Transcript DNA: The Hereditary Material

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 Experiment
1.
Mice injected with virulent strain of Pneumococcus
bacteria  died of blood poisoning
2. Mice infected with pneumococcus which looked
similar to the bacteria in exp. 1  mice lived
3. Heat destroyed bacteria were injected into the mice
 mice lived
4. 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 DNA
The 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.