Introduction to DNA Structure and Function

In 1868, Johann Miescher investigated the chemical composition of the nucleus. From cell nuclei, he isolated an organic acid that was high in phosphorus and called it nuclein. We now call it DNA (deoxyribonucleic acid).

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:

1. Inject mice with R (nonvirulent) cells and the mice lived.
2. Inject mice with S (virulent) cells and the mice died. Blood samples from the dead mice contained many S cells.
3. S cells were killed, then injected into mice and the mice lived.
4. Live R cells plus heat-killed S cells were injected into mice and the mice died. Live S cells were found in the blood.
5. The conclusion was that some substance from the dead S cells had transformed the harmless R cells into cells capable of causing death. Descendents of the transformed cells were also pathogenic.

This animation (Audio - Important) describes the Griffith experiment.

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
Answer viruses.