In 1951, Linus Pauling deduced the structure of proteins. He and others thought that proteins with their 20 amino acids were complex enough to code hereditary instructions.
However, additional experiments in the 1950s showed that DNA is the hereditary material. A number of scientists raced to determine the structure of DNA.
In 1953, Watson and Crick put together a model of DNA and proposed that DNA is a double helix . This turned out to be the real stuff of heredity.
In 1928, Fred Griffith was attempting to develop a vaccine for a pneumonia causing bacterium. He isolated two strains of Streptococcus pneumoniae. A rough strain (R or nonvirulent) that was harmless and a smooth strain (S or virulent) that was pathogenic.
His experiments are summarized here:
Oswald and Avery showed (in 1944) that the "Griffith substance" was nucleic acid, not protein as some people had proposed. They showed that cell extracts treated with protein-digesting enzymes could still transform bacteria. However, cell extracts treated with DNA-digesting enzymes lost their transforming ability They concluded that DNA, not protein, transforms bacteria
Viruses that infect bacteria are called bacteriophages. They consist of protein and DNA and can inject their hereditary material into bacteria for reproduction. Because they consist of only a protein coat and a nucleic acid core, these viruses were used in experiments by Hershey and Chase to prove which of these (protein or DNA) was the heredity material.
Hershey and Chase created labeled bacteriophages containing radioactive sulfur or radioactive phosphorus. Bacteriophage proteins contain sulfur but not phosphorus. Bacteriophage DNA contains phosphorus but not sulfur. They allowed the labeled viruses to infect bacteria. Afterwards, they found the radioactive sulfur outside the bacterial cell and the radioactive phosphorus within. This indicated that DNA was the hereditary material.
This animation (Audio - Important) describes the Hershey-Chase experiments.
REVIEW: Bacteriophages are
REVIEW: The significance of the experiments in which 32P and 35S were used is that
REVIEW: Sulfur is
PREVIOUS
NEXT
LECTURE 8 INDEX
MAIN INDEX