DNA: The Hereditary Material

Deoxyribonucleic Acid (DNA) – a double-stranded polymer of nucleotides, each consisting of a

deoxyribose sugar, a phosphate, and four nitrogenous bases that carries the genetic

information of an organism.

The Discovery of DNA

• Friedrich Miescher – investigated the chemical composition of DNA using pus cells.

• Discovered that the nuclei of cells contain large quantities of a substance that does not act like a protein.

• He called this substance nuclein because it was found in the nucleus of cells.

Where do cells store hereditary info?

• Joachim Hammerling (1930s)• Experimented on a large unicellular alga that had 3

distinct regions: foot, stalk, cap, where the nucleus was in the foot

• After amputations, the only region that regenerated was the foot

• Grafting the stalk of one species onto the foot of another, the first cap that regrew resembled the cap of A. When this cap was amputated, the caps that regrew were simlar to B Original info. In stalk A was expressed then used up.

The Location of Hereditary Material• Acetabularia – one-celled green alga• Experiment: Removed the cap from some cells

and the foot from others.

Griffith-Avery Experiment1. Mice injected with virulent strain of Pneumococcus

bacteria died of blood poisoning2. Mice infected with pneumococcus which looked

similar to the bacteria in exp. 1 mice lived3. Heat destroyed bacteria were injected into the mice

mice lived4. Mice injected with mix of heat destroyed bacteria

and living bacteria with missing coats many mice developed disease & died blood contained normal virulent Pneum. Bacteria

Conclusion: info for creating a coat was passed from the dead bacteria to the live coatless bacteria

The Transforming Principle• Oswald Avery, 1944: identified agent that

passed between the bacteria as the transforming principle

• Hershey and Chase (1952)– experiments with a T2 bacteriophage that infects a bacterial host.

• Bacteriophages consist of 2 components: DNA and a protein coat.

• Showed that the DNA, not the protein coat, enters the bacteria.

Hershey-Chase

• Bacteriophages bind to the cell surface then inject their hereditary information into the cell, where new viruses are produced causing the cell to lyse.

• Bacteriophage DNA labelled with P-32 and protein coat labelled with S-35

• Virus allowed to infect bacteria centrifuge S-35 was found in solution, P-32 found in the bacterial cell

Conclusion: hereditary info. Injected into the bacterial cells was DNA

Heinz Fraenkl-Conrat (1957)

Problem: some virus contain RNA not DNAThe protein coat of the tobacco mosaic virus

was combined with the RNA of the Homes ribgrass virus (HRV).

When this virus infected tobacco plants, the leaves developed lesions symptomatic of the HRV virus RNA transfers hereditary info.