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.