Chapter 12.1 DNA

Griffith and Transformation Fredrick Griffith was trying to figure out how bacteria made people sick in 1928. He looked at 2 types of pneumonia bacteria from mice. He wanted to know if the bacteria produced poison so he heat killed it and infected mice. It did not make the mice sick. He then mixed the heat-killed bacteria with harmless bacteria – this made the mice sick and die of pneumonia. Griffith called this process transformation – one strain of bacteria had been changed into another.

Avery and DNA Oswald Avery repeated Griffith’s work. He decided a molecule in the heat-killed bacteria changed the harmless bacteria into harmful bacteria. He found out that DNA stores and transmits genetic information from one generation to the next.

The Hershey-Chase Experiment They wanted to find out for sure what carried genetic information. They used viruses. Specifically a virus found in bacteria called a bacteriophage. Bacteriophages have a DNA or RNA core and a protein coat. The question was whether the genetic information was the protein coat or the DNA.

Radioactive Markers To figure out which part of the virus carried genes they grew viruses in cultures that contains either phosphorous-32 or sulfur-35. The sulfur marked the protein coat and the phosphorous marked the DNA. If sulfur was found in the bacteria, it means that the protein had been injected into the bacteria and that part carried genes. If phosphorous was found, then DNA had been injected. What they found was that all of the bacteria had phosphorous. That convinced Hershey and Chase that the genetic material of the bacteriophage was DNA, not protein.

The Components and Structure of DNA Scientists wanted to know how genes worked. Specifically3 things: 1. genes had to carry information from one generation to the next. 2. They had to put that information to work by determining the heritable characteristics of organisms. 3. Genes had to be easily copied because it needed to replicate every time the cell divided. DNA is made of nucleotides. Nucleotides have 3 parts: a 5 carbon sugar called deoxyribose, a phosphate group, and a nitrogenous base. There are 4 bases – adenine and guanine (these are called purines) and cytosine and thymine (these are called pyrimidines). The backbone of DNA is made from the sugar and phosphate. The nitrogenous bases stick out and they join together.

Chargaff’s Rules Erwin Chargaff found that there was always the same amount of guanine (G) and cytosine (C) in a DNA sample. He also found there were always the same amounts of adenine (A) and thymine (T). Chargaff’s rules state that A = T and C = G

X-Ray Evidence In the 50s Rosalind Franklin studied DNA. She used X-ray diffraction to get information about DNA. Her x-ray showed that the strands in DNA are twisted around each other. It also shows there are 2 strands.

The Double Helix Francis Crick and James Watson also were researching the shape of DNA. They found that DNA was a double helix – it was 2 strands wound around each other.

They found that the 2 sides were held together by hydrogen bonds between nitrogen bases. It also showed that only certain bases paired together. He found only A and T and then C and G paired together – this is called base pairing.