killed smooth strain bacteria were prepped

  • Cell contents of the dead Smooth strain bacteria were extracted and split into 3 samples”

    • one treated with protease (destroys proteins)

    • one treated with RNase (destroys RNA)

    • one treated with DNase (destroys DNA)

  • Each treated sample was mixed with live rough strain bacteria

  • RESULTS:

    • Protease treated —> rough(nonvirulent) strain became smooth strain (virulent)

    • RNase treated —> rough(nonvirulent) strain became smooth strain (virulent)

    • DNase treated sasmple —> R strain did NOT transform

  • CONCLUSION: DNA is the transofrming principle

  • DNA carries genetic information, not protein or RNA

Hershey Chase Experiment

• Goal: determine whether DNA of protein is the hereditary (genetic) material

• Studied bacteriophages —> viruses that inflect E.coli bacteria, made of DNA and protein only

Chargraffs rule

The amount of Adenine = the amount of Thymine, the amount of Guanine = Amount of Cytocine

Rosalind Franklin and Maurice Wilkins

a.) DNA was crystallized and X-ray diffraction/crstallography was performed

b.) Hit with X-rays, diffracted in a regular pattern as DNA is a repeating structure

c.) The pattern is produced on a photographic plate, which can then be visualized

James Watson and Francis Crick

a.) Watson and Crick found the 3 dimensional structure of the DNA molecule

b.) This structure explained how genetic material could be replicated and passed on to daughter cells

c.) The two strands of the parent molecule could each be used as a Template to produce two new daughter molecules

Meselson-Stahl Experiment

Proposed three ways that DNA could replicate;

1.) Semi conservative replication —> half of each daughter molecule of DNA has been conserved from the old strand, half has been newly synthesized.

2.) Conservative replication: One daighter DNA is composed of all the original nucleotides; the other composed of all the newer nucleotides

3.) Dispersive Replication; Each new strand is a combination of old and new nucleotides.

Meselston Stahl experiment procedure

1.) E.Coli grown in N nitrogen (heavy isotope-

2.) swuitch to N nitrogen (light) and after 1 or 2 replication cycles…

3.) take samples of DNA

4.) Mix with cesium chloride and separate heavy and light DNA

5.) DNA of heavy, light, and intermediate densities can be separated by centrifugation (rapidly spinning to separate components of a mixture)

6.) The patterns formed by the DNA molecules could only occur if DNA replication is semiconservative.