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DNA: The Information Molecule
How did scientists use evidence to discover that the DNA molecule
contains information?
A Historical Perspective
Type of Molecule Structure Function
Proteins
Carbohydrates
Fats
DNA
Complex A few known functions
Simple
Simple
Simple
Energy and Structural
Energy and Structural
Unknown
What molecule contains information in cells?
Experiment 1: Griffith (1928)
•Molecules from dead virulent Streptococcus can transform benign Streptococcus into a virulent strain.
There are two strains of Streptococcus pneumoniae.ROUGH COLONY (R) SMOOTH COLONY (S)
R strain is benign(Lacking a protectivecapsule, it is recognizedand destroyed byhost’s immune system)
S strain is virulent(Protective capsuleprevents detection byhost’s immune system)
Explain each treatment:
1. What data shows the heat-killed S strain did not cause death of the mouse in treatment #4?
2. What data shows S strain molecules cannot kill the mouse alone; the streptococcus must be alive?
3. What data shows that the R strain alone will not kill the mouse?
4. What data shows that the S strain IS virulent?
Griffith’s Conclusion
• The R strain was able to take in molecules from the dead S strain.
• These molecules somehow gave the R strain characteristics of the dead S strain. – Ability to make protective coating
So, we know that there is a molecule that transfers information…
BUT…we still haven’t proved what those molecules are!
What molecule contains information in cells?
• Experiment 2. (Avery et al., 1942)– Extracted material from dead virulent Streptococcus and treated it with enzymes to destroy either DNA or RNA or proteins.
Sample shouldContainNO PROTEIN
Adddeoxy-ribonuclease
Sample shouldContainNO DNA
Addribonuclease
Sample shouldContainNO RNA
Addproteinases
Sample shouldcontainNO PROTEIN
Adddeoxy-ribonuclease
Sample shouldcontainNO DNA
Addribonuclease
Sample shouldcontainNO RNA
Add R cellsAdd R cells
S cellsappear
S cellsappear
No S cellsappear
Transformation occurs No transformation occurs
Conclusion: Transformation cannot occur unless __________ is present.Therefore, ______________ must be the hereditary material.
Add R cells
Addproteinases
DETERMINING THAT DNA IS THE HEREDITARY MATERIAL
Heat-killedS cells
Lipids
Carbohydrates
1. Remove the lipidsand carbohydratesfrom a solution ofheat-killed S cells.Proteins, RNA, andDNA remain.
2. Subject thesolution totreatments ofenzymes to destroyeither the proteins,RNA, or DNA.
3. Add a small portion of each sample to a culturecontaining R cells.Observe whether transformation hasoccurred by testing for the presencevirulent S cells.
Are Genes Composed of DNA or Protein?
T2 Virus
Cell
Virus protein coat
Host cell membrane
Virus DNA
1. Start of infection.Virus DNA enters hostcells. Protein coatdoes not.
2. Virus DNA directsthe production of newvirus particles.
3. End of infection.New generation ofvirus particles burstfrom host cell.
Host cell genome
Virus particle
1. Viral genomeenters host cell.
Host cell genome
Virus particle
1. Viral genomeenters host cell.
2. Viral genomeis replicated andtranscribed.
DNAmRNA
Host cell genome
Virus particle
1. Viral genomeenters host cell.
2. Viral genomeis replicated andtranscribed.
DNAmRNA
3. Viral mRNA aretranslated and proteinsprocessed.
Protein
HOW DO VIRUSES WORK?
Free particlesin tissue orenviroment
4. Particles assembleinside host, then burstor bud to exterior.
Hershey Chase Experiment 1952
Two treatments:
1. Radioactive amino acids
2. Radioactive nucleotides
Treatment 1
T2 Phage
Bacterium
Radioactive protein (35S)
Radioactive phage infects bacterial cells
Blender separates protein coats from bacterial surface
Centrifuge and measure radioactivity in pellet and supernatant
Radioactivity in supernatant, but not pellet
Treatment 2
Radioactive DNA (32P)
Radioactivity in pellet, but not supernatant
Therefore, it is the viral DNA, and not protein, that programs cells to make copies of the virus.
Are Genes Composed of DNA or Protein?
• T2 virus experiment convinces skeptics that DNA is the genetic material since the material injected by the virus into host cells is DNA, not protein.
T2 Virus
What we learned
DNA is the molecule that carries informationInformation in DNA determines which
proteins can be made by an organismDNA can be passed between organisms in
nature.Bacteria can take in pieces of DNA from
their environment. This process is called transformation.