12-1 DNA

Griffith and Transformation In 1928, British scientist Fredereick Griffith was trying to figure out how bacteria make people sick with pneumonia. Griffith had isolated two different strains (types) of pneumonia bacteria from mice. Only one of the strains caused pneumonia. The disease-causing strain grew into smooth colonies on culture plates, the harmless strain had colonies w/ rough edges.

Griffith’s Experiments When Griffith injected mice with the disease-causing strain of bacteria, the mice developed pneumonia and died. When mice were injected with the harmless strain, they didn’t get sick at all.

Transformation Griffith’s next experiment produced an amazing result. He mixed heat killed-disease causing bacteria with live, harmless ones and injected the mixture into mice…and they developed pneumonia and died. The heat-killed bacteria had somehow passed their disease causing ability the harmless strain, and he called this “transformation”.

Transformation Griffith hypothesized that some factor was passed from the heat-killed cells into the live cells.

Avery and DNA Oswald Avery made an extract (juice) from the heat-killed bacteria. When they treated it with an enzyme that destroyed DNA, transformation did not occur. They concluded that DNA was the transforming factor. This is how Avery discovered that DNA stores and transmits genetic information.

The Hershey-Chase Experiment These 2 scientists collaborated in studying viruses, nonliving particles that can infect living organisms.

Bacteriophages One kind of virus that infects bacteria is known as a bacteriophage, which means “bacteria eater”. Bacteriophages are made of a DNA or RNA coat and a protein core. The virus attaches to the surface of a cell and injects its genetic information into it. Many new bacteriophages are made by the cell, and they burst out of the cell when it splits open.

Radioactive Markers They grew viruses in cultures containing radioactive isotopes of phosphorus-32 and sulfur-35. Proteins contain no phosphorus and DNA contains no sulfur. The radioactive isotopes were used as markers to determine that the genetic material of the bacteriophage was DNA, and not protein.

The Components and Structure of DNA DNA is a long molecule made up of units called nucleotides. Each nucleotide is made up of 3 basic components: a 5 carbon sugar, a phosphate group, and a nitrogenous base. There are four kinds of nitrogenous bases in DNA Purines have 2 rings, pyrimidines have 1

Chargaff’s Rules The observation that [A]=[T] and [G]=[C] in any sample of DNA is known as Chargaff’s Rules.

X-Ray Evidence Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule.

The Double Helix James Watson and Francis Crick tried to understand the structure of DNA by using 3 dimensional models of the molecule. Their final model of DNA was a double helix, with 2 strands wound around eachother.